Code Like It's 198x

Keep a Bounded Cell-edit Undo Log on Amiga

Undo should restore the previous cell value, including when the same cell was edited twice.

Undo should restore the previous cell value, including when the same cell was edited twice.

Amiga shows the working diagnostic row 01 01 00 01 01 09 00 00.
A completed native caller displays 01 01 00 01 01 09 00 00. The public field order below identifies these bytes; the capture is a state diagnostic, not a recording of a finished game.

Use it in a project

A sixteen-cell board has a four-entry undo stack: J stores cell indices and K their previous values. MODE=0 records then applies one CELL/VALUE edit; MODE=1 undoes the newest edit; MODE=2 clears history without changing the board. Full or empty history returns RESULT=0 with VALID=1 and leaves the board unchanged. Invalid mode, COUNT above four, edit index or popped log index returns VALID=0 without committing changes.

Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: RESULT, VALID, COUNT, MODE, CELL, VALUE, C0, C1, C2, C3, C4, C5, C6, C7, C8, C9, C10, C11, C12, C13, C14, C15, J0, J1, J2, J3, K0, K1, K2, K3. They occupy 30 contiguous bytes. Also reserve the private workspace: T0, T1, T2, EXPR_SAVE (4 bytes). Array names in the explanation refer to the numbered consecutive bindings, such as C0, C1 and so on.

Call bounded_undo_init once at the intended reset boundary. It clears inputs as well as outputs and workspace. Set your starting state and configuration afterwards, then call bounded_undo_step whenever the described event occurs. The helper reads no device, waits for no frame and installs no interrupt handler.

Ownership Fields
Retain between calls COUNT, C0–C15 and J0–J3/K0–K3
Supply before service MODE, CELL and VALUE
Read after service RESULT and VALID

Record the old value before writing the new one. Undo restores that saved value and shortens the stack only after validating the stored cell index.

