Undo should restore the previous cell value, including when the same cell was edited twice.
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
; 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:
ld a,0
ld (RESULT),a
ld (VALID),a
ld (COUNT),a
ld (MODE),a
ld (CELL),a
ld (VALUE),a
ld (C0),a
ld (C1),a
ld (C2),a
ld (C3),a
ld (C4),a
ld (C5),a
ld (C6),a
ld (C7),a
ld (C8),a
ld (C9),a
ld (C10),a
ld (C11),a
ld (C12),a
ld (C13),a
ld (C14),a
ld (C15),a
ld (J0),a
ld (J1),a
ld (J2),a
ld (J3),a
ld (K0),a
ld (K1),a
ld (K2),a
ld (K3),a
ld (T0),a
ld (T1),a
ld (T2),a
ld (EXPR_SAVE),a
ret
bounded_undo_step:
; RESULT = 0
ld a,0
ld (RESULT),a
; VALID = 0
ld a,0
ld (VALID),a
; if COUNT > 4:
ld a,(COUNT)
cp 4
jp z,bounded_undo_L2
jp nc,bounded_undo_L1
jp bounded_undo_L2
bounded_undo_L1:
; return
ret
jp bounded_undo_L3
bounded_undo_L2:
bounded_undo_L3:
; if MODE >= 3:
ld a,(MODE)
cp 3
jp nc,bounded_undo_L4
jp bounded_undo_L5
bounded_undo_L4:
; return
ret
jp bounded_undo_L6
bounded_undo_L5:
bounded_undo_L6:
; if MODE == 2:
ld a,(MODE)
cp 2
jp z,bounded_undo_L7
jp bounded_undo_L8
bounded_undo_L7:
; COUNT = 0
ld a,0
ld (COUNT),a
; VALID = 1
ld a,1
ld (VALID),a
; RESULT = 1
ld a,1
ld (RESULT),a
; return
ret
jp bounded_undo_L9
bounded_undo_L8:
bounded_undo_L9:
; if MODE == 0:
ld a,(MODE)
cp 0
jp z,bounded_undo_L10
jp bounded_undo_L11
bounded_undo_L10:
; if CELL >= 16:
ld a,(CELL)
cp 16
jp nc,bounded_undo_L13
jp bounded_undo_L14
bounded_undo_L13:
; return
ret
jp bounded_undo_L15
bounded_undo_L14:
bounded_undo_L15:
; VALID = 1
ld a,1
ld (VALID),a
; if COUNT == 4:
ld a,(COUNT)
cp 4
jp z,bounded_undo_L16
jp bounded_undo_L17
bounded_undo_L16:
; return
ret
jp bounded_undo_L18
bounded_undo_L17:
bounded_undo_L18:
; T0 = C[CELL]
ld a,(CELL)
ld e,a
ld d,0
ld hl,C0
add hl,de
ld a,(hl)
ld (T0),a
; J[COUNT] = CELL
ld a,(CELL)
ld c,a
ld a,(COUNT)
ld e,a
ld d,0
ld hl,J0
add hl,de
ld (hl),c
; K[COUNT] = T0
ld a,(T0)
ld c,a
ld a,(COUNT)
ld e,a
ld d,0
ld hl,K0
add hl,de
ld (hl),c
; C[CELL] = VALUE
ld a,(VALUE)
ld c,a
ld a,(CELL)
ld e,a
ld d,0
ld hl,C0
add hl,de
ld (hl),c
; COUNT = COUNT + 1
ld a,(COUNT)
add a,1
ld (COUNT),a
; RESULT = 1
ld a,1
ld (RESULT),a
; return
ret
jp bounded_undo_L12
bounded_undo_L11:
bounded_undo_L12:
; VALID = 1
ld a,1
ld (VALID),a
; if COUNT == 0:
ld a,(COUNT)
cp 0
jp z,bounded_undo_L19
jp bounded_undo_L20
bounded_undo_L19:
; return
ret
jp bounded_undo_L21
bounded_undo_L20:
bounded_undo_L21:
; T0 = COUNT - 1
ld a,(COUNT)
sub 1
ld (T0),a
; T1 = J[T0]
ld a,(T0)
ld e,a
ld d,0
ld hl,J0
add hl,de
ld a,(hl)
ld (T1),a
; if T1 >= 16:
ld a,(T1)
cp 16
jp nc,bounded_undo_L22
jp bounded_undo_L23
bounded_undo_L22:
; VALID = 0
ld a,0
ld (VALID),a
; return
ret
jp bounded_undo_L24
bounded_undo_L23:
bounded_undo_L24:
; T2 = K[T0]
ld a,(T0)
ld e,a
ld d,0
ld hl,K0
add hl,de
ld a,(hl)
ld (T2),a
; C[T1] = T2
ld a,(T2)
ld c,a
ld a,(T1)
ld e,a
ld d,0
ld hl,C0
add hl,de
ld (hl),c
; COUNT = T0
ld a,(T0)
ld (COUNT),a
; RESULT = 1
ld a,1
ld (RESULT),a
ret
component_end:Machine contract
AF/BC/DE/HL may change; IX/IY are preserved. A direct CALL needs two 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 48K diagnostic boots through ROM tape loading and USR, disables interrupts and owns the screen. The helper performs no port or screen writes.
