# RETRO: a Modern, Pragmatic Forth Welcome to RETRO, my personal take on the Forth language. This is a modern system primarily targetting desktop, mobile, and servers, though it can also be used on some larger (ARM, MIPS32) embedded systems. The language is Forth. It is untyped, uses a stack to pass data between functions called words, and a dictionary which tracks the word names and data structures. But it's not a traditional Forth. RETRO draws influences from many sources and takes a unique approach to the language. RETRO has a large vocabulary of words. Keeping a copy of the Glossary on hand is highly recommended as you learn to use RETRO. This book will hopefully help you develop a better understanding of RETRO and how it works. The text in these files is Copyright (c) 2018-2020 by Charles Childers. To the extent possible under law, Charles Childers has waived all copyright and related or neighboring rights to the RETRO Documentation. This work is published from: United States. The historical papers are Copyright (c) 1999-2000 by Tom Novelli. ## Legal Text See https://creativecommons.org/publicdomain/zero/1.0/legalcode The laws of most jurisdictions throughout the world automatically confer exclusive Copyright and Related Rights (defined below) upon the creator and subsequent owner(s) (each and all, an "owner") of an original work of authorship and/or a database (each, a "Work"). Certain owners wish to permanently relinquish those rights to a Work for the purpose of contributing to a commons of creative, cultural and scientific works ("Commons") that the public can reliably and without fear of later claims of infringement build upon, modify, incorporate in other works, reuse and redistribute as freely as possible in any form whatsoever and for any purposes, including without limitation commercial purposes. These owners may contribute to the Commons to promote the ideal of a free culture and the further production of creative, cultural and scientific works, or to gain reputation or greater distribution for their Work in part through the use and efforts of others. For these and/or other purposes and motivations, and without any expectation of additional consideration or compensation, the person associating CC0 with a Work (the "Affirmer"), to the extent that he or she is an owner of Copyright and Related Rights in the Work, voluntarily elects to apply CC0 to the Work and publicly distribute the Work under its terms, with knowledge of his or her Copyright and Related Rights in the Work and the meaning and intended legal effect of CC0 on those rights. 1. Copyright and Related Rights. A Work made available under CC0 may be protected by copyright and related or neighboring rights ("Copyright and Related Rights"). Copyright and Related Rights include, but are not limited to, the following: - the right to reproduce, adapt, distribute, perform, display, communicate, and translate a Work; - moral rights retained by the original author(s) and/or performer(s); - publicity and privacy rights pertaining to a person's image or likeness depicted in a Work; - rights protecting against unfair competition in regards to a Work, subject to the limitations in paragraph 4(a), below; - rights protecting the extraction, dissemination, use and reuse of data in a Work; - database rights (such as those arising under Directive 96/9/EC of the European Parliament and of the Council of 11 March 1996 on the legal protection of databases, and under any national implementation thereof, including any amended or successor version of such directive); and - other similar, equivalent or corresponding rights throughout the world based on applicable law or treaty, and any national implementations thereof. 2. Waiver. To the greatest extent permitted by, but not in contravention of, applicable law, Affirmer hereby overtly, fully, permanently, irrevocably and unconditionally waives, abandons, and surrenders all of Affirmer's Copyright and Related Rights and associated claims and causes of action, whether now known or unknown (including existing as well as future claims and causes of action), in the Work (i) in all territories worldwide, (ii) for the maximum duration provided by applicable law or treaty (including future time extensions), (iii) in any current or future medium and for any number of copies, and (iv) for any purpose whatsoever, including without limitation commercial, advertising or promotional purposes (the "Waiver"). Affirmer makes the Waiver for the benefit of each member of the public at large and to the detriment of Affirmer's heirs and successors, fully intending that such Waiver shall not be subject to revocation, rescission, cancellation, termination, or any other legal or equitable action to disrupt the quiet enjoyment of the Work by the public as contemplated by Affirmer's express Statement of Purpose. 3. Public License Fallback. Should any part of the Waiver for any reason be judged legally invalid or ineffective under applicable law, then the Waiver shall be preserved to the maximum extent permitted taking into account Affirmer's express Statement of Purpose. In addition, to the extent the Waiver is so judged Affirmer hereby grants to each affected person a royalty-free, non transferable, non sublicensable, non exclusive, irrevocable and unconditional license to exercise Affirmer's Copyright and Related Rights in the Work (i) in all territories worldwide, (ii) for the maximum duration provided by applicable law or treaty (including future time extensions), (iii) in any current or future medium and for any number of copies, and (iv) for any purpose whatsoever, including without limitation commercial, advertising or promotional purposes (the "License"). The License shall be deemed effective as of the date CC0 was applied by Affirmer to the Work. Should any part of the License for any reason be judged legally invalid or ineffective under applicable law, such partial invalidity or ineffectiveness shall not invalidate the remainder of the License, and in such case Affirmer hereby affirms that he or she will not (i) exercise any of his or her remaining Copyright and Related Rights in the Work or (ii) assert any associated claims and causes of action with respect to the Work, in either case contrary to Affirmer's express Statement of Purpose. 4. Limitations and Disclaimers. No trademark or patent rights held by Affirmer are waived, abandoned, surrendered, licensed or otherwise affected by this document. Affirmer offers the Work as-is and makes no representations or warranties of any kind concerning the Work, express, implied, statutory or otherwise, including without limitation warranties of title, merchantability, fitness for a particular purpose, non infringement, or the absence of latent or other defects, accuracy, or the present or absence of errors, whether or not discoverable, all to the greatest extent permissible under applicable law. Affirmer disclaims responsibility for clearing rights of other persons that may apply to the Work or any use thereof, including without limitation any person's Copyright and Related Rights in the Work. Further, Affirmer disclaims responsibility for obtaining any necessary consents, permissions or other rights required for any use of the Work. Affirmer understands and acknowledges that Creative Commons is not a party to this document and has no duty or obligation with respect to this CC0 or use of the Work. # Obtaining RETRO The RETRO source code can be obtained from http://forthworks.com/retro or gopher://forthworks.com/1/retro ## Stable Releases I periodically make stable releases. This will typically happen two to four times per year. These are good for those needing a solid base that doesn't change frequently. ## Snapshots A lot of development happens between releases. I make snapshots of my working source tree nightly (and often more often). This is what I personally recommend for most users. It reflects my latest system and is normally reliable as it's used daily in production. The latest snapshot can be downloaded from the following stable URLs: * http://forthworks.com/retro/r/latest.tar.gz * gopher://forthworks.com/9/retro/r/latest.tar.gz ## Repository I use a Fossil repository to manage development. To obtain a copy of the repository install Fossil and: ``` fossil clone http://forthworks.com:8000 retro.fossil mkdir retro cd retro fossil open /path/to/retro.fossil ``` See the Fossil documentation for details on using Fossil to keep your local copy of the repository current. This will let you stay current with my latest changes faster than the snapshots, but you may occasionally encounter bigger problems as some commits may be in a partially broken state. If you have problems, check the version of Fossil you are using. I am currently using Fossil 2.7, you may experience issues checking out or cloning if using older versions. # Building on BSD, Linux, macOS, and outher Inix Targets RETRO is well supported on BSD (tested on FreeBSD, NetBSD, OpenBSD), Linux, and macOS systems. It should build on any of these without issue. ## Requirements - c compiler & linker - standard headers - make ## Process For a standard 32-bit system: Run `make` For a 64-bit system: Run `make OPTIONS=-DBIT64` This will build the toolchain and then the main `retro` executable. ## Executables In the `bin/` directory: retro retro-unu retro-muri retro-extend retro-embedimage retro-describe ## Issues If you run into any build issues, please send details to crc@forth.works so I can work on addressing them as quickly as possible. # Building RETRO on Windows It is possible to build RETRO on Windows, though a few of the extensions are not supported: - no `unix:` words - no `gopher:` words This is currently more difficult than on a Unix host. If you have Windows 10 and WSL, it may be better to build under that (using the Linux instructions). ## Setup Build Environment RETRO on Windows is built with TCC. Go to http://download.savannah.gnu.org/releases/tinycc/ Download the *winapi-full* and *tcc-xxxx-bin* packages for your system. Decompress them, copy the headers from the winapi package into the tcc directory. ## Prepare Source Copy the `source/interfaces/retro-windows.c` and the `source/interfaces/retro-windows.c` to the directory you setup tcc into. ## Build Building will require use of the command line. Assuming that tcc.exe is in the current directory along with the RETRO sources: tcc retro-windows.c -o retro.exe # Building Alternative Systems In addition to the C implementation, there are a few other interfaces that can be built. ## Requirements - c compiler (tested: clang, tcc, gcc) - make - standard unix shell ## retro-repl A basic interactive system can be built by using: make bin/retro-repl This requires a copy of `ngaImage` to be in the current directory. ## Barebones This is a minimal version of the `retro-repl`. It keeps the C portion as short as possible, making it a useful starting point for new interfaces. To build: make bin/retro-barebones ## retro-compiler This is a turnkey compiler. It can compile a new executable bundling a Retro VM and image. Requirements: - BSD or Linux - objcopy in $PATH To build: make bin/retro-compiler Example use: 1. Given a source file like "Hello.forth": ~~~ :hello 'hello_world! s:put nl ; ~~~ 2. Use: ./bin/retro-compiler Hello.forth hello The first argument is the source file, the second is the word to run on startup. 