# Muri: a Minimalist Assembler for Nga

This is a small assembler used to build the initial image for
RETRO. The implementation here uses the runtime variant included
in the core RETRO system. See the glossary entries for `i`, `d`,
`r`, `as{`, and `}as` for details on these.

The full assembler has a postfix notation. Syntax is:

    <directive> <data>

Directives are a single character. Muri recognizes:

* **i** for instructions
* **d** for numeric data
* **s** for string data
* **:** for creating a label
* **r** for references to labels

Instructions are packed up to four instructions per location.
You can specify them using the first two characters of the
instruction name. For a non operation, use '..' instead of
'no'.

    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

E.g., for a sequence of dup, multiply, no-op, drop:

    i dumu..dr

An example of a small program:

    i liju....
    r main
    : square
    i dumure..
    : main
    i lilica..
    d 12
    r square
    i ha......

As mentioned earlier this requires knowledge of Nga architecture.
While you can pack up to four instructions per location, you
should not place anything after an instruction that modifies the
instruction pointer. These are: ju, ca, cc, re, and zr.

~~~
:file:operation #4 io:scan-for io:invoke ;

#0 'file:R  const
#1 'file:W  const
#2 'file:A  const
#3 'file:R+ const

:file:open  (sm-h) #0 file:operation ;
:file:close (h-) #1 file:operation ;
:file:read  (h-c) #2 file:operation ;
:file:write (ch-) #3 file:operation ;

{{
  'FID var
  'Size var
  'Action var
  'Buffer var
  :-eof? (-f) @FID file:tell @Size lt? ;
  :preserve (q-) &FID [ &Size &call v:preserve ] v:preserve ;
---reveal---
  :file:read-line (f-s)
    !FID
    [ here dup !Buffer buffer:set
      [ @FID file:read dup buffer:add
        [ ASCII:CR eq? ] [ ASCII:LF eq? ] [ ASCII:NUL eq? ] tri or or ] until
        buffer:get drop ] buffer:preserve
    @Buffer ;

  :file:for-each-line (sq-)
    [ !Action
      file:open-for-reading !FID !Size
      [ @FID file:read-line @Action call -eof? ] while
      @FID file:close
    ] preserve ;
}}
~~~

## Unu

This is documented in *example/retro-unu.forth*, but basically
it provides a combinator that runs a quote for each line in a
file, provided that the lines are in fenced blocks starting and
ending with `~~~`.

The RETRO sources are written in this style, so I include Unu
here to simplify the later workflow.

~~~
{{
  'Fenced var
  :toggle-fence @Fenced not !Fenced ;
  :fenced? (-f) @Fenced ;
  :handle-line (s-)
    fenced? [ over call ] [ drop ] choose ;
---reveal---
  :unu (sq-)
    swap [ dup '~~~ s:eq?
           [ drop toggle-fence ]
           [ handle-line       ] choose
         ] file:for-each-line drop ;
}}
~~~

## Muri

Now for the assembler. I create a couple of data structures: a
buffer for the assembled image and a pointer into this.

~~~
'Image d:create #8192 allot
'AP var
~~~

I then use these to implement `I,`, a word which stores a value
into the image buffer and increment the pointer.

~~~
:I, (n-) &Image @AP + store &AP v:inc ;
~~~

### Pass 1

Muri is a two pass assembler. The first pass handles most of the
work. It processes instrution bundles, data, strings, and creates
labels pointing to specific addresses in the image. References
are compiled as dummy values, to be resolved later.

~~~
#0 !AP
'retro.muri
  [ dup s:length n:zero? [ drop #0 ] if 0;
    fetch-next &n:inc dip
    $i [ i here n:dec fetch I, ] case
    $d [ s:to-number I, ] case
    $r [ drop #-1 I, ] case
    $: [ @AP swap 'muri! s:prepend const ] case
    $s [ &I, s:for-each #0 I, ] case
    'ERROR s:put nl
  ] unu
~~~

### Pass 2

The second pass skips over everything except references, which
get resolved and filled in. This allows for forward references.

~~~
#0 !AP
'retro.muri
  [ dup s:length n:zero? [ drop #0 ] if 0;
    fetch-next &n:inc dip
    $i [ drop &AP v:inc ] case
    $d [ drop &AP v:inc ] case
    $r [ 'muri! s:prepend d:lookup d:xt fetch I, ] case
    $: [ drop ] case
    $s [ s:length n:inc &AP v:inc-by ] case
    'ERROR s:put nl
  ] unu
~~~

### Save Image

Saving the image is pretty straightforward. For each cell,
convert to bytes and write them to the output file.

~~~
'FID var

:write-byte (n-)  @FID file:write ;
:mask       (n-)  #255 and ;

:write-cell (n-)
           dup mask write-byte
  #8 shift dup mask write-byte
  #8 shift dup mask write-byte
  #8 shift     mask write-byte ;

:image:save (s-)
  file:W file:open !FID
  &Image @AP [ fetch-next write-cell ] times drop
  @FID file:close ;

'retro.nga image:save
~~~