Reusable initialisation and service

component.inc
; SPDX-License-Identifier: MIT
; Keep a Bounded Cell-edit Undo Log. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
bounded_undo_init:
    move.b #0,d0
    move.b d0,RESULT
    move.b d0,VALID
    move.b d0,COUNT
    move.b d0,MODE
    move.b d0,CELL
    move.b d0,VALUE
    move.b d0,C0
    move.b d0,C1
    move.b d0,C2
    move.b d0,C3
    move.b d0,C4
    move.b d0,C5
    move.b d0,C6
    move.b d0,C7
    move.b d0,C8
    move.b d0,C9
    move.b d0,C10
    move.b d0,C11
    move.b d0,C12
    move.b d0,C13
    move.b d0,C14
    move.b d0,C15
    move.b d0,J0
    move.b d0,J1
    move.b d0,J2
    move.b d0,J3
    move.b d0,K0
    move.b d0,K1
    move.b d0,K2
    move.b d0,K3
    move.b d0,T0
    move.b d0,T1
    move.b d0,T2
    move.b d0,EXPR_SAVE
    rts
bounded_undo_step:
; RESULT = 0
    move.b #0,d0
    move.b d0,RESULT
; VALID = 0
    move.b #0,d0
    move.b d0,VALID
; if COUNT > 4:
    move.b COUNT,d0
    cmp.b #4,d0
    bhi bounded_undo_L1
    jmp bounded_undo_L2
bounded_undo_L1:
; return
    rts
    jmp bounded_undo_L3
bounded_undo_L2:
bounded_undo_L3:
; if MODE >= 3:
    move.b MODE,d0
    cmp.b #3,d0
    bcc bounded_undo_L4
    jmp bounded_undo_L5
bounded_undo_L4:
; return
    rts
    jmp bounded_undo_L6
bounded_undo_L5:
bounded_undo_L6:
; if MODE == 2:
    move.b MODE,d0
    cmp.b #2,d0
    beq bounded_undo_L7
    jmp bounded_undo_L8
bounded_undo_L7:
; COUNT = 0
    move.b #0,d0
    move.b d0,COUNT
; VALID = 1
    move.b #1,d0
    move.b d0,VALID
; RESULT = 1
    move.b #1,d0
    move.b d0,RESULT
; return
    rts
    jmp bounded_undo_L9
bounded_undo_L8:
bounded_undo_L9:
; if MODE == 0:
    move.b MODE,d0
    cmp.b #0,d0
    beq bounded_undo_L10
    jmp bounded_undo_L11
bounded_undo_L10:
; if CELL >= 16:
    move.b CELL,d0
    cmp.b #16,d0
    bcc bounded_undo_L13
    jmp bounded_undo_L14
bounded_undo_L13:
; return
    rts
    jmp bounded_undo_L15
bounded_undo_L14:
bounded_undo_L15:
; VALID = 1
    move.b #1,d0
    move.b d0,VALID
; if COUNT == 4:
    move.b COUNT,d0
    cmp.b #4,d0
    beq bounded_undo_L16
    jmp bounded_undo_L17
bounded_undo_L16:
; return
    rts
    jmp bounded_undo_L18
bounded_undo_L17:
bounded_undo_L18:
; T0 = C[CELL]
    moveq #0,d1
    move.b CELL,d1
    lea C0,a0
    move.b 0(a0,d1.w),d0
    move.b d0,T0
; J[COUNT] = CELL
    move.b CELL,d0
    move.b d0,d2
    moveq #0,d1
    move.b COUNT,d1
    lea J0,a0
    move.b d2,0(a0,d1.w)
; K[COUNT] = T0
    move.b T0,d0
    move.b d0,d2
    moveq #0,d1
    move.b COUNT,d1
    lea K0,a0
    move.b d2,0(a0,d1.w)
; C[CELL] = VALUE
    move.b VALUE,d0
    move.b d0,d2
    moveq #0,d1
    move.b CELL,d1
    lea C0,a0
    move.b d2,0(a0,d1.w)
; COUNT = COUNT + 1
    move.b COUNT,d0
    add.b #1,d0
    move.b d0,COUNT
; RESULT = 1
    move.b #1,d0
    move.b d0,RESULT
; return
    rts
    jmp bounded_undo_L12
bounded_undo_L11:
bounded_undo_L12:
; VALID = 1
    move.b #1,d0
    move.b d0,VALID
; if COUNT == 0:
    move.b COUNT,d0
    cmp.b #0,d0
    beq bounded_undo_L19
    jmp bounded_undo_L20
bounded_undo_L19:
; return
    rts
    jmp bounded_undo_L21
bounded_undo_L20:
bounded_undo_L21:
; T0 = COUNT - 1
    move.b COUNT,d0
    sub.b #1,d0
    move.b d0,T0
; T1 = J[T0]
    moveq #0,d1
    move.b T0,d1
    lea J0,a0
    move.b 0(a0,d1.w),d0
    move.b d0,T1
; if T1 >= 16:
    move.b T1,d0
    cmp.b #16,d0
    bcc bounded_undo_L22
    jmp bounded_undo_L23
bounded_undo_L22:
; VALID = 0
    move.b #0,d0
    move.b d0,VALID
; return
    rts
    jmp bounded_undo_L24
bounded_undo_L23:
bounded_undo_L24:
; T2 = K[T0]
    moveq #0,d1
    move.b T0,d1
    lea K0,a0
    move.b 0(a0,d1.w),d0
    move.b d0,T2
; C[T1] = T2
    move.b T2,d0
    move.b d0,d2
    moveq #0,d1
    move.b T1,d1
    lea C0,a0
    move.b d2,0(a0,d1.w)
; COUNT = T0
    move.b T0,d0
    move.b d0,COUNT
; RESULT = 1
    move.b #1,d0
    move.b d0,RESULT
    rts
component_end:

Machine contract

D0–D3/A0–A1/CCR may change; D4–D7/A2–A6 are preserved. A direct BSR/JSR needs four return-address stack bytes. The helper makes no nested calls and adds no other stack use. Its named scratch is private workspace, not a pointer, a ROM variable or shared zero-page space that the game may borrow concurrently. The samples use fixed symbol bindings. Scope or rename every label and binding, including the component_end measurement marker, and allocate a separate block for each independent instance. Different helpers can also use common field names, so combining them needs separate namespaces or explicitly renamed bindings. Give each block one mutation owner.

The A500 OCS diagnostic cold-boots an OFS disk, owns the Copper and bitplane and disables interrupt/DMA sources. The helper does not take over hardware or allocate Chip RAM.

Adapt the caller

demo.asm supplies the bindings and include order. exercise.inc first runs table cases, then a separate client retaining state between calls. A game can call the public step entry directly and remove the diagnostic packet and display. Do not copy the diagnostic’s hardware takeover into an existing game loop.

Build with make ASM198X=/path/to/asm198x and BUILD198X=/path/to/build198x. The Makefile produces normal, custom, deliberately wrong and repaired programs. Tools must already be installed; the sample downloads nothing.