3. Run the generated `a.out` Limits: This only supports the core words ('all' interface) and the file i/o words. Support for other I/O extensions will be added in the future. ## Pascal There is a Pascal version of `retro-repl`. Dependencies: - freepascal Building: cd vm/nga-pascal fpc listener.lpr This will require a copy of the `ngaImage` in the current directory. ## Python: retro.py This is an implementation of `retro-repl` in Python. As with `retro-repl` it requires the `ngaImage` in the current directory when starting. ## C#: retro.cs This is an implementation of `retro-repl` in C#. As with `retro-repl` it requires the `ngaImage` in the current directory when starting. Building: cd vm\nga-csharp csc retro.cs You'll need to make sure your path has the CSC.EXE in it, or provide a full path to it. Something like this should reveal the path to use: dir /s %WINDIR%\CSC.EXE I've only tested building this using Microsoft's .NET tools. It should also build and run under Mono. # Advanced Builds ## Custom Image For users of BSD, Linux, macOS, you can customize the image at build time. In the top level directory is a `package` directory containing a file named `list.forth`. You can add files to compile into your system by adding them to the `list.forth` and rebuilding. Example: If you have wanted to include the NumbersWithoutPrefixes.forth example, add: ~~~ 'example/NumbersWithoutPrefixes.forth include ~~~ To the start of the `list.forth` file and then run `make` again. The newly built `bin/retro` will now include your additions. # Starting RETRO RETRO can be run for scripting or interactive use. ## Interactive To start it interactively, run: `retro` without any command line arguments, or with `-i`, `-s`, or `-i,c`. Starting the interactive system: ``` retro ``` Or: ``` retro -i ``` This should be sufficient for most uses. Starting the interactive system (without displaying the startup banner): ``` retro -s ``` ## Using In a Pipe If using a Unix shell and piping input between processes, you will probably want to use `-s` to supress the startup messages and `Ok` prompt that normally appear. E.g., ``` echo "'lol s:put nl" | retro -s ``` ## Running A Program In A File You can run code in a file very easily. This is simply: ``` retro filename ``` You can follow the filename with any arguments that it may need. These will be accessible to the program via the `sys:argc` and `sys:argv` words. Source files must be written in Unu format. ## Scripting You can use RETRO to write scripts. Add a shebang: ``` #!/usr/bin/env retro ``` And make the file executable. Source files must be written in Unu format. ## Command Line Arguments For a summary of the full command line arguments available: Scripting Usage: retro filename [script arguments...] Interactive Usage: retro [-h] [-i] [-c] [-s] [-f filename] [-t] -h Display this help text -i Interactive mode (line buffered) -s Suppress the startup text -f filename Run the contents of the specified file -t Run tests (in ``` blocks) in any loaded files # Basic Interactions Start RETRO in interactive mode: ``` retro -i ``` You should see something similar to this: RETRO 12 (rx-2019.6) 8388608 MAX, TIB @ 1025, Heap @ 9374 At this point you are at the *listener*, which reads and processes your input. You are now set to begin exploring RETRO. To exit, run `bye`: ``` bye ``` # Unu: Simple, Literate Source Files RETRO is written in a literate style. Most of the sources are in a format called Unu. This allows easy mixing of commentary and code blocks, making it simple to document the code. As an example, # Determine The Average Word Name Length To determine the average length of a word name two values are needed. First, the total length of all names in the Dictionary: ~~~ #0 [ d:name s:length + ] d:for-each ~~~ And then the number of words in the Dictionary: ~~~ #0 [ drop n:inc ] d:for-each ~~~ With these, a simple division is all that's left. ~~~ / ~~~ Finally, display the results: ~~~ 'Average_name_length:_%n\n s:format s:put ~~~ This illustrates the format. Only code in the fenced blocks (between \~~~ pairs) get extracted and run. (Note: this only applies to *source files*; fences are not used when entering code interactively). ## On The Name The name Unu comes from the Maori language, where it means: (verb) (-hia) pull out, withdraw, draw out, extract. Taken from https://maoridictionary.co.nz/ # A Quick Tutorial Programming in RETRO is all about creating words to solve the problem at hand. Words operate on data, which can be kept in memory or on the stack. Let's look at this by solving a small problem: writing a word to determine if a string is a palindrome. A palindrome is a phrase which reads the same backward and forward. We first need a string to look at. Starting with something easy: ``` 'anna ``` Looking in the Glossary, there is a `s:reverse` word for reversing a string. We can find `dup` to copy a value, and `s:eq?` to compare two strings. So testing: ``` 'anna dup s:reverse s:eq? ``` This yields -1 (`TRUE`) as expected. So we can easily name it: ``` :palindrome? dup s:reverse s:eq? ; ``` Naming uses the `:` prefix to add a new word to the dictionary. The words that make up the definition are then placed, with a final word (`;`) ending the definition. We can then use this: ``` 'anna palindrome? ``` Once defined there is no difference between our new word and any of the words already provided by the RETRO system. # Syntax RETRO has more syntax than a traditional Forth due to ideas borrowed from ColorForth and some design decisions. This has some useful traits, and helps to make the language more consistent. ## Tokens Input is divided into a series of whitespace delimited tokens. Each of these is then processed individually. There are no parsing words in RETRO. Tokens may have a single character *prefix*, which RETRO will use to decide how to process the token. ## Prefixes Prefixes are single characters added to the start of a token to guide the compiler. The use of these is a major way in which RETRO differs from traditional Forth. When a token is passed to `interpret`, RETRO first takes the intitial character and looks to see if there is a word that matches this. If so, it will pass the rest of the token to that word to handle. In a traditional Forth, the interpret process is something like: get token is token in the dictionary? yes: is it immediate? yes: call the word. no: are we interpreting? yes: call the word no: compile a call to the word no: is it a number? yes: are we interpreting? yes: push the number to the stack no: compile the number as a literal no: report an error ("not found") In RETRO, the interpret process is basically: get token does the first character match a `prefix:` word? yes: pass the token to the prefix handler no: is token a word in the dictionary? yes: push the XT to the stack and call the class handler no: report an error ("not found") All of the actual logic for how to deal with tokens is moved to the individual prefix handlers, and the logic for handling words is moved to word class handlers. This means that prefixes are used for a lot of things. Numbers? Handled by a `#` prefix. Strings? Use the `'` prefix. Comments? Use `(`. Making a new word? Use the `:` prefix. The major prefixes are: | Prefix | Used For | | ------ | ----------------------------- | | @ | Fetch from variable | | ! | Store into variable | | & | Pointer to named item | | # | Numbers | | $ | ASCII characters | | ' | Strings | | ( | Comments | | : | Define a word | The individual prefixes will be covered in more detail in the later chapters on working with different data types. ## Word Classes Word classes are words which take a pointer and do something with it. These are covered in detail in their own chapter, but essentially they decide *how* to execute or compile specific types of words. # Additional Tools In addition to the core `retro` binary, the `bin` directory will contain a few other tools. ## retro This is the main RETRO binary. ## retro-describe This is a program that looks up entries in the Glossary. At the command line, you can use it like: ``` retro-describe s:for-each ``` ## retro-embedimage This is a program which generates a C file with the ngaImage contents. It's used when building `retro`. ``` retro-embedimage ngaImage ``` The output is written to stdout; redirect it as needed. ## retro-extend This is a program which compiles code into the ngaImage. It's used when building `retro` and when you want to make a standalone image with custom additions. Example command line: ``` retro-extend ngaImage example/rot13.forth ``` Pass the image name as the first argument, and then file names as susequent ones. Do *not* use this for things relying on I/O apart from the basic console output as it doesn't emulate other devices. If you need to load in things that rely on using the optional I/O devices, see the **Advanced Builds** chapter. ## retro-muri This is the assembler for Nga. It's used to build the initial RETRO kernel and can be used by other tools as well. ## retro-tags TBD ## retro-unu This is the literate source extraction tool for RETRO. It is used in building `retro`. Example usage: ``` retro-unu literate/RetroForth.md ``` Output is written to stdout; redirect as neeeded. # The Optional Retro Compiler In addition to the base system, users of RETRO on Unix hosts with ELF executables can build and use the `retro-compiler` to generate turnkey executables. ## Requirements - Unix host - ELF executable support - objcpy in the $PATH ## Building BSD users: make bin/retro-compiler Linux users: make -f Makefile.linux bin/retro-compiler ## Installing Copy `bin/retro-compiler` to somewhere in your $PATH. ## Using `retro-compiler` takes two arguments: the source file to compile and the name of the word to use as the main entry point. Example: Given a `hello.forth`: ``` ~~~ :hello 'Hello_World! s:put nl ; ~~~ ``` Use: ``` retro-compiler hello.forth hello ``` The compiler will generate an `a.out` file which you can then rename. ## Known Limitations This does not provide the scripting support for command line arguments that the standard `retro` interface offers. A copy of `objcopy` needs to be in the path for compilation to work. The current working directory must be writable. This only supports hosts using ELF executables. The output file name is fixed to `a.out`. # Working With Arrays RETRO offers a number of words for operating on statically sized arrays. ## Namespace The words operating on arrays are kept in an `a:` namespace. ## Creating Arrays The easiest way to create an array is to wrap the values in a `{` and `}` pair: ``` { #1 #2 #3 #4 } { 'this 'is 'an 'array 'of 'strings } { 'this 'is 'a 'mixed 'array #1 #2 #3 } ``` You can also make an array from a quotation which returns values and the number of values to store in the a: ``` [ #1 #2 #3 #3 ] a:counted-results [ #1 #2 #3 #3 ] a:make ``` ## Accessing Elements You can access a specific value with `a:th` and `fetch` or `store`: ``` { #1 #2 #3 #4 } #3 a:th fetch ``` ## Find The Length Use `a:length` to find the size of the array. ``` { #1 #2 #3 #4 } a:length ``` ## Duplicate Use `a:dup` to make a copy of an a: ``` { #1 #2 #3 #4 } a:dup ``` ## Filtering RETRO provides `a:filter` which extracts matching values from an array. This is used like: ``` { #1 #2 #3 #4 #5 #6 #7 #8 } [ n:even? ] a:filter ``` The quote will be passed each value in the array and should return TRUE or FALSE. Values that lead to TRUE will be collected into a new array. ## Mapping `a:map` applies a quotation to each item in an array and constructs a new array from the returned values. Example: ``` { #1 #2 #3 } [ #10 * ] a:map ``` ## Reduce `a:reduce` takes an array, a starting value, and a quote. It executes the quote once for each item in the array, passing the item and the value to the quote. The quote should consume both and return a new value. ``` { #1 #2 #3 } #0 [ + ] a:reduce ``` ## Search RETRO provides `a:contains?` and `a:contains-string?` to search an array for a value (either a number or string) and return either TRUE or FALSE. ``` #100 { #1 #2 #3 } a:contains? 