retroforth/RETRO-Book.md

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.

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 RETRO on BSD

RETRO is well supported on BSD (FreeBSD, NetBSD, OpenBSD) systems. It should build on a base install of any of these without issue.

Requirements

• c compiler
• make

Process

Run make

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

Building RETRO on Linux

Building on Linux is pretty easy. You'll need to make sure you have a C compiler, headers, and make.

Requirements

• c compiler (tested: clang, tcc, gcc)
• development headers
• make

Process

Run:

make

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

Troubleshooting

I've had issues building on Linux in the past. The GNU toolchain can be quite problematic.

If the build fails please contact Charles with details and he will work on finding a fix.

Building RETRO on macOS

To build on macOS, you will need the command line tools from Xcode. Install these and you should be able to build and use RETRO.

Requirements

• command line tools from Xcode

Process

Run make

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

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

You'll need to comment out (or remove) some things before RETRO will build.

In rre.c:

• remove includes for unistd.h, sys/sockets.h, netinet/in.h, netdb.h, errno.h, sys/wait.h, signal.h
• remove the #define USE_TERMIOS line
• change the #define NUM_DEVICES to 6
• remove io_unix_handler and io_gopher_handler from IO_deviceHandlers
• remove io_unix_query and io_gopher_query from IO_queryHandlers

In image-functions.c:

• remove includes for unistd.h

In image-functions.h:

• remove includes for unistd.h

In io\filesystem.c:

• remove includes for unistd.h

In io\floatingpoint.c:

• remove includes for unistd.h

Build

\path\to\tcc rre.c image-functions.c io\filesystem.c io\floatingpoint.c -o retro.exe

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 Ok prompt or startup banner):

retro -s

RETRO also has a character breaking mode, in which input is processed directly as entered, this is started with the -c option:

retro -i,c

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)
  -i,c         Interactive mode (character buffered)
  -i,fs        Interactive mode (fullscreen)
  -s           Suppress the 'ok' prompt and keyboard
               echo in interactive mode
  -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

Ok

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

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.

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.

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

Programming Techniques

The upcoming chapters provide helpful information on using RETRO with different types of data and hints on how to solve problems in a way consistent with the RETRO system.

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/

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
• Use short names for indices

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

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 (-)

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 . . .

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<with-index> variation that provides access to the loop index (via I) and parent loop indexes (via J and K).

#10 [ I n:put sp ] times<with-index>

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.

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.

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?

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.

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<xt> will give the contents of the d:xt field for the latest header. There are also d:last<class> and d:last<name>.

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

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?

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.

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.

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 end
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 en
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 as{ 'dumure.. i }as ;

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..

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.

Internals

The next few chapters dive into RETRO's architecture. If you seek to implement a port to a new platform or to extend the I/O functionality you'll find helpful information here.

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.

Instrution 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 1 Opcode 2 Opcode 3 Opcode 4
00000000:00000000:00000000:00000000

If we have a bundle of lidumu.., it would look like:

      li       du       mu       ..
00000001:00000010:00010011:00000000

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:

00000001:00000001:00000000:00000000
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

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: 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.

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-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.

Advanced Builds

Reduced Memory

RETRO can be built for reduced memory targets. This is primarily intendeded for use with embedded targets, but can be useful on large machines as well.

To do this, make with CFLAGS set to -DMEM1024K, -DMEM512K, -DMEM256K, -DMEM192K, -DMEM128K' or -DMEM96K. These will target memory sizes as specified per the following table.

	1024kB	512kB	256kB	192kB	128kB	96kB
Total Bytes	1,048,579	524,288	262,144	196,608	131,072	98,304
Image	968,000	450,000	192,000	126,000	96,000	72,000
Data Stack	512	512	512	512	512	512
Address Stack	1,024	1,024	1,024	1,024	1,024	1,024
Remaining	79,040	72,752	68,608	69,072	33,536	24,768

The remaining memory is available for use by the VM.

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. You can add files to compile into your system by adding them to the list and rebuilding.

Example:

If you have wanted to include the NumbersWithoutPrefixes.forth example, add:

~~~
'example/NumbersWithoutPrefixes.forth include
~~~

To the start of the list file and then run make again. The newly built bin/retro will now include your additions.

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.

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.

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.

Technical Notes and Reflections

This is a collection of short papers providing some additional background and reflections on design decisions.

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
  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

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.

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.

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 tcn@tunes.org

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