SplitBit assembles SplitBit: M1, a single file with no includes
Programs/CosmOS/Assembler/ is an assembler written in SplitBit assembly. It
runs under CosmOS, reads source off a SplitBit disk, and writes a binary back
to it with no host involved anywhere:
> run Asm.sbx hello.asm
wrote hello.bin: program 17, data 14, labels 2
THE ACCEPTANCE TEST IS THE BYTES. Tests/native.sh assembles Programs/hello.asm
both ways and compares the two files byte for byte, then runs the one the
machine built. "It ran" and "the sizes look right" both pass for a binary with
a label one byte out, which is a program that jumps into the middle of an
instruction - so the only honest test is the one SplitDisk and sbfs.asm
already work under: two implementations of one written specification, each
checking the other. The files are identical and the result prints Hello,
World! in 70 cycles.
hello.asm is the target because it is the oldest program in the repository.
The first thing this machine ever ran is now the first thing it assembles for
itself.
TWO PASSES OVER STREAMED SOURCE. The C assembler reads every token of every
file into one array; that cannot port, because cosmos.asm alone is 56,047
bytes against 64K of Data Memory. The native one streams through a 256 byte
window, twice, and keeps only the label table between the passes. Two passes
suffice because every length is known without resolving anything - an
instruction's from its shape, a value's is one, a string's is its characters
and a zero - so the first pass fixes every address and the second never needs
a fixup list. A forward reference stops being a special case and becomes the
reason there are two passes at all.
The parts, each checked before anything was built on it:
source.asm characters out of a file of any size, with a line number
token.asm tokens out of characters, one character of lookahead
classify.asm what a token is, in the C assembler's order, which IS the
language: keyword, instruction, value, string, label
labels.asm names packed in an arena, four bytes of index each
numbers.asm sixteen bit arithmetic, since sbfs.asm's cannot be reached
table.asm the instruction set, generated by the same script the
monitor's copy is, and now BOTH are checked by docs.sh
readTest.asm and tokenTest.asm check the reader and the tokenizer on their
own, recorded as cosmosSource and cosmosTokens. A wrong classification does
not produce a wrong byte somewhere obvious; it produces a right looking
program of the wrong length, so it is worth catching where it happens.
WHAT IT REFUSES: #Include, #Base, #Align, #Reserve and #Vectors are refused
by name rather than ignored. Skipping a directive would produce a file that
looked right and was the wrong length, which is the worst thing an assembler
can do.
Two traps worth recording, both already known to this project and both hit
again: CALL restores A, B and DP0-DP2, so three routines returning an answer
in A had it undone by their own return; and numStep works on DP0, so three
sites that set DP1 left a pointer that never advanced.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E2JrLzFvuFX9fgi1LDRjrW
This commit is contained in:
Anachronaut
co-authored by
Claude Opus 5
Claude Opus 5
parent
a131a90c67
commit
dcb331c151
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; Tokens out of characters.
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;
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; A token is a run of characters with whitespace or a comment on either side, or anything
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; between a pair of quotes. That is the whole of the lexical grammar: SplitBit assembly has
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; no operators, no punctuation and no line continuation, so there is nothing here that has
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; to look ahead more than one character.
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;
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; ONE CHARACTER OF LOOKAHEAD, and it is held here rather than in the reader. A word ends
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; when something that is not part of it turns up, and that something has already been read
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; by the time anyone knows - so it is put in TokPending and taken again next time. Keeping
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; it at this level rather than pushing it back into the reader means the line number needs
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; no arithmetic: srcNext counted the newline when it handed it out, and it stays counted.
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;
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; Zero means "nothing held", which is safe because a source file is text and a text file
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; has no zero bytes in it. A file that did would be rejected as unassemblable long before
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; the difference showed.
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;
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; Written by Anachronaut
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#Program
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; The next token, into TokText with a zero after it. Q is zero if there was one.
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;
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; TokLength is how long it is, TokString says whether it arrived in quotes, and TokLine is
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; the line it STARTED on - captured before the token is read, because a token ending in a
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; newline has already moved the reader on to the next line by the time it is finished.
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tokNext:
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RSTA
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SETD.0 TokString
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STA.0
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tokSkip:
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CALL tokGet
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BNQ tokEnded
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CALL tokIsSpace
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BRQ tokSkip
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SETD.0 TokChar
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LDA.0
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INIB 0x3B ; A semicolon starts a comment.
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XOR
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BNQ tokBegin
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tokComment:
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CALL tokGet
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BNQ tokEnded
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SETD.0 TokChar
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LDA.0
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INIB 0x0A
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XOR
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BNQ tokComment ; Everything up to the newline belongs to the comment.