'test { 'abc 'def 'test 'ghi } a:contains-string? ``` ## Implementation In memory, an array is a count followed by the values. As an example, if you have an array: { #10 #20 #30 } In memory this would be setup as: | Offset | Value | | ------ | ----- | | 000 | 3 | | 001 | 10 | | 002 | 20 | | 003 | 30 | You can construct one on the fly by keeping a pointer to `here` and using `,` to place the values. E.g., here [ #3 , #10 , #20 , #30 , ] dip An example of this can be seen in this excerpt from an example (*example/Primes.forth*): :create-set (-a) here #3000 , #2 #3002 [ dup , n:inc ] times drop ; # Working With Assembly Language RETRO runs on a virtual machine called Nga. It provides a standard assembler for this called *Muri*. Muri is a simple, multipass model that's not fancy, but suffices for RETRO's needs. ## Assembling A Standalone File A small example (*test.muri*) ~~~ i liju.... r main : c:put i liiire.. i 0 : main i lilica.. d 97 i liju.... r main ~~~ Assembling it: retro-muri test.muri So breaking down: Muri extracts the assembly code blocks to assemble, then proceeds to do the assembly. Each source line starts with a directive, followed by a space, and then ending with a value. The directives are: : value is a label i value is an instruction bundle d value is a numeric value r value is a reference s value is a string to inline Instructions for Nga are provided as bundles. Each memory location can store up to four instructions. And each instruction gets a two character identifier. From the list of instructions: 0 nop 5 push 10 ret 15 fetch 20 div 25 zret 1 lit 6 pop 11 eq 16 store 21 and 26 halt 2 dup 7 jump 12 neq 17 add 22 or 27 ienum 3 drop 8 call 13 lt 18 sub 23 xor 28 iquery 4 swap 9 ccall 14 gt 19 mul 24 shift 29 iinvoke This reduces to: 0 .. 5 pu 10 re 15 fe 20 di 25 zr 1 li 6 po 11 eq 16 st 21 an 26 ha 2 du 7 ju 12 ne 17 ad 22 or 27 ie 3 dr 8 ca 13 lt 18 su 23 xo 28 iq 4 sw 9 cc 14 gt 19 mu 24 sh 29 ii Most are just the first two letters of the instruction name. I use `..` instead of `no` for `NOP`, and the first letter of each I/O instruction name. So a bundle may look like: dumure.. (This would correspond to `dup multiply return nop`). ## Runtime Assembler RETRO also has a runtime variation of Muri that can be used when you need to generate more optimal code. So one can write: :n:square dup * ; Or: :n:square \dumure.. ; The second one will be faster, as the entire definition is one bundle, which reduces memory reads and decoding by 2/3. Doing this is less readable, so I only recommend doing so after you have finalized working RETRO level code and determined the best places to optimize. The runtime assembler has the following directives: i value is an instruction bundle d value is a numeric value r value is a reference Additionally, in the runtime assembler, these are reversed: 'dudumu.. i Instead of: i dudumu.. # Working With a Buffer RETRO provides words for operating on a linear memory area. This can be useful in building strings or custom data structures. ## Namespace Words operating on the buffer are kept in the `buffer:` namespace. ## Implementation A buffer is a linear sequence of memory. The buffer words provide a means of incrementally storing and retrieving values from it. The buffer words keep track of the start and end of the buffer. They also ensure that an `ASCII:NULL` is written after the last value, which make using them for string data easy. ## Limitations Only one buffer can be active at a time. RETRO provides a `buffer:preserve` combinator to allow using a second one before returning to the prior one. ## Set The Active Buffer To set a buffer as the active one use `buffer:set`. This takes an address. The buffer will be assumed to be empty. The inital value will be set to ASCII:NULL. ## Add Value Use `buffer:add` to append a value to the buffer. This takes a single value and will also add an ASCII:NULL after the end of the buffer. ## Fetch Last Value To return the last value in the buffer you can use `buffer:get`. This removes the value and sets an ASCII:NULL in the memory location the returned value occupied. ## Get Data About The Buffer RETRO provides `buffer:start` to get the initial address in the buffer, `buffer:end` to get the last address (ignoring the ASCII:NULL), and `buffer:size` to return the number of values in the buffer. ## Reset You can reset a buffer to the empty state using `buffer:empty`. ## Example To begin, create a memory region to use as a buffer. ``` 'Test d:create #1025 allot ``` Then you can set this as the current buffer: ``` &Test buffer:set ``` When a buffer is set, the vocabulary sets an internal index to the first address in it. This will be incremented when you add data and decremented when you remove data. Let's add some stuff using `buffer:add`: ``` #100 buffer:add #200 buffer:add #300 buffer:add ``` And then retreive the values: ``` buffer:get n:put nl buffer:get n:put nl buffer:get n:put nl ``` You can remove all values using `buffer:empty`: ``` #100 buffer:add #200 buffer:add #300 buffer:add buffer:empty ``` And ask the buffer how many items it contains: ``` buffer:size n:put nl #100 buffer:add #200 buffer:add #300 buffer:add buffer:size n:put nl buffer:empty ``` The other functions are `buffer:start`, which returns the address of the buffer, `buffer:end`, which returns the address of the last value, and `buffer:preserve`. The first is easy to demo: ``` buffer:start Test eq? n:put nl ``` The last one is useful. Only one buffer is ever active at a given time. The `buffer:preserve` combinator lets you execute a word, saving and restoring the current buffer indexes. So the word could assign and use a new buffer and this will reset the previous one after control returns. There are a few notes that need to be considered. The preserve combinator saves the start and current index but *not* the contents. If the word you call uses the same buffer, the contents will remain altered. Finally, the buffer words have one interesting trait: they store an ASCII NULL after adding each item to the buffer. This lets one use them to build strings easily. ``` Test buffer:set $h buffer:add $e buffer:add $l buffer:add $l buffer:add $o buffer:add $, buffer:add #32 buffer:add $w buffer:add $o buffer:add $r buffer:add $l buffer:add $d buffer:add buffer:start s:put nl ``` # Working With Characters RETRO provides words for working with ASCII characters. ## Prefix Character constants are returned using the `$` prefix. ## Namespace Words operating on characters are in the `c:` namespace. ## Classification RETRO provides a number of words to determine if a character fits into predefined groups. The primary words for this are: * `c:consonant?` * `c:digit?` * `c:letter?` * `c:lowercase?` * `c:uppercase?` * `c:visible?` * `c:vowel?` * `c:whitespace?` There are also corresponding "not" forms: * `c:-consonant?` * `c:-digit?` * `c:-lowercase?` * `c:-uppercase?` * `c:-visible?` * `c:-vowel?` * `c:-whitespace?` All of these take a character and return either a `TRUE` or `FALSE` flag. ## Conversions A few words are provided to convert case. Each takes a character and returns the modified character. * `c:to-lower` * `c:to-number` * `c:to-upper` * `c:toggle-case` RETRO also has `c:to-string`, which takes a character and creates a new temporary string with the character. ## I/O Characters can be displayed using `c:put`. ``` $a c:put ``` With the default system on BSD, Linux, and macOS (and other Unix style hosts), `c:get` is provided to read input. This may be buffered, depending on the host. # Checking The Version RETRO releases add and change things. You can use the `Version` variable to determine the version in use and react accordingly. ``` @Version #201906 eq? [ 'Needs_2019.6! s:put nl bye ] if ``` This can be also be used to conditionally load compatibility files: ``` (If_newer_than_2016.6,_load_aliases_for_renamed_words) @Version #201906 gt? [ 'Renamed_2019.6.forth include ] if ``` ## Version Number Format The version is a six digit number encoding the year and month of the release. So: 201901 is 2019.1 201906 is 2019.6 201911 is 2019.11 A `#100 /mod` will suffice to split these if needed. # Working With The Dictionary The Dictionary is a linked list containing the dictionary headers. ## Namespace Words operating on the dictionary are in the `d:` namespace. ## Variables `Dictionary` is a variable holding a pointer to the most recent header. ## Header Structure Each entry follows the following structure: Offset Contains ------ --------------------------- 0000 Link to Prior Header 0001 Link to XT 0002 Link to Class Handler 0003+ Word name (null terminated) RETRO provides words for accessing the fields in a portable manner. It's recommended to use these to allow for future revision of the header structure. ## Accessing Fields Given a pointer to a header, you can use `d:xt`, `d:class`, and `d:name` to access the address of each specific field. There is no `d:link`, as the link will always be the first field. ## Shortcuts For The Latest Header RETRO provides several words for operating on the most recent header. `d:last` returns a pointer to the latest header. `d:last` will give the contents of the `d:xt` field for the latest header. There are also `d:last` and `d:last`. ## Adding Headers Two words exist for making new headers. The easy one is `d:create`. This takes a string for the name and makes a new header with the class set to `class:data` and the XT field pointing to `here`. Example: ``` 'Base d:create ``` The other is `d:add-header`. This takes a string, a pointer to the class handler, and a pointer for the XT field and builds a new header using these. Example: ``` 'Base &class:data #10000 d:add-header ``` ## Searching RETRO provides two words for searching the dictionary. `d:lookup` takes a string and tries to find it in the dictionary. It will return a pointer to the dictionary header or a value of zero if the word was not found. `d:lookup-xt` takes a pointer and will return the dictionary header that has this as the `d:xt` field, or zero if no match is found. ## Iteration You can use the `d:for-each` combinator to iterate over all entries in the dictionary. For instance, to display the names of all words: ``` [ d:name s:put sp ] d:for-each ``` For each entry, this combinator will push a pointer to the entry to the stack and call the quotation. ## Listing Words Most Forth systems provide WORDS for listing the names of all words in the dictionary. RETRO does as well, but this is named `d:words`. This isn't super useful as looking through several hundred names is annoying. RETRO also provides `d:words-with` to help in filtering the results. Example: ``` 'class: d:words-with ``` # Errors RETRO does only minimal error checking. ## Non-Fatal A non-fatal error will be reported on *word not found* during interactive or compile time. Note that this only applies to calls: if you try to get a pointer to an undefined word, the returned pointer will be zero. ## Fatal A number of conditions are known to cause fatal errors. The main ones are stack overflow, stack underflow, and division by zero. On these, RETRO will generally exit. For stack depth issues, the VM will attempt to display an error prior to exiting. In some cases, the VM may get stuck in an endless loop. If this occurs, try using CTRL+C to kill the process, or kill it using whatever means your host system provides. ## Rationale Error checks are useful, but slow - especially on a minimal system like RETRO. The overhead of doing depth or other checks adds up quickly. As an example, adding a depth check to `drop` increases the time to use it 250,000 times in a loop from 0.16 seconds to 1.69 seconds. # Working With Floating Point Some RETRO systems include support for floating point numbers. When present, this is built over the system `libm` using the C `double` type. Floating point values are typically 64 bit IEEE 754 double precision (1 bit for the sign, 11 bits for the exponent, and the remaining 52 bits for the value), i.e. 15 decimal digits of precision. ## Prefix Floating point numbers start with a `.