162 lines
4 KiB
Forth
162 lines
4 KiB
Forth
# Muri, Extended
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Muri is my minimalist assembler for Nga. Using it requires
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some knowledge of the Nga architecture to be useful.
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Nga has 30 instructions. These are:
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0 nop 5 push 10 ret 15 fetch 20 div 25 zret
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1 lit 6 pop 11 eq 16 store 21 and 26 halt
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2 dup 7 jump 12 neq 17 add 22 or 27 ienum
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3 drop 8 call 13 lt 18 sub 23 xor 28 iquery
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4 swap 9 ccall 14 gt 19 mul 24 shift 29 iinvoke
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The mnemonics allow for each name to be reduced to just two
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characters. In the same order as above:
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0 .. 5 pu 10 re 15 fe 20 di 25 zr
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1 li 6 po 11 eq 16 st 21 an 26 ha
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2 du 7 ju 12 ne 17 ad 22 or 27 ie
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3 dr 8 ca 13 lt 18 su 23 xo 28 iq
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4 sw 9 cc 14 gt 19 mu 24 sh 29 ii
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Up to four instructions can be packed into a single memory
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location. (You can only use *no*p after a *ju*mp, *ca*ll,
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*cc*all, *re*t, or *zr*et as these alter the instruction
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pointer.)
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So a bundled sequence like:
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lit 100
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lit 200
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add
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ret
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Would look like:
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'liliadre i
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100 d
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200 d
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And:
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lit s:eq?
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call
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Would become:
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'lica.... i
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's:eq? r
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Note the use of `..` instead of `no` for the nop's; this is
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done to improve readability a little.
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Instruction bundles are specified as strings, and are converted
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to actual instructions by the `i` word. As in the standard Muri
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assembler, the RETRO version uses `d` for decimal values and `r`
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for references to named functions.
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----
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This implements an extended version of `i`, the instruction
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assembler. It allows for use of hex constants (in uppercase)
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in place of (or in addition to) the instruction names. This can
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be useful if you are running on a VM with an extended instruction
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set.
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When loaded, it will *replace* the original `i` with a jump to
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the one provided here, allowing existing words (like `sigil:\`)
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to use this instead.
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----
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I'm keeping everything in a private namespace to keep the final
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dictionary clean.
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~~~
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{{
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~~~
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It begins with an array of instruction names. The index matches
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the opcode, so these must be in order.
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~~~
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{ '.. 'li 'du 'dr 'sw 'pu 'po 'ju 'ca 'cc 're 'eq 'ne 'lt
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'gt 'fe 'st 'ad 'su 'mu 'di 'an 'or 'xo 'sh 'zr 'ha 'ie
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'iq 'ii } 'Instructions const
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~~~
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Then I define a `quad` combinator to simplify the later debundling
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of the instruction names.
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~~~
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:quad (xqqqq-)
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'abcde 'abacadae reorder
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\pupupupu \pupuca..
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\popoca.. \popoca..
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\popoca.. ;
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~~~
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Next, a word to handle hex numbers. A standard Retro system only
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handles decimal by default, so this just implements a quick hex
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conversion.
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~~~
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'0123456789ABCDEF 'DIGITS s:const
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'Number var
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:convert (c-) &DIGITS swap s:index/char @Number #16 * + !Number ;
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:check-sign (s-ns) dup fetch $- eq? [ #-1 swap n:inc ] [ #1 swap ] choose ;
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:s:to-hex-number (s-n)
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#0 !Number check-sign [ convert ] s:for-each @Number * ;
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~~~
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Decoding an instruction is simple. If it's in the `Instructions`
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array, return the index. If not, convert to a number using the
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hex conversion above.
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~~~
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:decode (s-n)
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dup &Instructions a:contains/string?
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[ &Instructions swap a:index/string ]
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[ s:to-hex-number ] choose ;
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~~~
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The `debundle` word breaks a string into four two byte substrings
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and runs `decode` against each.
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~~~
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:debundle (s-abcd)
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[ #0 #2 s:substr decode ]
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[ #2 #2 s:substr decode ]
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[ #4 #2 s:substr decode ]
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[ #6 #2 s:substr decode ] quad ;
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~~~
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Once debundled and decoded, I can then pack the opcodes into a
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single cell. This is simple, just some quick shifts and addition.
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~~~
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:pack (abcd-n)
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#-24 shift swap #-16 shift + swap #-8 shift + + ;
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~~~
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Nearing completion, I wrap everything up into a single word and
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then patch the original `i` to jump to this.
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(The 1793 corresponds to the liju.... instruction sequence)
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~~~
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:assemble (s-) debundle pack , ;
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#1793 &i store
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&assemble &i n:inc store
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~~~
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And finally, close off the namespace leaving the dictionary clean
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of all the words used to implement this.
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~~~
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}}
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'muri s:put nl
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~~~
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