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
2026-08-21 12:03:31 -04:00
parent 20989c3439
commit fb335681d2
12 changed files with 180 additions and 55 deletions
@@ -1358,9 +1358,9 @@ layOutImage:
  CALL checkImageRoom
  BNQ layOutNo

-  SETD.0 Image
-  SETD.1 ImgWalk
-  STD.0.1
+  SETD.0 ImgWalk
+  SETD.2 ScratchImage
+  CALL numSet
  SETD.0 MagicSPBT
  INIA 0d4
  CALL putBytes
@@ -1412,9 +1412,9 @@ layOutLoadable:
  CALL checkImageRoom
  BNQ layOutNo

-  SETD.0 Image
-  SETD.1 ImgWalk
-  STD.0.1
+  SETD.0 ImgWalk
+  SETD.2 ScratchImage
+  CALL numSet
  SETD.0 MagicSBEX
  INIA 0d4
  CALL putBytes
@@ -1674,9 +1674,9 @@ writeVectorsHeader:
  SETD.0 Loadable
  LDA.0
  BRA writeVectorsNone
-  SETD.0 Image
-  SETD.1 ImgWalk
-  STD.0.1
+  SETD.0 ImgWalk
+  SETD.2 ScratchImage
+  CALL numSet
  SETD.0 ImgWalk
  INIA 0d4
  CALL numAddByte
@@ -1693,9 +1693,9 @@ writeVectorsNone:
  SETD.0 Loadable
  LDA.0
  BRA writeVectorsOut
-  SETD.0 Image
-  SETD.1 ImgWalk
-  STD.0.1
+  SETD.0 ImgWalk
+  SETD.2 ScratchImage
+  CALL numSet
  SETD.0 ImgWalk
  INIA 0d8
  CALL numAddByte
@@ -1773,7 +1773,8 @@ vectorNotEntry:

 writeImage:
  SETD.0 OutName
-  SETD.1 Image
+  SETD.1 ScratchImage
+  LDD.1.1
  SETD.2 ImgTotal
  LDA.2
  INCD.2
@@ -2046,9 +2047,10 @@ DropWalk:
  0x00 0x00

 ; How big a binary this can build. Everything the assembler makes has to fit here at once,
-; because a file is written in one call and there is nowhere to put half of one.
+; because a file is written in one call and there is nowhere to put half of one. CosmOS
+; itself comes to 9,564 bytes.
 ImgRoom:
-  0x10 0x00
+  0x34 0x00

 MagicSPBT:
 "SPBT"
@@ -2169,9 +2171,8 @@ LabelsEnd:
 "
 "

-Image:
-  #Reserve 0d4096

+#Include scratch.asm
 #Include numbers.asm
 #Include source.asm
 #Include token.asm
@@ -25,12 +25,12 @@ labReset:
  CALL numZero
  SETD.0 LabUsed
  CALL numZero
-  SETD.0 LabArena
-  SETD.1 LabNext
-  STD.0.1
-  SETD.0 LabIndex
-  SETD.1 LabBase
-  STD.0.1
+  SETD.0 LabNext
+  SETD.2 ScratchLabArena
+  CALL numSet
+  SETD.0 LabBase
+  SETD.2 ScratchLabIndex
+  CALL numSet
  RET

 ; Adds the name at DP0, meaning the address in A and B. Q is zero if it went in.
@@ -294,13 +294,16 @@ LabLenWalk:
 LabEnd:
  0x00 0x00

-; How many labels there may be, and how many bytes of name between them. Sized for a
-; single program rather than for CosmOS: raising either is changing a number here, and
-; running into one says so rather than writing past the end of the table.
+; How many labels there may be, and how many bytes of name between them.
+;
+; SIZED FOR THE ASSEMBLER ITSELF, which turns out to be the largest thing it is asked to
+; build: 555 labels and about 6,800 bytes of name, against CosmOS's 475 and 5,881. Running into
+; either limit says so rather than writing past the end of the table. The buffers live
+; above the program rather than inside it - see the scratch map in Asm.asm.
 LabLimit:
-  0x01 0x00
+  0x03 0x00
 LabRoom:
-  0x08 0x00
+  0x20 0x00

 LabNamed:
 ": "
@@ -316,7 +319,3 @@ LabFull:
 LabNoRoom:
 "no room left for label names"