` Examples: Token Value .1 1.0 .0.5 0.5 .-.4 -0.4 .1.3 1.3 ## Namespace Floating point words are in the `f:` namespace. There is also a related `e:` namespace for *encoded values*, which allows storing of floats in standard memory. ## Operation Floating point values exist on a separate stack, and are bigger than the standard memory cells, so can not be directly stored and fetched from memory. The floating point system also provides an alternate stack that can be used to temporarily store values. The following words exist for arranging values on the floating point stack. These are direct analogs to the non-prefiexd words for dealing with the data stack. - `f:nip` - `f:over` - `f:depth` - `f:drop` - `f:drop-pair` - `f:dup` - `f:dup-pair` - `f:dump-stack` - `f:tuck` - `f:swap` - `f:rot` For the secondary floating point stack, the following words are provided: - `f:push` - `f:pop` - `f:adepth` - `f:dump-astack` ## Constants | Name | Returns | | -------- | ----------------- | | `f:E` | Euler's number | | `f:-INF` | Negative infinity | | `f:INF` | Positive infinity | | `f:NAN` | Not a Number | | `f:PI` | PI | ## Comparisons The basic set of comparators are the same as those for operating on integers. These are: - `f:-eq?` - `f:between?` - `f:eq?` - `f:gt?` - `f:lt?` - `f:negative?` - `f:positive?` - `f:case` There are also a few additions for comparing to special values like infinity and NaN. - `f:-inf?` - `f:inf?` - `f:nan?` ## Basic Math - `f:*` - `f:+` - `f:-` - `f:/` - `f:abs` - `f:floor` - `f:inc` - `f:limit` - `f:max` - `f:min` - `f:negate` - `f:power` - `f:ceiling` - `f:dec` - `f:log` - `f:sqrt` - `f:square` - `f:round` - `f:sign` - `f:signed-sqrt` - `f:signed-square` ## Geometry RETRO provides a small number of words for doing geometric related calculations. | Word | Returns | | -------- | ------------ | | `f:acos` | arc cosine | | `f:asin` | arc sine | | `f:atan` | arc tangent | | `f:cos` | cosine | | `f:sin` | sine | | `f:tan` | tangent | ## Storage and Retrieval By leveraging the encoded value functions, RETRO is able to allow storage of floating point values in memory. This does have a tradeoff in accuracy as the memory cells are considerably smaller than a full floating point size. You can use `f:fetch` to fetch a floating point value and `f:store` to store one. If you need more precision, try Kiyoshi Yoneda's FloatVar example (`example/FloatVar.forth`), which includes words to store and retrieve values using multiple cells. - `f:to-number` - `f:to-string` ## I/O The floating point vocabulary has a single I/O word, `f:put`, for the display of floating point numbers. ## Encoded Values RETRO provides a means of encoding and decoding floating point values into standard integer cells. This is based on the paper "Encoding floating point values to shorter integers" by Kiyoshi Yoneda and Charles Childers. - `f:E1` - `f:to-e` - `e:-INF` - `e:-inf?` - `e:INF` - `e:MAX` - `e:MIN` - `e:NAN` - `e:clip` - `e:inf?` - `e:max?` - `e:min?` - `e:n?` - `e:nan?` - `e:put` - `e:to-f` - `e:zero?` # Lexical Scope RETRO has a single dictionary, but does provide a means of using lexical scope to keep this dictionary clean. ## Example ``` {{ 'A var :++A &A v:inc ; ---reveal--- :B ++A ++A @A n:put nl ; }} ``` In this example, the lexical namespace is created with `{{`. A variable (`A`) and word (`++A`) are defined. Then a marker is set with `---reveal---`. Another word (`B`) is defined, and the lexical area is closed with `}}`. The headers between `{{` and `---reveal---` are then hidden from the dictionary, leaving only the headers between `---reveal---` and `}}` exposed. ## Notes This only affects word visibility within the scoped area. As an example: ``` :a #1 ; {{ :a #2 ; ---reveal--- :b 'a s:evaluate n:put ; }} ``` In this, after `}}` closes the area, the `:a #2 ;` is hidden and the `s:evaluate` will find the `:a #1 ;` when `b` is run. # Naming Conventions Word names in RETRO generally follow the following conventions. ## General Guidelines * Readability is important * Be consistent * Don't use a prefix as the first character of a name * Don't use underscores in word names * Use short names for indices * Word names start with a `-` for "not" * Words returning a flag end in ? ## Typical Format The word names will generally follow a form like: [namespace:]name The `namespace:` is optional, but recommended for consistency with the rest of the system and to make it easier to identify related words. ## Case Word names are lowercase, with a dash (-) for compound names. ``` hello drop-pair s:for-each ``` Variables use TitleCase, with no dash between compound names. ``` Base Heap StringBuffers ``` Constants are UPPERCASE, with a dash (-) for compound names. ``` TRUE FALSE f:PI MAX-STRING-LENGTH ``` ## Namespaces Words are grouped into broad namespaces by attaching a short prefix string to the start of a name. The common namespaces are: | Prefix | Contains | | ------- | ------------------------------------------------------ | | a: | Words operating on simple arrays | | ASCII: | ASCII character constants for control characters | | buffer: | Words for operating on a simple linear LIFO buffer | | c: | Words for operating on ASCII character data | | class: | Contains class handlers for words | | d: | Words operating on the Dictionary | | err: | Words for handling errors | | io: | General I/O words | | n: | Words operating on numeric data | | prefix: | Contains prefix handlers | | s: | Words operating on string data | | v: | Words operating on variables | | file: | File I/O words | | f: | Floating Point words | | gopher: | Gopher protocol words | | unix: | Unix system call words | ## Tips Avoid using a prefix as the first character of a word name. RETRO will look for prefixes first, this will prevent direct use of the work in question. To find a list of prefix characters, do: ``` 'prefix: d:words-with ``` Underscores in strings are replaced by spaces. This is problematic, especially with variables. Consider: 'test_name var #188 !test_name In this, the string for the name is converted to "test name". The store in the second line will not add the space, so resolves to an incorrect address. I personally recommend avoiding the use of underscores in any word names. # Working With Numbers Numbers in RETRO are signed, 32 bit integers with a range of -2,147,483,648 to 2,147,483,647. ## Token Prefix All numbers start with a `#` prefix. ## Namespace Most words operating on numbers are in the `n:` namespace. # Working With Pointers ## Prefix Pointers are returned by the `&` prefix. ## Examples ``` 'Base var &Base fetch #10 &Base store #10 &n:inc call ``` ## Notes The use of `&` to get a pointer to a data structure (with a word class of `class:data`) is not required. I like to use it anyway as it makes my intent a little clearer. Pointers are useful with combinators. Consider: ``` :abs dup n:negative? [ n:negate ] if ; ``` Since the target quote body is a single word, it is more efficient to use a pointer instead: ``` :abs dup n:negative? &n:negate if ; ``` The advantages are speed (saves a level of call/return by avoiding the quotation) and size (for the same reason). This may be less readable though, so consider the balance of performance to readability when using this approach. # The Return Stack RETRO has two stacks. The primary one is used to pass data beween words. The second one primarily holds return addresses. Each time a word is called, the next address is pushed to the return stack. # Stack Diagrams Most words in RETRO have a stack comment. These look like: (-) (nn-n) As with all comments, a stack comment begins with `(` and should end with a `)`. There are two parts to the comment. On the left side of the `-` is what the word *consumes*. On the right is what it *leaves*. RETRO uses a short notation, with one character per value taken or left. In general, the following symbols represent certain types of values. | Notation | Represents | | ------------------- | ----------------------- | | b, n, m, o, x, y, z | generic numeric values | | s | string | | v | variable | | p, a | pointers | | q | quotation | | d | dictionary header | | f | `TRUE` or `FALSE` flag. | In the case of something like `(xyz-m)`, RETRO expects z to be on the top of the stack, with y below it and x below the y value. And after execution, a single value (m) will be left on the stack. Words with no stack effect have a comment of (-) # Working With Strings Strings in RETRO are NULL terminated sequences of values representing characters. Being NULL terminated, they can't contain a NULL (ASCII 0). The character words in RETRO are built around ASCII, but strings can contain UTF8 encoded data if the host platform allows. Words like `s:length` will return the number of bytes, not the number of logical characters in this case. ## Prefix Strings begin with a single `'`. ``` 'Hello 'This_is_a_string 'This_is_a_much_longer_string_12345_67890_!!! ``` RETRO will replace spaces with underscores. If you need both spaces and underscores in a string, escape the underscores and use `s:format`: ``` 'This_has_spaces_and_under\_scored_words. s:format ``` ## Namespace Words operating on strings are in the `s:` namespace. ## Lifetime At the interpreter, strings get allocated in a rotating buffer. This is used by the words operating on strings, so if you need to keep them around, use `s:keep` or `s:copy` to move them to more permanent storage. In a definition, the string is compiled inline and so is in permanent memory. You can manually manage the string lifetime by using `s:keep` to place it into permanent memory or `s:temp` to copy it to the rotating buffer. ## Mutability Strings are mutable. If you need to ensure that a string is not altered, make a copy before operating on it or see the individual glossary entries for notes on words that may do this automatically. ## Searching RETRO provides four words for searching within a string. `s:contains-char?` `s:contains-string?` `s:index-of` `s:index-of-string` ## Comparisons `s:eq?