Bindings and include order

demo.asm
    section code,code_c
CUSTOM = 0
BROKEN = 0
COYOTE_STEPS = 4
BUFFER_STEPS = 4
TEXT_CAP = 8
PROPERTY_COUNT = 16
STREAM_BYTES = 13
CRC_POLY = 7
    include "caller.inc"
    include "exercise.inc"
component_start:
    include "component.inc"
    even
    include "data.inc"
    include "cases.inc"
    even
    include "display.inc"
    include "../../framework/turn-deadline/font.inc"
code_end:
STATE:
RESULT: dc.b 0
VALID: dc.b 0
COUNT: dc.b 0
MODE: dc.b 0
CELL: dc.b 0
VALUE: dc.b 0
C0: dc.b 0
C1: dc.b 0
C2: dc.b 0
C3: dc.b 0
C4: dc.b 0
C5: dc.b 0
C6: dc.b 0
C7: dc.b 0
C8: dc.b 0
C9: dc.b 0
C10: dc.b 0
C11: dc.b 0
C12: dc.b 0
C13: dc.b 0
C14: dc.b 0
C15: dc.b 0
J0: dc.b 0
J1: dc.b 0
J2: dc.b 0
J3: dc.b 0
K0: dc.b 0
K1: dc.b 0
K2: dc.b 0
K3: dc.b 0
T0: dc.b 0
T1: dc.b 0
T2: dc.b 0
EXPR_SAVE: dc.b 0
    even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
    even
signature: dc.b "198XWORLDKIT000",0
    even
copperlist:
    dc.w $08e,$2c81,$090,$2cc1,$092,$0038,$094,$00d0
    dc.w $100,$1200,$102,0,$104,0,$108,0,$10a,0
    dc.w $180,$012,$182,$fff
copbpl:
    dc.w $0e0,0,$0e2,0
    dc.w $ffff,$fffe
plane: dcb.b 10240,0

A client retaining state between calls

exercise.inc, excerpt
preview_rules:
    ifne CUSTOM
; CELL = 0
    move.b #0,d0
    move.b d0,CELL
; VALUE = 7
    move.b #7,d0
    move.b d0,VALUE
    ifne BROKEN
; COUNT = 0
    move.b #0,d0
    move.b d0,COUNT
    endif
    bsr checked_step
    bsr emit_state
; VALUE = 9
    move.b #9,d0
    move.b d0,VALUE
    ifne BROKEN
; COUNT = 0
    move.b #0,d0
    move.b d0,COUNT
    endif
    bsr checked_step
    bsr emit_state
; MODE = 1
    move.b #1,d0
    move.b d0,MODE
    ifne BROKEN
; COUNT = 0
    move.b #0,d0
    move.b d0,COUNT
    endif
    bsr checked_step
    bsr emit_state
    rts
    else
; CELL = 0
    move.b #0,d0
    move.b d0,CELL
; VALUE = 7
    move.b #7,d0
    move.b d0,VALUE
    ifne BROKEN
; COUNT = 0
    move.b #0,d0
    move.b d0,COUNT
    endif
    bsr checked_step
    bsr emit_state
; CELL = 1
    move.b #1,d0
    move.b d0,CELL
; VALUE = 9
    move.b #9,d0
    move.b d0,VALUE
    ifne BROKEN
; COUNT = 0
    move.b #0,d0
    move.b d0,COUNT
    endif
    bsr checked_step
    bsr emit_state
; MODE = 1
    move.b #1,d0
    move.b d0,MODE
    ifne BROKEN
; COUNT = 0
    move.b #0,d0
    move.b d0,COUNT
    endif
    bsr checked_step
    bsr emit_state

    ifne BROKEN
; COUNT = 0
    move.b #0,d0
    move.b d0,COUNT
    endif
    bsr checked_step
    bsr emit_state
    endif
    rts

Customise it

Edit the same cell twice, then undo only the latest edit. The maintained custom caller changes its inputs or client sequence rather than hiding a different implementation behind the same label. Compare every state of the two clients, not only their final display.

Custom client selection

custom.asm
    section code,code_c
CUSTOM = 1
BROKEN = 0
COYOTE_STEPS = 2
BUFFER_STEPS = 2
TEXT_CAP = 4
PROPERTY_COUNT = 16
STREAM_BYTES = 13
CRC_POLY = 49
    include "caller.inc"
    include "exercise.inc"
component_start:
    include "component.inc"
    even
    include "data.inc"
    include "cases.inc"
    even
    include "display.inc"
    include "../../framework/turn-deadline/font.inc"
code_end:
STATE:
RESULT: dc.b 0
VALID: dc.b 0
COUNT: dc.b 0
MODE: dc.b 0
CELL: dc.b 0
VALUE: dc.b 0
C0: dc.b 0
C1: dc.b 0
C2: dc.b 0
C3: dc.b 0
C4: dc.b 0
C5: dc.b 0
C6: dc.b 0
C7: dc.b 0
C8: dc.b 0
C9: dc.b 0
C10: dc.b 0
C11: dc.b 0
C12: dc.b 0
C13: dc.b 0
C14: dc.b 0
C15: dc.b 0
J0: dc.b 0
J1: dc.b 0
J2: dc.b 0
J3: dc.b 0
K0: dc.b 0
K1: dc.b 0
K2: dc.b 0
K3: dc.b 0
T0: dc.b 0
T1: dc.b 0
T2: dc.b 0
EXPR_SAVE: dc.b 0
    even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
    even
signature: dc.b "198XWORLDKIT000",0
    even
copperlist:
    dc.w $08e,$2c81,$090,$2cc1,$092,$0038,$094,$00d0
    dc.w $100,$1200,$102,0,$104,0,$108,0,$10a,0
    dc.w $180,$012,$182,$fff
copbpl:
    dc.w $0e0,0,$0e2,0
    dc.w $ffff,$fffe
plane: dcb.b 10240,0

