Step a Data Pointer with INCD and DECD, not DPUP and DPDN by one

DPUP takes an immediate, so an offset of one is legal and does exactly the
right thing. It is also three bytes where INCD is two, and reads as "offset
the pointer up by one" where INCD reads as "step the pointer".

56 of them across 15 files: the system, the assembler, the editor, and eight
test programs. CosmOS is 9,564 bytes to 9,537, the native assembler 11,648
to 11,635, and every program in the repository together 49 bytes lighter.

The worst offender was numbers.asm, written this week, where every sixteen
bit helper reaches the low byte and comes back the long way round. It is the
file every other part of the assembler includes, so it is the first thing
anybody reads when they go looking - and it was teaching them the long way.
Pattern matched off sbfs.asm rather than off the instruction table I had
just embedded in two programs.

THIS IS NOT TWO WAYS TO DO ONE THING. DPUP takes an arbitrary number, so one
is inevitably among them; INCD earns its place by making the common case a
byte cheaper. The overlap is structural and the choice is a usage question,
which is a linter's job rather than an ISA's - "DPUP.n 0d01: INCD.n does
this in a byte less" is a mechanical rule with no judgement in it.

Nothing needed re-recording, which was not a foregone conclusion: cosmosBreak
prints the system addresses the registers happened to hold, and they did not
move. Both assemblers still produce identical bytes and CosmOS still builds
itself to a fixed point.

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

Programs/CosmOS/Apps/Edit.asm

@@ -653,12 +653,12 @@ appendNode:
 
 ; DP2 is a two byte count. Takes one off it.
 takeOneOff:
-  DPUP.2 0d01
+  INCD.2
   LDA.2
   DECA
   STA.2
   BNC takeOneDone           ; No borrow, so the high half is untouched.
-  DPDN.2 0d01
+  DECD.2
   LDA.2
   DECA
   STA.2
@@ -766,12 +766,12 @@ printWord:
 
 ; DP0 is a two byte number, A is a byte. Adds the one to the other.
 addByteToWord:
-  DPUP.0 0d01
+  INCD.0
   LDB.0
   CCF
   ADD
   STQ.0
-  DPDN.0 0d01
+  DECD.0
   LDA.0
   RSTB
   ADD

Programs/CosmOS/Apps/Stream.asm

@@ -290,11 +290,11 @@ checksumLoop:
   INCD.2
   LDA.2
   BNA checksumLow
-  DPDN.2 0d01
+  DECD.2
   LDA.2
   DECA
   STA.2                     ; Borrow out of the high byte.
-  DPUP.2 0d01
+  INCD.2
   INIA 0xFF
   STA.2
   BRI checksumTest
@@ -333,7 +333,7 @@ stepIndex:
   INCA
   STA.2
   BNC stepIndexDone
-  DPDN.2 0d01
+  DECD.2
   LDA.2
   INCA
   STA.2

Programs/CosmOS/Assembler/Asm.asm

@@ -735,7 +735,7 @@ emitStringLoop:
   CALL numStep
   SETD.1 EmitWalk
   LDD.0.1
-  DPDN.0 0d01
+  DECD.0
   LDA.0
   BNA emitStringLoop        ; The zero goes out with the rest and then stops the loop.
   BRI emitDone

Programs/CosmOS/Assembler/numbers.asm

@@ -28,16 +28,16 @@ numSet:
 
 ; The two byte number at DP0 becomes itself plus the one at DP2.
 numAdd:
-  DPUP.0 0d01
+  INCD.0
-  DPUP.2 0d01
+  INCD.2
   LDA.0
   LDB.2
   CCF
   ADD
   MVQA
   STA.0
-  DPDN.0 0d01
+  DECD.0
-  DPDN.2 0d01
+  DECD.2
   LDA.0
   LDB.2
   ADD                       ; Carries in from the low half. Nothing between touches it.
@@ -47,16 +47,16 @@ numAdd:
 
 ; The two byte number at DP0 becomes itself less the one at DP2.
 numTake:
-  DPUP.0 0d01
+  INCD.0
-  DPUP.2 0d01
+  INCD.2
   LDA.0
   LDB.2
   CCF
   SUB
   MVQA
   STA.0
-  DPDN.0 0d01
+  DECD.0
-  DPDN.2 0d01
+  DECD.2
   LDA.0
   LDB.2
   SUB                       ; Borrows in from the low half.
@@ -66,14 +66,14 @@ numTake:
 
 ; Adds the byte in A to the two byte number at DP0.
 numAddByte:
-  DPUP.0 0d01
+  INCD.0
   LDB.0
   CCF
   ADD
   MVQA
   STA.0
   BNC numAddByteDone
-  DPDN.0 0d01
+  DECD.0
   LDA.0
   INCA
   STA.0
@@ -82,12 +82,12 @@ numAddByteDone:
 