` `s:case` ## Extraction To obtain a new string containing the first `n` characters from a source string, use `s:left`: ``` 'Hello_World #5 s:left ``` To obtain a new string containing the last `n` characters from a source string, use `s:right`: ``` 'Hello_World #5 s:right ``` If you need to extract data from the middle of the string, use `s:substr`. This takes a string, the offset of the first character, and the number of characters to extract. ``` 'Hello_World #3 #5 s:substr ``` ## Joining You can use `s:append` or `s:prepend` to merge two strings. ``` 'First 'Second s:append 'Second 'First s:prepend ``` ## Tokenization `s:tokenize` `s:tokenize-on-string` `s:split` `s:split-on-string` ## Conversions To convert the case of a string, RETRO provides `s:to-lower` and `s:to-upper`. `s:to-number` is provided to convert a string to an integer value. This has a few limitations: - only supports decimal - non-numeric characters will result in incorrect values ## Cleanup RETRO provides a handful of words for cleaning up strings. `s:chop` will remove the last character from a string. This is done by replacing it with an ASCII:NULL. `s:trim` removes leading and trailing whitespace from a string. For more control, there is also `s:trim-left` and `s:trim-right` which let you trim just the leading or trailing end as desired. ## Combinators `s:for-each` `s:filter` `s:map` ## Other `s:evaluate` `s:copy` `s:reverse` `s:hash` `s:length` `s:replace` `s:format` `s:empty` ## Controlling The Temporary Buffers As dicussed in the Lifetime subsection, temporary strings are allocated in a rotating buffer. The details of this can be altered by updating two variables. | Variable | Holds | | ------------- | ---------------------------------------- | | TempStrings | The number of temporary strings | | TempStringMax | The maximum length of a temporary string | For example, to increase the number of temporary strings to 48: ``` #48 !TempStrings ``` The defaults are: | Variable | Default | | ------------- | ------- | | TempStrings | 32 | | TempStringMax | 512 | It's also important to note that altering these will affect the memory map for all temporary buffers. Do not use anything already in the buffers after updating these or you will risk data corruption and possible crashes. # The Stacks The stacks are a defining feature of Forth. They are are used to pass data between words and to track return addresses for function calls. RETRO always has two stacks, and optionally (if built with floating point support) a third. ## Data Stack This is the primary stack. Values are placed here, passed to words which consume them and then return results. When I refer to "the stack", this is the one I mean. Learning to use the stack is a crucial part to making effective use of RETRO. ### Placing Values On The Stack Values can be placed on the stack directly. | Example | Action | | -------------- | ---------------------------------------- | | `#300123` | Push the number `300123` to the stack | | `$h` | Push the ASCII code for `h` to the stack | | `'hello_world` | Push a pointer to a string to the stack | | `&fetch` | Push the address of `fetch` to the stack | ### Reordering The Stack RETRO provides a number of *shufflers* for reordering items on the stack. Some of the most common ones are: | Word | Before | After | | ------- |--------- | -------- | | dup | #1 | #1 #1 | | drop | #1 #2 | #1 | | swap | #1 #2 | #2 #1 | | over | #1 #2 | #1 #2 #1 | | tuck | #1 #2 | #2 #1 #2 | | nip | #1 #2 | #2 | | rot | #1 #2 #3 | #3 #1 #2 | You can use `push` and `pop` to move values to and from the address stack. Make sure you `pop` them back before the word ends or RETRO will crash. These two words can not be used at the interpreter. There is also a special one, `reorder`, which allows for big stack restructuring. This is slow but can be very useful. As an example, let's say we have four values: ``` #1 #2 #3 #4 ``` And we want them to become: ``` #4 #3 #2 #1 ``` Doing this with the basic shufflers is difficult. You could end up with something similar to: ``` swap rot push rot pop swap ``` But with `reorder`, you can just express the before and after states: ``` 'abcd 'dcba reorder ``` ### Resetting The Stack If you need to quickly empty the stack, use `reset`. ### Get The Stack Depth To find out how many items are on the stack, use `depth`. ### Displaying The Stack You can display the stack by running `dump-stack`. ### Data Flow Combinators RETRO provides *combinators* for working with data order on the stack. These are covered in a later chapter and are worth learning to use as they can help provide a cleaner, more structured means of working. ### Tips The stack is *not* an array in addressable memory. Don't try to treat it like one. ## Address Stack This stack primarily holds return addresses for function calls. You normally won't need to directly interact with this stack, but you can use `push` and `pop` to move values between the data stack and this. ## Floating Point Stack If you are using a build with floating point support a third stack will be present. Floating point values are kept and passed between words using this. See the Floating Point chapter for more details on this. ## Tips I recommend keeping the data stack shallow. Don't try to juggle too much; it's better to factor definitions into shorter ones that deal with simpler parts of the stack values than to have a big definition with a lot of complex shuffling. ## Notes The standard system is configured with a very deep data stack (around 2,000 items) and an address stack that is 3x deeper. In actual use, your programs are unlikely to ever need this, but if you do, keep the limits in mind. # Using Combinators A combinator is a function that consumes functions as input. They are used heavily by the RETRO system. ## Types of Combinators Combinators are divided into three primary types: compositional, execution flow, and data flow. ## Compositional A compositional combinator takes elements from the stack and returns a new quote. `curry` takes a value and a quote and returns a new quote applying the specified quote to the specified value. As an example, ``` :acc (n-) here swap , [ dup v:inc fetch ] curry ; ``` This would create an accumulator function, which takes an initial value and returns a quote that will increase the accumulator by 1 each time it is invoked. It will also return the latest value. So: ``` #10 acc dup call n:put dup call n:put dup call n:put ``` ## Execution Flow Combinators of this type execute other functions. ### Fundamental `call` takes a quote and executes it immediately. ``` [ #1 n:put ] call &words call ``` ### Conditionals RETRO provides three primary combinators for use with conditional execution of quotes. These are `choose`, `if`, and `-if`. `choose` takes a flag and two quotes from the stack. If the flag is true, the first quote is executed. If false, the second quote is executed. ``` #-1 [ 'true s:put ] [ 'false s:put ] choose #0 [ 'true s:put ] [ 'false s:put ] choose ``` `if` takes a flag and one quote from the stack. If the flag is true, the quote is executed. If false, the quote is discarded. ``` #-1 [ 'true s:put ] if #0 [ 'true s:put ] if ``` `-if` takes a flag and one quote from the stack. If the flag is false, the quote is executed. If true, the quote is discarded. ``` #-1 [ 'false s:put ] -if #0 [ 'false s:put ] -if ``` RETRO also provides `case` and `s:case` for use when you have multiple values to check against. This is similar to a `switch` in C. `case` takes two numbers and a quote. The initial value is compared to the second one. If they match, the quote is executed. If false, the quote is discarded and the initial value is left on the stack. Additionally, if the first value was matched, `case` will exit the calling function, but if false, it returns to the calling function. `s:case` works the same way, but for strings instead of simple values. ``` :test (n-) #1 [ 'Yes s:put ] case #2 [ 'No s:put ] case drop 'No idea s:put ; ``` ### Looping Several combinators are available for handling various looping constructs. `while` takes a quote from the stack and executes it repeatedly as long as the quote returns a true flag on the stack. This flag must be well formed and equal -1 or 0. ``` #10 [ dup n:put sp n:dec dup 0 -eq? ] while ``` `times` takes a count and quote from the stack. The quote will be executed the number of times specified. No indexes are pushed to the stack. ``` #1 #10 [ dup n:put sp n:inc ] times drop ``` There is also a `times` variation that provides access to the loop index (via `I`) and parent loop indexes (via `J` and `K`). ``` #10 [ I n:put sp ] times ``` ## Data Flow These combinators exist to simplify stack usage in various circumstances. ### Preserving Preserving combinators execute code while preserving portions of the data stack. `dip` takes a value and a quote, moves the value off the main stack temporarily, executes the quote, and then restores the value. ``` #10 #20 [ n:inc ] dip ``` Would yield the following on the stack: ``` 11 20 ``` `sip` is similar to `dip`, but leaves a copy of the original value on the stack during execution of the quote. So: ``` #10 [ n:inc ] sip ``` Leaves us with: ``` 11 10 ``` ### Cleave Cleave combinators apply multiple quotations to a single value or set of values. `bi` takes a value and two quotes, it then applies each quote to a copy of the value. ``` #100 [ n:inc ] [ n:dec ] bi ``` `tri` takes a value and three quotes. It then applies each quote to a copy of the value. ``` #100 [ n:inc ] [ n:dec ] [ dup * ] tri ``` ### Spread Spread combinators apply multiple quotations to multiple values. The asterisk suffixed to these function names signifies that they are spread combinators. `bi*` takes two values and two quotes. It applies the first quote to the first value and the second quote to the second value. ``` #1 #2 [ n:inc ] [ #2 * ] bi* ``` `tri*` takes three values and three quotes, applying the first quote to the first value, the second quote to the second value, and the third quote to the third value. ``` #1 #2 #3 [ n:inc ] [ #2 * ] [ n:dec ] tri* ``` ### Apply Apply combinators apply a single quotation to multiple values. The @ sign suffixed to these function names signifies that they are apply combinators. `bi@` takes two values and a quote. It then applies the quote to each value. ``` #1 #2 [ n:inc ] bi@ ``` `tri@` takes three values and a quote. It then applies the quote to each value. ``` #1 #2 #3 [ n:inc ] tri@ ``` RETRO also provides `for-each` combinators for various data structures. The exact usage of these varies; consult the Glossary and relevant chapters for more details on these. # Word Classes Word classes are one of the two elements at the heart of RETRO's interpreter. There are different types of words in a Forth system. At a minimum there are data words, regular words, and immediate words. There are numerous approaches to dealing with this. In RETRO I define special words which receive a pointer and decide how to deal with it. These are grouped into a `class:` namespace. ## How It Works When a word is found in the dictionary, RETRO will push a pointer to the definition (the `d:xt` field) to the stack and then call the word specified by the `d:class` field. The word called is responsible for processing the pointer passed to it. As a simple case, let's look at `immediate` words. These are words which will always be called when encountered. A common strategy is to have an immediacy bit which the interpreter will look at, but RETRO uses a class for this. The class is defined: ``` :class:immediate (a-) call ; ``` Or a normal word. These should be called at interpret time or compiled into definitions. The handler for this can look like: ``` :class:word (a-) compiling? [ compile:call ] [ call ] choose ; ``` ## Using Classes The ability to add new classes is useful. If I wanted to add a category of word that preserves an input value, I could do it with a class: ``` :class:duplicating (a-) compiling? [ &dup compile:call ] [ &dup dip ] choose class:word ; :duplicating &class:duplicating reclass ; :. n:put nl ; duplicating #100 . . . ``` # Internals: Nga Virtual Machine ## Overview At the heart of RETRO is a simple MISC (minimal instruction set computer) processor for a dual stack architecture. This is a very simple and straightforward system. There are 30 instructions. The memory is a linear array of signed 32 bit values. And there are two stacks: one for data and one for return addresses. ## Instruction Table | Opcode | Muri | Full Name | Data Stack | Address Stack | | ------ | ---- | ------------------ | ---------- | ------------- | | 0 | .. | nop | - | - | | 1 | li | lit | -n | - | | 2 | du | dup | n-nn | - | | 3 | dr | drop | n- | - | | 4 | sw | swap | xy-yx | - | | 5 | pu | push | n- | -n | | 6 | po | pop | -n | n- | | 7 | ju | jump | a- | - | | 8 | ca | call | a- | -A | | 9 | cc | conditional call | af- | -A | | 10 | re | return | - | A- | | 11 | eq | equality | xy-f | - | | 12 | ne | inequality | xy-f | - | | 13 | lt | less than | xy-f | - | | 14 | gt | greater than | xy-f | - | | 15 | fe | fetch | a-n | - | | 16 | st | store | na- | - | | 17 | ad | addition | xy-n | - | | 18 | su | subtraction | xy-n | - | | 19 | mu | multiplication | xy-n | - | | 20 | di | divide & remainder | xy-rq | - | | 21 | an | bitwise and | xy-n | - | | 22 | or | bitwise or | xy-n | - | | 23 | xo | bitwise xor | xy-n | - | | 24 | sh | shift | xy-n | - | | 25 | zr | zero return | n-? | - | | 26 | en | end | - | - | | 27 | ie | i/o enumerate | -n | - | | 28 | iq | i/o query | n-xy | - | | 29 | ii | i/o invoke | ...n- | - | ## Encoding Up to four instructions can be packed into each memory cell. As an example, Opcode 4 Opcode 3 Opcode 2 Opcode 1 00000000:00000000:00000000:00000000 If we have a bundle of `duliswst`, it would look like: st sw li du 00010000:00000100:00000001:00000010 Each `li` should have a value in the following cell(s). These values will be pushed to the stack. E.g., `lili....