-LabIndex:
-  #Reserve 0d1024
-LabArena:
-  #Reserve 0d2048
@@ -80,5 +80,6 @@ NoFileText:
 ; The libraries go last, after both segments have been based. An included file that carries
 ; code brings its own #Program and #Data lines with it, and a #Base has to come before
 ; anything is in the segment it bases - so the bases are set here and the code arrives after.
+#Include scratch.asm
 #Include numbers.asm
 #Include source.asm
@@ -0,0 +1,44 @@
+; Where the assembler's big buffers live.
+;
+; A map rather than a set of declarations, and it has a file of its own because the reader
+; and the label table both need addresses out of it while neither includes the other.
+;
+#Data
+
+; NOT #Reserve, AND THAT IS THE WHOLE POINT. Reserved space in a segment is written into
+; the file as zeroes and copied at load, so 22K of scratch made a 34K file - and a loaded
+; program is staged at 0x8000 before being put in place, which leaves exactly 32,768 bytes
+; for the whole of it. The assembler could not load itself.
+;
+; None of this is initialised data. It is scratch, wanted only while the assembler is
+; running, and while it is running everything above its own data is free: the system keeps
+; below 0x1000, the staging area is only in use during a load, and the Stack comes down
+; from the top. So the addresses are written down here and the file carries none of it.
+;
+;   0x8000  3072   the label index, 768 entries of four
+;   0x8C00  8192   the label names, packed end to end
+;   0xAC00 13312   the binary being built
+;   0xE000  1792   the vector names, 64 entries of twenty eight
+;   0xE700  1758   the reader's stack, six levels of 293
+;   0xEE00   368   which files have been included, sixteen names of 23
+;
+; That ends at 0xEF70, with the Stack coming down from 0xFFFF above it - about four
+; kilobytes, against the tens of bytes of CALL frames this ever nests.
+;
+; THE TWO THINGS THAT DECIDE THESE SIZES are the largest program it will be asked to build
+; and the largest one it will be asked to read. CosmOS is 475 labels and 9,564 bytes of
+; output; the assembler itself is 555 labels, about 6,800 bytes of name and 11,648 of output. The
+; second is bigger than the first, which is worth knowing: the hardest thing this assembles
+; is not the operating system, it is itself.
+ScratchLabIndex:
+  0x80 0x00
+ScratchLabArena:
+  0x8C 0x00
+ScratchImage:
+  0xAC 0x00
+ScratchVecNames:
+  0xE0 0x00
+ScratchSrcStack:
+  0xE7 0x00
+ScratchIncNames:
+  0xEE 0x00
@@ -278,9 +278,9 @@ srcStepBackAt:

 ; Where the slot for the current depth is, into SrcSlot.
 srcSlot:
-  SETD.0 SrcStack
-  SETD.1 SrcSlot
-  STD.0.1                   ; WHERE the stack is, not what is in it.
+  SETD.0 SrcSlot
+  SETD.2 ScratchSrcStack
+  CALL numSet               ; WHERE the stack is, not what is in it.
  SETD.0 SrcSlotLeft
  SETD.2 SrcDepth
  LDA.2
@@ -388,9 +388,9 @@ srcRemember:
 ; Where name number IncLeft sits, into IncSlot. Fixed fields of 23 bytes: there are few of
 ; these and they are short, so an arena would cost more code than it saved.
 srcSeenSlot:
-  SETD.0 IncNames
-  SETD.1 IncSlot
-  STD.0.1
+  SETD.0 IncSlot
+  SETD.2 ScratchIncNames
+  CALL numSet
  SETD.0 IncSlotLeft
  SETD.2 IncLeft
  LDA.2
@@ -549,10 +549,4 @@ TooDeepText:
 "included files are nested deeper than this assembler will follow
 "

-; Six levels of nesting, at 293 bytes each. CosmOS itself nests three deep.
-SrcStack:
-  #Reserve 0d1758

-; Sixteen names of 23 bytes, which is more separate files than anything here includes.
-IncNames:
-  #Reserve 0d368
@@ -169,6 +169,7 @@ NoFileText:
 "no such file
 "

+#Include scratch.asm
 #Include numbers.asm
 #Include source.asm
 #Include token.asm
@@ -225,9 +225,9 @@ vecWriteHandler:

 ; Where entry number VecWhich sits, into VecSlot. Twenty eight bytes an entry.
 vecSlotAt:
-  SETD.0 VecNames
-  SETD.1 VecSlot
-  STD.0.1
+  SETD.0 VecSlot
+  SETD.2 ScratchVecNames
+  CALL numSet
  SETD.0 VecSlotLeft
  SETD.2 VecWhich
  CALL numSet
@@ -293,5 +293,3 @@ VecFull:
 VecName:
  #Reserve 0d23

-VecNames:
-  #Reserve 0d1792