 ; Adds one to the two byte number at DP0.
 numStep:
-  DPUP.0 0d01
+  INCD.0
   LDA.0
   INCA
   STA.0
   BNC numStepDone           ; It did not wrap, so the high byte is untouched.
-  DPDN.0 0d01
+  DECD.0
   LDA.0
   INCA
   STA.0

Programs/CosmOS/Source/cosmos.asm

@@ -262,7 +262,7 @@ dirCheck:
   ; A file's length is its block count times 256 plus its tail, which is the block count
   ; in the high byte and the tail in the low one. Nothing has to multiply anything.
   SETD.0 SbfsFileBlocks
-  DPUP.0 0d01
+  INCD.0
   LDA.0
   SETD.1 DirSize
   STA.1
@@ -1281,14 +1281,14 @@ breakAddress:
   SETD.0 SRegText
   PSHD.3
   POPD.1
-  DPUP.1 0d1
+  INCD.1
   CALL breakByte
 
   ; And which bits of it those are, since a debug dump that makes you look the number up is
   ; only half of one. Halt is not among them: the machine is plainly not halted.
   PSHD.3
   POPD.1
-  DPUP.1 0d1
+  INCD.1
   LDA.1
   PSHA
   INIB 0x01
@@ -2283,12 +2283,12 @@ assembleNeedsValue:
 
 ; DP0 is a two byte address and A is how far to move it on.
 stepCursor:
-  DPUP.0 0d01
+  INCD.0
   LDB.0
   CCF
   ADD
   STQ.0
-  DPDN.0 0d01
+  DECD.0
   LDA.0
   RSTB
   ADD

Programs/CosmOS/Source/sbfs.asm

@@ -101,7 +101,7 @@ sbfsFind:
   SETD.1 SbfsBlock
   CALL sbfsCopyWord
   SETD.0 SbfsDirBlocks
-  DPUP.0 0d01
+  INCD.0
   LDA.0
   SETD.1 SbfsLeft
   STA.1                 ; Only the low byte: a directory of 256 blocks is 2048 files.
@@ -163,7 +163,7 @@ sbfsFindFound:
 
   PSHD.3
   POPD.0
-  DPUP.0 0d01
+  INCD.0
   SETD.1 SbfsFileStart
   CALL sbfsCopyWord
 
@@ -208,7 +208,7 @@ sbfsFirst:
   SETD.1 SbfsWalkBlock
   CALL sbfsCopyWord
   SETD.0 SbfsDirBlocks
-  DPUP.0 0d01
+  INCD.0
   LDA.0
   SETD.1 SbfsWalkLeft
   STA.1                 ; Only the low byte, the same as sbfsFind.
@@ -308,7 +308,7 @@ sbfsWalkEnd:
 sbfsWalkTake:
   SETD.1 SbfsWalkAt
   LDD.0.1
-  DPUP.0 0d01
+  INCD.0
   SETD.1 SbfsFileStart
   CALL sbfsCopyWord
 
@@ -428,7 +428,7 @@ sbfsRead:
   CALL sbfsCopyWord
 
   SETD.0 SbfsFileBlocks
-  DPUP.0 0d01
+  INCD.0
   LDA.0
   SETD.1 SbfsLeft
   STA.1
@@ -456,7 +456,7 @@ sbfsReadGot:
 
   ; On by a whole block. DPUP carries one byte at a time, so 256 takes two of them.
   DPUP.3 0xFF
-  DPUP.3 0x01
+  INCD.3
   SETD.0 SbfsBlock
   CALL sbfsStepWord
 
@@ -534,7 +534,7 @@ sbfsEntryBounds:
   POPD.3
   PSHD.3
   POPD.0
-  DPUP.0 0d01
+  INCD.0
   SETD.1 SbfsEntryStart
   CALL sbfsCopyWord
   SETD.0 SbfsEntryEnd
@@ -586,7 +586,7 @@ sbfsAllocTry:
   SETD.1 SbfsBlock
   CALL sbfsCopyWord
   SETD.0 SbfsDirBlocks
-  DPUP.0 0d01
+  INCD.0
   LDA.0
   SETD.1 SbfsLeft
   STA.1
@@ -682,16 +682,16 @@ sbfsSetWord:
 
 ; The two byte number at DP0 becomes itself less the one at DP2.
 sbfsSubWord:
-  DPUP.0 0d01
+  INCD.0
-  DPUP.2 0d01
+  INCD.2
   LDA.0
   LDB.2
   CCF
   SUB
   MVQA
   STA.0
-  DPDN.0 0d01
+  DECD.0
-  DPDN.2 0d01
+  DECD.2
   LDA.0
   LDB.2
   SUB                   ; Borrows in from the low half.
@@ -730,7 +730,7 @@ sbfsCreate:
   SETD.1 SbfsBlock
   CALL sbfsCopyWord
   SETD.0 SbfsDirBlocks
-  DPUP.0 0d01
+  INCD.0
   LDA.0
   SETD.1 SbfsLeft
   STA.1
@@ -781,7 +781,7 @@ sbfsCreateFill:
 