` and 1, 2: 00000000:00000000:00000001:00000001 00000000 00000000 00000000 00000001 (1) 00000000 00000000 00000000 00000010 (2) ## Shifts `sh` performs a bitwise arithmetic shift operation. This takes two values: xy And returns a single one: z If y is positive, this shifts `x` right by `y` bits. If negative, it shifts left. ## Queries: Memory, Stacks The `fe` instruction allows queries of some data related to the Nga VM state. These are returned by reading from negative addresses: | Address | Returns | | ------- | ---------------------- | | -1 | Data stack depth | | -2 | Address stack depth | | -3 | Maximum Image Size | | -4 | Minimum Integer Value | | -5 | Maximum Integer Value | ## I/O Devices Nga provides three instructions for interacting with I/O devices. These are: ie i/o enumerate returns the number of attached devices iq i/o query returns information about a device ii i/o interact invokes an interaction with a device As an example, with an implementation providing an output source, a block storage system, and keyboard: ie will return `3` since there are three i/o devices 0 iq will return 0 0, since the first device is a screen (0) with a version of 0 1 iq will return 1 3, since the second device is a block storage (3), with a version of 1 2 iq will return 0 1, since the third device is a keyboard (1), with a version of 0 In this case, some interactions can be defined: : c:put i liiire.. d 0 : c:get i liiire.. d 2 Setup the stack, push the device ID to the stack, and then use `ii` to invoke the interaction. A RETRO system requires one I/O device (a generic output for a single character). This must be the first device, and must have a device ID of 0. All other devices are optional and can be specified in any order. The currently supported and reserved device identifiers are: | ID | Device Type | Notes | | ---- | ---------------- | -------------------------- | | 0000 | Generic Output | Always present as device 0 | | 0001 | Keyboard | | | 0002 | Floating Point | | | 0003 | Block Storage | Raw, 1024 cell blocks | | 0004 | Filesystem | Unix-style Files | | 0005 | Network: Gopher | Make gopher requests | | 0006 | Network: HTTP | Make HTTP requests | | 0007 | Network: Sockets | | | 0008 | Syscalls: Unix | | | 0009 | Scripting Hooks | | | 0010 | Random Number | | This list may be revised in the future. The only guaranteed stable indentifier is 0000 for generic output. ## Trivia There are 810,000 possible combinations of instructions. Only 73 are used in the implementation of RETRO. # Internals: Interface Layers Nga provides a virtual processor and an extensible way of adding I/O devices, but does not provide any I/O itself. Adding I/O is the responsability of the *interface layer*. An interface layer will wrap Nga, providing at least one I/O device (a generic output target), and a means of interacting with the *retro image*. It's expected that this layer will be host specific, adding any system interactions that are needed via the I/O instructions. The image will typically be extended with words to use these. # Internals: I/O RETRO provides three words for interacting with I/O. These are: io:enumerate returns the number of attached devices io:query returns information about a device io:invoke invokes an interaction with a device As an example, with an implementation providing an output source, a block storage system, and keyboard: io:enumerate will return `3` since there are three i/o devices #0 io:query will return 0 0, since the first device is a screen (type 0) with a version of 0 #1 io:query will return 1 3, since the second device is block storage (type 3), with a version of 1 #2 io:query will return 0 1, since the last device is a keyboard (type 1), with a version of 0 In this case, some interactions can be defined: :c:put #0 io:invoke ; :c:get #2 io:invoke ; Setup the stack, push the device ID, and then use `io:invoke` to invoke the interaction. A RETRO system requires one I/O device (a generic output for a single character). This must be the first device, and must have a device ID of 0. All other devices are optional and can be specified in any order. # Internals: The Retro Image The actual RETRO language is stored as a memory image for Nga. ## Format The image file is a flat, linear sequence of signed 32-bit values. Each value is stored in little endian format. The size is not fixed. An interface should check when loading to ensure that the physical image is not larger than the emulated memory. ## Header The image will start with two cells. The first is a liju.... instruction, the second is the target address for the jump. This serves to skip over the rest of the data and reach the actual entry point. This is followed by a pointer to the most recent dictionary header, a pointer to the next free address in memory, and then the RETRO version number. | Offset | Contains | | ------ | --------------------------- | | 0 | lit call nop nop | | 1 | Pointer to main entry point | | 2 | Dictionary | | 3 | Heap | | 4 | RETRO version | The actual code starts after this header. The version number is the year and month. As an example, the 12.2019.6 release will have a version number of `201906`. ## Layout Assuming an Nga built with 524287 cells of memory: | RANGE | CONTAINS | | --------------- | ---------------------------- | | 0 - 1024 | rx kernel | | 1025 - 1535 | token input buffer | | 1536 + | start of heap space | | ............... | free memory for your use | | 506879 | buffer for string evaluate | | 507904 | temporary strings (32 * 512) | | 524287 | end of memory | The buffers at the end of memory will resize when specific variables related to them are altered. # Using The Glossary The Glossary is a valuable resource. It provides information on the RETRO words. ## Example Entry f:+ Data: - Addr: - Float: FF-F Add two floating point numbers, returning the result. Class: class:word | Namespace: f | Interface Layer: rre Example #1: .3.1 .22 f:+ ## Reading The Entry An entry starts with the word name. This is followed by the stack effect for each stack. All RETRO systems have Data and Address stacks, some also include a floating point stack). The stack effect diagrams are followed by a short description of the word. After the description is a line providing some useful data. This includes the class handler, namespace prefix, and the interface layer that provides the word. Words in all systems will be listed as `all`. Some words (like the `pb:` words) are only on specific systems like iOS. These can be identified by looking at the interface layer field. At the end of the entry may be an example or two. ## Access Online The latest Glossary can be browsed at http://forthworks.com:9999 or gopher://forthworks.com:9999 # Historical Papers and Notes ## On the Naming of RETRO Taken from http://lists.tunes.org/archives/tunes-lll/1999-July/000121.html On Fri, Jul 30, 1999 at 07:43:54PM -0400, Paul Dufresne wrote: > My brother did found it funny that Retro is called like that. > For him retro means going back (generally in time) so this > does not looks like a name of a OS to come. So he'd like to > know from where the name came. Heheh, here's the story: When I started playing with OS stuff last year (not seriously), I was reading about some old things like FORTH and ITS, dating back to the 1960's and 70's. The past few years in America, there's been a revival of disco music (along with bell bottoms, platform shoes, and all that crap) and they call it "retro". Now, my OS was named by musicians.. I was telling a fellow musician about my ideas, how it would be cool to have a small OS that isn't bloated and unmanageable like Windows... go back to the 70's and resurrect a line of software that died out. He goes "hmm.. sounds kinda retro.." I think it sounds kinda rebellious, which is a Good Thing now that everybody hates the M$ empire. :) It seems like other people are as sick of the future as I am. Look at TUNES, the idea there isn't to make some great new invention, just take some decades-old ideas and combine them in one OS. The first time I saw Knuth's "Art of Computer Programming" in the library I thought "god that looks old.. 1973!!! nevermind.." Now it's my programming bible. Find me something better published in the 90's.. if such a thing exists, it'll be like a needle in a haystack. "Newer" doesn't necessarily mean "better". New cars = flimsier New farming methods = more devastating New version of Netscape = more bloat, more bullshit One thing is better now: computer hardware. Give me 70's software on 90's and 00's hardware :) - Tom Novelli ## The Design Philosophy of RETRO Native Forth Computer software is a technology in its infancy, a mere fifty years old. The last 25 years in particular have seen an explosion in the software business. However, software has seen little innovation while hardware technology has improved phenomenally (notwithstanding the advent of lousy slave-made parts). Proven software techniques of forty years ago have yet to reach widespread use, in deference to the "latest and greatest" proprietary solutions of dubious value. Thanks to agressive marketing, we make huge investments in these dead-end technologies (through our businesses and governments, if not personally) and we end up with a reliance on a heap of complicated, error-prone, poorly understood junk software. Complexity will dominate the software industry for the foreseeable future. The Retro philosophy is a simple alternative for those willing to make a clean break with legacy software. A Retro system can communicate with other systems, but it won't run much legacy software, especially proprietary software without source code. An emulation layer could be added, but doing so would defeat the purpose of a simple operating system. I think TCP/IP support is all the compatibility that's needed. At first Retro will appeal to computer hobbyists and electronic engineers. Once the rough edges are smoothed out, it could catch on with ordinary folks who don't like waiting five minutes just to check their email (not to mention the long hours of setup and maintenance). Game programmers who take their craft seriously may also be interested. Businesses might even see a use for it, if the managers decide it's more cost-effective to carefully design software for specific needs, rather than buying off-the-shelf crap and spending countless manhours working around the bugs. Since it's not practical for businesses to make a clean break, my advice is to run Retro (and its ilk) on separate machines connected by a network. Retro is efficient enough to run on older machines that would otherwise sit idle, being too slow for the latest Microsoft bloatware (or Linux, for that matter). I strive to avoid the extraneous. That applies even to proven technologies, if I don't need them. If my computer isn't set up for people to log in over the network, I don't want security features; they just get in the way. If I'm only running programs I wrote, I should be able to run them with full access to the hardware; I don't need protection