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BRI tokSkip
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tokBegin:
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; Where it starts, for anything that has to complain about it later.
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SETD.0 TokLine
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SETD.2 SrcLine
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CALL numSet
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RSTA
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SETD.0 TokLength
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STA.0
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SETD.0 TokText
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SETD.1 TokPointer
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STD.0.1
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SETD.0 TokChar
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LDA.0
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INIB 0x22 ; A quote starts a string.
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XOR
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BNQ tokWord
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INIA 0d1
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SETD.0 TokString
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STA.0
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tokStringLoop:
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CALL tokGet
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BNQ tokDone ; The file ended inside a string. Take what there is; the
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; classifier will have something to complain about.
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SETD.0 TokChar
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LDA.0
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INIB 0x22
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XOR
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BRQ tokDone
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CALL tokAppend
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BRI tokStringLoop
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tokWord:
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CALL tokAppend
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tokWordLoop:
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CALL tokGet
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BNQ tokDone
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CALL tokIsSpace
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BRQ tokHoldDone
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SETD.0 TokChar
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LDA.0
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INIB 0x3B
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XOR
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BRQ tokHoldDone ; A comment butting straight up against a word ends it.
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CALL tokAppend
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BRI tokWordLoop
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tokHoldDone:
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; Whatever ended the word was not part of it, so it goes back to be looked at again.
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SETD.0 TokChar
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LDA.0
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SETD.0 TokPending
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STA.0
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tokDone:
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SETD.1 TokPointer
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LDD.0.1
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RSTA
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STA.0 ; The zero that makes it a string.
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RSTA
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RSTB
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CCF
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ADD ; Q is zero: there was a token.
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RET
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tokEnded:
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RSTA
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SETD.0 TokLength
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STA.0
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SETD.0 TokText
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STA.0
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RSTA
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INIB 0d1
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CCF
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ADD ; Q is not zero: the source is finished.
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RET
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; The next character, into TokChar. Q is zero if there was one. Takes the held one first.
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tokGet:
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SETD.0 TokPending
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LDA.0
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BRA tokGetFresh
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SETD.0 TokChar
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STA.0
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RSTA
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SETD.0 TokPending
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STA.0
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RSTA
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RSTB
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CCF
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ADD
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RET
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tokGetFresh:
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CALL srcNext
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BNQ tokGetNone
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SETD.0 SrcChar
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LDA.0
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SETD.0 TokChar
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STA.0
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RSTA
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RSTB
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CCF
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ADD
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RET
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tokGetNone:
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RSTA
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INIB 0d1
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CCF
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ADD
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RET
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; Adds TokChar to the token being built, unless it is already as long as one may be.
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;
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; A token that runs over is truncated rather than refused, and the classifier refuses it
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; afterwards: nothing 255 characters long is a valid mnemonic, literal or label, so the
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; error that comes out names what was wrong with it rather than only how long it was.
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tokAppend:
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SETD.0 TokLength
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LDA.0
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INIB 0xFF
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XOR
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BRQ tokAppendFull
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SETD.1 TokPointer
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LDD.0.1
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SETD.2 TokChar
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LDA.2
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STA.0
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INCD.0
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STD.0.1
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SETD.0 TokLength
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LDA.0
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INCA
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STA.0
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tokAppendFull:
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RET
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; Q is zero if TokChar is whitespace: a space, or anything in the run from tab to carriage
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; return, which is what the C library calls a space and what the other assembler uses.
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tokIsSpace:
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SETD.0 TokChar
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LDA.0
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INIB 0x20
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XOR
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BRQ tokSpaceYes
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SETD.0 TokChar
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LDA.0
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INIB 0x09
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CCF
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SUB
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BRC tokSpaceNo ; Below a tab.
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SETD.0 TokChar
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LDA.0
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INIB 0x0E
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CCF
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SUB
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BNC tokSpaceNo ; Past a carriage return.
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tokSpaceYes:
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RSTA
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RSTB
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CCF
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ADD
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RET
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tokSpaceNo:
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RSTA
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INIB 0d1
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CCF
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ADD
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RET
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#Data
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TokLine:
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0x00 0x00
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TokPointer:
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0x00 0x00
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TokLength:
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0x00
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TokString:
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0x00
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TokChar:
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0x00
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TokPending:
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0x00
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; As long as a token may be, and one more for the zero. The other assembler stops at the
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; same 255, and the limit is worth matching rather than choosing again.
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TokText:
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#Reserve 0d256
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