The custom diagnostic row is 01 01 01 01 00 09 07 00. The full client trace in the source explains how it gets there.

Mistake and repair

The wrong caller clears COUNT before each operation. Edits lose their history and undo has nothing to restore. Keep the log and its count together until an explicit clear or restart.

The BROKEN branch lives in the sequential client, not the reusable helper. wrong.asm selects it; repaired.asm restores the normal call sequence. The repaired executable matches the normal program (for Spectrum, compare loaded CODE rather than the tape filename).

Select the deliberately wrong client

wrong.asm
    section code,code_c
CUSTOM = 0
BROKEN = 1
COYOTE_STEPS = 4
BUFFER_STEPS = 4
TEXT_CAP = 8
PROPERTY_COUNT = 16
STREAM_BYTES = 13
CRC_POLY = 7
    include "caller.inc"
    include "exercise.inc"
component_start:
    include "component.inc"
    even
    include "data.inc"
    include "cases.inc"
    even
    include "display.inc"
    include "../../framework/turn-deadline/font.inc"
code_end:
STATE:
RESULT: dc.b 0
VALID: dc.b 0
COUNT: dc.b 0
MODE: dc.b 0
CELL: dc.b 0
VALUE: dc.b 0
C0: dc.b 0
C1: dc.b 0
C2: dc.b 0
C3: dc.b 0
C4: dc.b 0
C5: dc.b 0
C6: dc.b 0
C7: dc.b 0
C8: dc.b 0
C9: dc.b 0
C10: dc.b 0
C11: dc.b 0
C12: dc.b 0
C13: dc.b 0
C14: dc.b 0
C15: dc.b 0
J0: dc.b 0
J1: dc.b 0
J2: dc.b 0
J3: dc.b 0
K0: dc.b 0
K1: dc.b 0
K2: dc.b 0
K3: dc.b 0
T0: dc.b 0
T1: dc.b 0
T2: dc.b 0
EXPR_SAVE: dc.b 0
    even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
    even
signature: dc.b "198XWORLDKIT000",0
    even
copperlist:
    dc.w $08e,$2c81,$090,$2cc1,$092,$0038,$094,$00d0
    dc.w $100,$1200,$102,0,$104,0,$108,0,$10a,0
    dc.w $180,$012,$182,$fff
copbpl:
    dc.w $0e0,0,$0e2,0
    dc.w $ffff,$fffe
plane: dcb.b 10240,0
Amiga shows the deliberate error row 00 01 00 01 01 09 07 09.
The wrong client completes with 00 01 00 01 01 09 07 09; its changed inputs and state explain the result.

Cost and alternatives

The measured helper extent is 642 code bytes; public state is 30 bytes and private scratch is 4 bytes. There are no immutable helper tables. Caller data, the test cases, return-address stack, alignment and display buffers cost additional space. These are assembled sizes, not cycle measurements.

Constant work per edit or undo, with eight log bytes for four changes. Whole-state checkpoints are simpler for a tiny state; a larger transaction log needs capacity preflight and explicit grouping.

This is a LIFO edit log, not redo or a full game-state rewind. A move changing several cells needs a grouped transaction or several log entries committed together. Editing an unchanged value still consumes an entry in this deliberately simple contract.

Execution evidence

Four variants run on A500 OCS PAL, Kickstart1.3 OFS cold boot, using Asm198x 0.0.72, Emu198x 0.44.2 and Build198x 0.2.9. Each executes 33 boundary/varied/error table cases, dirty initialisation and the separate retained-state client. An independent mathematical/container model checks every public byte after every call. This is bounded example coverage, not exhaustive correctness for every possible state.

The records retain loaded immutable code, completion, register sentinels, full packets, source/tool/firmware hashes and captures. The public gate validates these retained observations and rejects altered evidence; it does not run restricted firmware. No firmware is distributed. The eight displayed bytes are the first eight public fields, zero-padded when shorter; the packet includes every field.

Where it fits

Checkpoints: A checkpoint restores one coherent snapshot; an undo log remembers a bounded sequence of individual changes.

The Craft · Maths for Games · Source and native evidence.