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

README.md

@@ -16,6 +16,7 @@ SplitBit is a custom 8 bit system designed for hobbyist projects and experimenta
 - Loadable Programs: A program that was not booted from carries a header saying where it belongs, and Programs/loader.asm reads one off a disk, puts it there, and runs it.
 - An Operating System: CosmOS boots the machine, mounts a disk, lists what is on it, loads a program and runs it, and takes the machine back when it finishes. It comes with a library of programs to run, including a game and a line editor that writes files a person typed.
 - System Services: A loaded program reaches the console and the disk through numbered software interrupts rather than carrying a copy of the code that drives them. The numbers are written down in one file that both sides include, so neither ever types one. It took the editor from 4941 bytes to 1983 without changing a line of what it does.
+- A Native Assembler: SplitBit assembles SplitBit. Programs/CosmOS/Assembler/ is an assembler written in SplitBit assembly that runs under CosmOS, reads source off a SplitBit disk, and writes a binary back to it with no host involved. Its output has to be byte for byte identical to what the C assembler produces from the same source, which is what Tests/native.sh checks.
 - Streaming Reads: A file bigger than the machine's memory is read a block at a time, through services that keep nothing open between calls. CosmOS's own source is 104K against 64K of Data Memory, so this is what a self-hosted assembler will stand on.
 - Storage: A block device with 256 byte blocks and 16 megabytes of them, backed by an image file on the host. It knows blocks and not files, because a filesystem is meant to be software SplitBit runs.
 - Memory Controller: Reads and writes Program Memory, moves blocks between memory banks, reaches memory that devices bring with them, and guards a range against being written by accident. It is how a SplitBit machine loads a program.