from viruses. If I download something I don't trust, then I can run it in an isolated process, which is customary with Unix and kin. But that's not core functionality. All that's needed is the flexibility to add things like security, graphical interfaces, and distributed processing - if the need ever arises. In programming languagues, I was misled. It's the Tower of Babel all over again. The thousands of languages in existence all fall into a handful of archetypes: Assembler, LISP, FORTRAN and FORTH represent the earliest descendants of nearly all languages. I hesitate to name a definitive "object-oriented" language, and here's why: Object-Oriented programming is just a technique, and any language will suffice, even Assembler. The complexites of fancy languages like Ada and C++ are a departure from reality -- the reality of the actual physical machine. When it all boils down, even LISP, FORTRAN and FORTH are only extensions of the machine. I chose FORTH as the "native tongue" of Retro. LISP, FORTRAN, and other languages can be efficiently implemented as extensions of FORTH, but the reverse isn't so efficient. Theoretically all languages are equivalent, but when design time, compilation time, and complexity are accounted for, FORTH is most efficient. FORTH also translates most directly to the hardware. (In fact, FORTH has been implemented in hardware; these "stack machines" are extremely efficient.) FORTH is also the easiest language to implement from scratch - a major concern when you're trying to make a clean break. So with simplicity in mind, FORTH was the obvious choice. I'm perfectly happy working with text only, and I go to great lengths to avoid using the standard graphical environments, which have major problems: windows, pulldown menus, and mice. Windows can't share the screen nicely; that idea is hopeless. Pulldowns are tedious. Mice get in the way of typing without reducing the need for it; all they give me is tendonitis. Their main use is for drawing. Some of my favorite interfaces: Telix, Telegard BBS, Pine, Pico, Lynx, and ScreamTracker. All "hotkey" interfaces where you press a key or two to perform an action. Usually the important commands are listed at the bottom of the screen, or at least on a help screen. The same principles apply to graphical interfaces: use the full screen, except for a status and menu area on one edge. Resist the temptation to clutter up the screen. As for switching between programs, the Windows methods suck; the only thing worse is Unix job control (jobs, fg, and such). The Linux method is tolerable: Alt-Arrows, Alt-F1, Alt-F2, etc. Still, things could be better: F11 and F12 cycle back and forth through all open programs; Alt-F1 assigns the currently selected program to F1, and likewise for the other function keys. Programs just won't use function keys - Control and Alt combinations are less awkward and easier to remember, besides. I'll also want a "last channel" key and a "task list" key; maybe I'll borrow those stupid Win95 keys. The Pause key will do like it says - pause the current program - and Ctrl-Pause (Break) will kill it. One more thing: consistency. I like programs to look different so I can tell them apart, but the keys should be the same as much as possible. Keys should be configured in one place, for all programs. Finally, remember the most consistent interface, one of the few constants throughout the history of computing - the text screen and keyboard, and the teletypewriter before that. Don't overlook it. More to come, maybe... :) "If it's on line, it's a work in progress." Tom Novelli, 3/4/2000 ## Metacompilation and Assembly RETRO 10 and 11 were written in themselves using a metacompiler. I had been fascinated by this idea for a long time and was able to explore it heavily. While I still find it to be a good idea, the way I ended up doing it was problematic. The biggest issue I faced was that I wanted to do this in one step, where loading the RETRO source would create a new image in place of the old one, switch to the new one, and then load the higher level parts of the language over this. In retrospect, this was a really bad idea. My earlier design for RETRO was very flexible. I allowed almost everything to be swapped out or extended at any time. This made it extremely easy to customize the language and environment, but made it crucial to keep track of what was in memory and what had been patched so that the metacompiler wouldn't refer to anything in the old image during the relocation and control change. It was far too easy to make a mistake, discover that elements of the new image were broken, and then have to go and revert many changes to try to figure out what went wrong. This was also complicated by the fact that I built new images as I worked, and, while a new image could be built from the last built one, it wasn't always possible to build a new image from the prior release version. (Actually, it was often worse - I failed to check in every change as I went, so often even the prior commits couldn't rebuild the latest images). For RETRO 12 I wanted to avoid this problem, so I decided to go back to writing the kernel ("Rx") in assembly. I actually wrote a Machine Forth dialect to generate the initial assembly, before eventually hand tuning the final results to its current state. I could (and likely will eventually) write the assembler in RETRO, but the current one is in C, and is built as part of the standard toolchain. My VM actually has two assemblers. The older one is Naje. This was intended to be fairly friendly to work with, and handles many of the details of packing instructions for the user. Here is an example of a small program in it: :square dup mul ret :main lit 35 lit &square call end The other assembler is Muri. This is a far more minimalistic assembler, but I've actually grown to prefer it. The above example in Muri would become: i liju.... r main : square i dumure.. : main i lilica.. d 35 r square i en...... In Muri, each instruction is reduced to two characters, and the bundlings are listed as part of an instruction bundle (lines starting with `i`). This is less readable if you aren't very familiar with Nga's assembly and packing rules, but allows a very quick, efficient way of writing assembly for those who are. I eventually rewrote the kernel in the Muri style as it's what I prefer, and since there's not much need to make changes in it. ## The Path to Self Hosting RETRO is an image based Forth system running on a lightweight virtual machine. This is the story of how that image is made. The first RETRO to use an image based approach was RETRO 10. The earliest images were built using a compiler written in Toka, an earlier experimental stack language I had written. It didn't take long to want to drop the dependency on Toka, so I rewrote the image compiler in RETRO and then began development at a faster pace. RETRO 11 was built using the last RETRO 10 image and an evolved version of the metacompiler. This worked well, but I eventually found it to be problematic. One of the issues I faced was the inability to make a new image from the prior stable release. Since I develop and test changes incrementally, I reached a point where the current metacompiler and image required each other. This wasn't a fatal flaw, but it was annoying. Perhaps more critical was the fragility of the system. In R11 small mistakes could result in a corrupt image. The test suite helped identify some of these, but there were a few times I was forced to dig back through the version control history to recover a working image. The fragile nature was amplified by some design decisions. In R11, after the initial kernel was built, it would be moved to memory address 0, then control would jump into the new kernel to finish building the higher level parts. Handling this was a tricky task. In R11 almost everything could be revectored, so the metacompiler had to ensure that it didn't rely on anything in the old image during the move. This caused a large number of issues over R11's life. So on to RETRO 12. I decided that this would be different. First, the kernel would be assembly, with an external tool to generate the core image. The kernel is in `Rx.md` and the assembler is `Muri`. To load the standard library, I wrote a second tool, `retro-extend`. This separation has allowed me many fewer headaches as I can make changes more easily and rebuild from scratch when necessary. But I miss self-hosting. So last fall I decided to resolve this. And today I'm pleased to say that it is now done. There are a few parts to this. **Unu**. I use a Markdown variation with fenced code blocks. The tool I wrote in C to extract these is called `unu`. For a self hosting RETRO, I rewrote this as a combinator that reads in a file and runs another word against each line in the file. So I could display the code block contents by doing: 'filename [ s:put nl ] unu This made it easier to implement the other tools. **Muri**. This is my assembler. It's minimalistic, fast, and works really well for my purposes. RETRO includes a runtime version of this (using `as{`, `}as`, `i`, `d`, and `r`), so all I needed for this was to write a few words to parse the lines and run the corresponding runtime words. As with the C version, this is a two pass assembler. Muri generates a new `ngaImage` with the kernel. To create a full image I needed a way to load in the standard library and I/O extensions. This is handled by **retro-extend**. This is where it gets more complex. I implemented the Nga virtual machine in RETRO to allow this to run the new image in isolation from the host image. The new ngaImage is loaded, the interpreter is located, and each token is passed to the interpreter. Once done, the new image is written to disk. So at this point I'm pleased to say that I can now develop RETRO using only an existing copy of RETRO (VM+image) and tools (unu, muri, retro-extend, and a line oriented text editor) written in RETRO. This project has delivered some additional side benefits. During the testing I was able to use it to identify a few bugs in the I/O extensions, and the Nga-in-RETRO will replace the older attempt at this in the debugger, allowing a safer testing environment. What issues remain? The extend process is *slow*. On my main development server (Linode 1024, OpenBSD 6.4, 64-bit) it takes a bit over five minutes to complete loading the standard library, and a few additional depending on the I/O drivers selected. Most of the performance issues come from running Nga-in-RETRO to isolate the new image from the host one. It'd be possible to do something a bit more clever (e.g., running a RETRO instance using the new image via a subprocess and piping in the source, or doing relocations of the data), but this is less error prone and will work on all systems that I plan to support (including, with a few minor adjustments, the native hardware versions [assuming the existance of mass storage]). Sources: **Unu** - http://forth.works/c8820f85e0c52d32c7f9f64c28f435c0 - gopher://forth.works/0/c8820f85e0c52d32c7f9f64c28f435c0 **Muri** - http://forth.works/09d6c4f3f8ab484a31107dca780058e3 - gopher://forth.works/0/09d6c4f3f8ab484a31107dca780058e3 **retro-extend** - http://forth.works/c812416f397af11db58e97388a3238f2 - gopher://forth.works/0/c812416f397af11db58e97388a3238f2 ## Prefixes as a Language Element A big change in RETRO 12 was the elimination of the traditional parser from the language. This was a sacrifice due to the lack of an I/O model. RETRO has no way to know *how* input is given to the `interpret` word, or whether anything else will ever be passed into it. And so `interpret` operates only on the current token. The core language does not track what came before or attempt to guess at what might come in the future. This leads into the prefixes. RETRO 11 had a complicated system for prefixes, with different types of prefixes for words that parsed ahead (e.g., strings) and words that operated on the current token (e.g., `@`). RETRO 12 eliminates all of these in favor of just having a single prefix model. The first thing `interpret` does is look to see if the first character in a token matches a `prefix:` word. If it does, it passes the rest of the token as a string pointer to the prefix specific handler to deal with. If there is no valid prefix found, it tries to find it in the dictionary. Assuming that it finds the words, it passes the `d:xt` field to the handler that `d:class` points to. Otherwise it calls `err:notfound`. This has an important implication: *words can not reliably have names that start with a prefix character.