   PSHD.3
   POPD.1
-  DPUP.1 0d01
+  INCD.1
   SETD.0 SbfsFileStart
   CALL sbfsCopyWord
 
@@ -859,7 +859,7 @@ sbfsWriteFile:
   CALL sbfsSetWord
   CALL sbfsFileExtent
   SETD.0 SbfsWantBlocks
-  DPUP.0 0d01
+  INCD.0
   LDA.0
   SETD.1 SbfsLeft
   STA.1
@@ -872,7 +872,7 @@ sbfsWriteLoop:
   CALL sbfsWriteBlock
   BNQ sbfsWriteFailed
   DPUP.3 0xFF
-  DPUP.3 0x01
+  INCD.3
   SETD.0 SbfsBlock
   CALL sbfsStepWord
   SETD.0 SbfsLeft
@@ -983,12 +983,12 @@ sbfsCopyWord:
 
 ; Adds one to the two byte number at DP0.
 sbfsStepWord:
-  DPUP.0 0d01
+  INCD.0
   LDA.0
   INCA
   STA.0
   BNC sbfsStepDone      ; It did not wrap, so the high byte is untouched.
-  DPDN.0 0d01
+  DECD.0
   LDA.0
   INCA
   STA.0
@@ -998,16 +998,16 @@ sbfsStepDone:
 ; The two byte number at DP0 becomes itself plus the one at DP2. Only memory changes, so
 ; it survives the return: a routine cannot hand back a pointer or a register.
 sbfsAddWord:
-  DPUP.0 0d01
+  INCD.0
-  DPUP.2 0d01
+  INCD.2
   LDA.0
   LDB.2
   CCF
   ADD
   MVQA
   STA.0
-  DPDN.0 0d01
+  DECD.0
-  DPDN.2 0d01
+  DECD.2
   LDA.0
   LDB.2
   ADD                   ; Carries in from the low half. Nothing between the two touches it.

Programs/testPrograms/blitTest.asm

@@ -179,7 +179,7 @@ refusedHandler:
   BRC refusedCarried
   RETI
 refusedCarried:
-  DPDN.0 0d01
+  DECD.0
   LDA.0
   INCA
   STA.0

Programs/testPrograms/controllerWriteTest.asm

@@ -102,7 +102,7 @@ stepPastRefusal:
   BRC carried
   RETI
 carried:
-  DPDN.0 0d01
+  DECD.0
   LDA.0
   INCA
   STA.0

Programs/testPrograms/faultResumeTest.asm

@@ -41,7 +41,7 @@ faultHandler:
   RETI
 
 carried:
-  DPDN.0 0d01
+  DECD.0
   LDA.0
   INCA
   STA.0

Programs/testPrograms/fenceTest.asm

@@ -143,7 +143,7 @@ guardHandler:
   BRC stepCarried
   RETI
 stepCarried:
-  DPDN.0 0d01
+  DECD.0
   LDA.0
   INCA
   STA.0

Programs/testPrograms/refusalTest.asm

@@ -50,7 +50,7 @@ refusalHandler:
   RETI
 
 carried:
-  DPDN.0 0d01
+  DECD.0
   LDA.0
   INCA
   STA.0

Programs/testPrograms/registerBankTest.asm

@@ -114,7 +114,7 @@ bankHandler:
   BRC bankCarried
   RETI
 bankCarried:
-  DPDN.0 0d01
+  DECD.0
   LDA.0
   INCA
   STA.0

Programs/testPrograms/sbfsReadTest.asm

@@ -89,7 +89,7 @@ showFound:
 showContents:
   SETD.0 Landing
   SETD.1 SbfsFileBlocks
-  DPUP.1 0d01
+  INCD.1
   LDA.1
   SETD.1 LeftHigh
   STA.1

Programs/testPrograms/sbfsWalkTest.asm

@@ -60,7 +60,7 @@ walkCheck:
   ; A file is its blocks times 256 plus its tail, which is the block count in the high
   ; byte and the tail in the low one. Nothing has to multiply anything.
   SETD.0 SbfsFileBlocks
-  DPUP.0 0d01
+  INCD.0
   LDA.0
   SETD.1 Size
   STA.1

Programs/testPrograms/stackPointerTest.asm

@@ -24,7 +24,7 @@ start:
   ; The Stack Pointer points at the next free slot, so the byte pushed last sits one
   ; above it, and the one before that sits two above.
   MVSD.0
-  DPUP.0 0d01
+  INCD.0
   LDA.0             ; 'K', the last one pushed.
   INCD.0
   LDB.0             ; 'O', the one before it.