123 lines
3 KiB
Forth
123 lines
3 KiB
Forth
RETRO runs on a virtual machine called Nga. The instruction set is MISC
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inspired, consisting of just 27 instructions:
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0 nop 7 jump 14 gt 21 and
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1 lit <v> 8 call 15 fetch 22 or
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2 dup 9 ccall 16 store 23 xor
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3 drop 10 return 17 add 24 shift
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4 swap 11 eq 18 sub 25 zret
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5 push 12 neq 19 mul 26 end
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6 pop 13 lt 20 divmod
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Four instructions are packed per 32-bit memory location. The Muri
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assembler allows the instructions to be specified like:
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'lica.... i
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#100 d
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I shorten the instructions to two letter abbreviations, with '..' for
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'nop'.
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The only special case here is that each 'li' instruction requires a
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value to push in the following cell.
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So to implement this, I first need to unpack a cell into the individual
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instructions.
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~~~
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{{
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:mask #255 and ;
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:next #8 shift ;
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---reveal---
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:unpack (n-dcba)
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dup mask swap next
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dup mask swap next
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dup mask swap next
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'abcd 'dcba reorder ;
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}}
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~~~
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With that out of the way, I make a variable to track the number of 'li'
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instructions in the bundle.
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~~~
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'LitCount var
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~~~
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And then a string with the two letter identifiers for each instruction.
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Here I append ?? at the end; I'll use this to identify unknown
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instructions.
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~~~
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'..lidudrswpupojucaccreeqneltgtfestadsumudianorxoshzren?? 'INST s:const
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~~~
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And then a simple word to return a name for the current instruction. I
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have this increment the `LitCount` when it finds a 'li' instruction.
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~~~
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:name-for (n-cc)
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dup #1 eq? [ &LitCount v:inc ] if
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#27 n:min #2 * &INST + fetch-next swap fetch swap ;
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~~~
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To actually display a bundle, I need to decide on what it is. So I have
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a `validate` word to look at each instruction and make sure all are
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actual instructions.
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~~~
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:valid? (c-f)
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unpack [ #0 #26 n:between? ] bi@ and
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[ [ #0 #26 n:between? ] bi@ and ] dip and ;
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~~~
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With this and the `LitCount`, I can determine how to render a bundle.
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So the next step is a word to pad raw values so they're at least as
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long as instruction bundle strings (8 characters).
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~~~
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:pad (s-)
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s:length #30 swap - #0 n:max [ sp ] times ;
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~~~
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I split out each type (instruction, reference/raw, and data) into a
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separate handler.
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~~~
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:render-inst (n-)
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$' putc unpack #4 [ name-for putc putc ] times sp '________ pad $i putc ;
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:render-data (n-)
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$# putc n:to-string dup puts pad sp $d putc ;
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:render-ref (n-)
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dup d:lookup-xt n:-zero?
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[ dup render-data sp $( putc d:lookup-xt d:name puts ]
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[ render-data ] choose ;
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~~~
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Then I use these and my `valid?` checker to implement a single word to
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render the packed cell in a meaningful manner.
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~~~
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:render-packed (n-)
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@LitCount n:zero?
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[ dup valid?
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[ render-inst ]
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[ render-ref ] choose ]
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[ render-ref &LitCount v:dec ] choose ;
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~~~
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And now to tie it all together:
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~~~
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:disassemble (an-)
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[ fetch-next
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over $( putc putn sp (address)
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render-packed nl (inst_or_data)
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] times drop ;
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~~~
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~~~
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&disassemble here over - disassemble
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~~~
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