M4: SplitBit assembles SplitBit, and then assembles itself

> load Asm.sbx
    > run cosmos.asm
    wrote cosmos.bin: program 7036, data 2448, labels 475
    > run Asm.asm
    wrote Asm.sbx: program 7533, data 4099, labels 555

Both byte for byte identical to what the host assembler builds from the
same source. The machine now builds the operating system it is running on,
and builds the thing that built it.

THE CHECK THAT MATTERS MOST IS THE THIRD ONE. A binary that matches could
still have come from an assembler wrong in some way this particular source
happens not to exercise. So Tests/native.sh boots the CosmOS that CosmOS
built and has THAT assemble CosmOS again - and the second generation is
identical to the first, down to the cycle count. It is a fixed point: the
machinery has been through itself. After this the host is a convenience
rather than a necessity.

WHAT STOOD IN THE WAY was not the assembler. It loaded, faulted at 7,780
cycles, and the fault was in CosmOS: a loaded program is staged at 0x8000
before being blitted into place, so the whole FILE has to fit in the 32,768
bytes above it. The assembler's file was 33,983, and 22K of that was
zeroed scratch buffers - because #Reserve emits what it reserves.

None of that is initialised data. It is scratch, wanted only while the
assembler runs, and while it runs everything above its own data is free.
So the buffers are a MAP now rather than declarations - Assembler/scratch.asm
writes down six addresses and the file carries none of it. 33,983 bytes
became 11,648, and the assembler could load itself.

The map has a file of its own because the reader and the label table both
need addresses out of it while neither includes the other.

The sizes are cut to the largest thing it is asked to build, and that turns
out not to be the operating system: the assembler is 555 labels and 11,648
bytes of output against CosmOS's 475 and 9,564. The hardest thing this
assembles is itself.

Also: sizing it for CosmOS meant raising the label table, and raising the
label table is what pushed the file over the staging limit. The two facts
only met because the first one was tried.

Speed, measured rather than guessed: CosmOS takes 80,168,646 cycles, which
is eighty seconds of emulated time and under a second under --fast. Most of
it is a straight walk of 475 label names, several thousand times. Sorting
or bucketing that is easy and was deliberately not written before there was
something to measure.

make run-cosmos now puts every source file on the disk, so the whole thing
can be done rather than read about.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E2JrLzFvuFX9fgi1LDRjrW

README.md

@@ -16,7 +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. It builds boot images and loadable applications, following every directive the language has - including #Vectors, so a program that brings its own interrupt handlers can be assembled on the machine, loaded by it, and have its vectors installed and taken back again. 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.
+- 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. It builds boot images and loadable applications, following every directive the language has. IT ASSEMBLES COSMOS, AND IT ASSEMBLES ITSELF, both byte for byte identical to what the C assembler produces from the same source. Tests/native.sh then boots the CosmOS that CosmOS built and has that one assemble CosmOS again, so the machinery has been through itself: after that the host is a convenience rather than a necessity.
 - 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.