* It also simplifies things. Anything that would normally parse becomes a prefix handler. So creating a new word? Use the `:` prefix. Strings? Use `'`. Pointers? Try `&`. And so on. E.g., In ANS | In RETRO : foo ... ; | :foo ... ; ' foo | &foo : bar ... ['] foo ; | :bar ... &foo ; s" hello world!" | 'hello_world! If you are familiar with ColorForth, prefixes are a similar idea to colors, but can be defined by the user as normal words. After doing this for quite a while I rather like it. I can see why Chuck Moore eventually went towards ColorForth as using color (or prefixes in my case) does simplify the implementation in many ways. ## On The Kernel Wordset In implementing the RETRO 12 kernel (called Rx) I had to decide on what functionality would be needed. It was important to me that this be kept clean and minimalistic, as I didn't want to spend a lot of time changing it as time progressed. It's far nicer to code at the higher level, where the RETRO language is functional, as opposed to writing more assembly code. So what made it in? Primitives These are words that map directly to Nga instructions. dup drop swap call eq? -eq? lt? gt? fetch store + - * /mod and or xor shift push pop 0; Memory fetch-next store-next , s, Strings s:to-number s:eq? s:length Flow Control choose if -if repeat again Compiler & Interpreter Compiler Heap ; [ ] Dictionary d:link d:class d:xt d:name d:add-header class:word class:primitive class:data class:macro prefix:: prefix:# prefix:& prefix:$ interpret d:lookup err:notfound Assembler i d r I *could* slightly reduce this. The $ prefix could be defined in higher level code, and I don't strictly *need* to expose the `fetch-next` and `store-next` here. But since the are already implemented as dependencies of the words in the kernel, it would be a bit wasteful to redefine them later in higher level code. A recent change was the addition of the assembler into the kernel. This allows the higher levels to use assembly as needed, which gives more flexibility and allows for more optimal code in the standard library. With these words the rest of the language can be built up. Note that the Rx kernel does not provide any I/O words. It's assumed that the RETRO interfaces will add these as best suited for the systems they run on. There is another small bit. All images start with a few key pointers in fixed offsets of memory. These are: | Offset | Contains | | ------ | --------------------------- | | 0 | lit call nop nop | | 1 | Pointer to main entry point | | 2 | Dictionary | | 3 | Heap | | 4 | RETRO version identifier | An interface can use the dictionary pointer and knowledge of the dictionary format for a specific RETRO version to identify the location of essential words like `interpret` and `err:notfound` when implementing the user facing interface. ## On The Evolution Of Ngaro Into Nga When I decided to begin work on what became RETRO 12, I knew the process would involve updating Ngaro, the virtual machine that RETRO 10 and 11 ran on. Ngaro rose out of an earlier experimental virtual machine I had written back in 2005-2006. This earlier VM, called Maunga, was very close to what Ngaro ended up being, though it had a very different approach to I/O. (All I/O in Maunga was intended to be memory mapped; Ngaro adopted a port based I/O system). Ngaro itself evolved along with RETRO, gaining features like automated skipping of NOPs and a LOOP opcode to help improve performance. But the I/O model proved to be a problem. When I created Ngaro, I had the idea that I would always be able to assume a console/terminal style environment. The assumption was that all code would be entered via the keyboard (or maybe a block editor), and that proved to be the fundamental flaw as time went on. As RETRO grew it was evident that the model had some serious problems. Need to load code from a file? The VM and language had functionality to pretend it was being typed in. Want to run on something like a browser, Android, or iOS? The VM would need to be implemented in a way that simulates input being typed into the VM via a simulated keyboard. And RETRO was built around this. I couldn't change it because of a promise to maintain, as much as possible, source compatibility for a period of at least five years. When the time came to fix this, I decided at the start to keep the I/O model separate from the core VM. I also decided that the core RETRO language would provide some means of interpreting code without requiring an assumption that a traditional terminal was being used. So Nga began. I took the opportunity to simplify the instruction set to just 26 essential instructions, add support for packing multiple instructions per memory location (allowing a long due reduction in memory footprint), and to generally just make a far simpler design. I've been pleased with Nga. On its own it really isn't useful though. So with RETRO I embed it into a larger framework that adds some basic I/O functionality. The *interfaces* handle the details of passing tokens into the language and capturing any output. They are free to do this in whatever model makes most sense on a given platform. So far I've implemented: - a scripting interface, reading input from a file and offering file i/o, gopher, and reading from stdin, and sending output to stdout. - an interactive interface, built around ncurses, reading input from stdin, and displaying output to a scrolling buffer. - an iOS interface, built around a text editor, directing output to a separate interface pane. - an interactive block editor, using a gopher-based block data store. Output is displayed to stdout, and input is done via the blocks being evaluated or by reading from stdin. In all cases, the only common I/O word that has to map to an exposed instruction is `putc`, to display a single character to some output device. There is no requirement for a traditional keyboard input model. By doing this I was able to solve the biggest portability issue with the RETRO 10/11 model, and make a much simpler, cleaner language in the end. ## RETRO 11 (2011 - 2019): A Look Back So it's now been about five years since the last release of RETRO 11. While I still see some people obtaining and using it, I've moved on to the twelth generation of RETRO. It's time for me to finally retire RETRO 11. As I prepare to do so, I thought I'd take a brief look back. RETRO 11 began life in 2011. It grew out of RETRO 10, which was the first version of RETRO to not be written in x86 assembly language. For R10 and R11, I wrote a portable virtual machine (with numerous implementations) and the Forth dialect was kept in an image file which ran on the VM. RETRO 10 worked, but was always a bit too sloppy and changed drastically between releases. The major goal of RETRO 11 was to provide a stable base for a five year period. In retrospect, this was mostly achieved. Code from earlier releases normally needed only minor adjustments to run on later releases, though newer releases added significantly to the language. There were seven releases. - Release 11.0: 2011, July - Release 11.1: 2011, November - Release 11.2: 2012, January - Release 11.3: 2012, March - Release 11.4: 2012, July - Release 11.5: 2013, March - Release 11.6: 2014, August Development was fast until 11.4. This was the point at which I had to slow down due to RSI problems. It was also the point which I started experiencing some problems with the metacompiler (as discussed previously). RETRO 11 was flexible. All colon definitions were setup as hooks, allowing new functionality to be layered in easily. This allowed the later releases to add things like vocabularies, search order, tab completion, and keyboard remapping. This all came at a cost though: later things could use the hooks to alter behavior of existing words, so it was necessary to use a lot of caution to ensure that the layers didn't break the earlier code. The biggest issue was the I/O model. RETRO 11 and the Ngaro VM assumed the existence of a console environment. All input was required to be input at the keyboard, and all output was to be shown on screen. This caused some problems. Including code from a file required some tricks, temporarily rewriting the keyboard input function to read from the file. It also became a major issue when I wrote the iOS version. The need to simulate the keyboard and console complicated everything and I had to spend a considerable amount of effort to deal with battery performance resulting from the I/O polling and wait states. But on the whole it worked well. I used RETRO 11.6 until I started work on RETRO 12 in late 2016, and continued running some tools written in R11 until the first quarter of last year. The final image file was 23,137 cells (92,548 bytes). This was bloated by keeping some documentation (stack comments and short descriptions) in the image, which started in 11.4. This contained 269 words. I used RETRO 11 for a wide variety of tasks. A small selection of things that were written includes: - a pastebin - front end to ii (irc client) - small explorations of interactive fiction - irc log viewer - tool to create html from templates - tool to automate creation of an SVCD from a set of photos - tools to generate reports from data sets for my employer In the end, I'm happy with how RETRO 11 turned out. I made some mistakes in embracing too much complexity, but despite this it was a successful system for many years. # Security Concerns The standard RETRO is not a good choice for applications needing to be highly secure. ## Runtime Checks The RETRO system performs only minimal checks. It will not load an image larger than the max set at build time. And stack over/underflow are checked for as code executes. The system does not attempt to validate anything else, it's quite easy to crash. ## Isolation The VM itself and the core code is self contained. Nga does not make use of malloc/free, and uses only standard system libraries. It's possible for buffer overruns within the image (overwriting Nga code), but the RETRO image shouldn't leak into the C portions. I/O presents a bigger issue. Anything involving I/O, especially with the `unix:` words, may be a vector for attacks. ## Future Direction I'm not planning to add anything to the *image* side as, for me, the performance hit due to added checks is bigger than the benefits. The story is different on the VM side. I've already begun taking steps to address some of the issues, using functions that check for overruns with strings and doing some minor testing for these conditions. I will be gradually addressing the various I/O related extensions, though it's unlikely to ever be fully guarded against attacks. ## Rationale RETRO is, primarily, a personal system. I'm running code I wrote to solve problems I face. On the occasions where I run code sent to me by others, I read it carefully first and then run inside a sandboxed environment if I'm worried about anything in it.