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. The Makefile produces normal, custom, deliberately wrong and repaired programs. Tools must already be installed; the sample downloads nothing.
Bindings and include order
org $8000
CUSTOM equ 0
BROKEN equ 0
COYOTE_STEPS equ 4
BUFFER_STEPS equ 4
TEXT_CAP equ 8
PROPERTY_COUNT equ 16
STREAM_BYTES equ 13
CRC_POLY equ 7
include "caller.inc"
include "exercise.inc"
component_start:
include "component.inc"
include "data.inc"
include "cases.inc"
include "../../physics/rectangle-overlap/display.inc"
code_end:
STATE:
RESULT: defb 0
VALID: defb 0
COUNT: defb 0
MODE: defb 0
CELL: defb 0
VALUE: defb 0
C0: defb 0
C1: defb 0
C2: defb 0
C3: defb 0
C4: defb 0
C5: defb 0
C6: defb 0
C7: defb 0
C8: defb 0
C9: defb 0
C10: defb 0
C11: defb 0
C12: defb 0
C13: defb 0
C14: defb 0
C15: defb 0
J0: defb 0
J1: defb 0
J2: defb 0
J3: defb 0
K0: defb 0
K1: defb 0
K2: defb 0
K3: defb 0
T0: defb 0
T1: defb 0
T2: defb 0
EXPR_SAVE: defb 0
input_ptr: defw 0
output_ptr: defw 0
cases_left: defb 0
ready: defb 0
preview: defs 8,0
OUTPUT: defs 1300,0
end startA client retaining state between calls
preview_rules:
if CUSTOM
; CELL = 0
ld a,0
ld (CELL),a
; VALUE = 7
ld a,7
ld (VALUE),a
if BROKEN
; COUNT = 0
ld a,0
ld (COUNT),a
endif
call checked_step
call emit_state
; VALUE = 9
ld a,9
ld (VALUE),a
if BROKEN
; COUNT = 0
ld a,0
ld (COUNT),a
endif
call checked_step
call emit_state
; MODE = 1
ld a,1
ld (MODE),a
if BROKEN
; COUNT = 0
ld a,0
ld (COUNT),a
endif
call checked_step
call emit_state
ret
else
; CELL = 0
ld a,0
ld (CELL),a
; VALUE = 7
ld a,7
ld (VALUE),a
if BROKEN
; COUNT = 0
ld a,0
ld (COUNT),a
endif
call checked_step
call emit_state
; CELL = 1
ld a,1
ld (CELL),a
; VALUE = 9
ld a,9
ld (VALUE),a
if BROKEN
; COUNT = 0
ld a,0
ld (COUNT),a
endif
call checked_step
call emit_state
; MODE = 1
ld a,1
ld (MODE),a
if BROKEN
; COUNT = 0
ld a,0
ld (COUNT),a
endif
call checked_step
call emit_state
if BROKEN
; COUNT = 0
ld a,0
ld (COUNT),a
endif
call checked_step
call emit_state
endif
retCustomise 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
org $8000
CUSTOM equ 1
BROKEN equ 0
COYOTE_STEPS equ 2
BUFFER_STEPS equ 2
TEXT_CAP equ 4
PROPERTY_COUNT equ 16
STREAM_BYTES equ 13
CRC_POLY equ 49
include "caller.inc"
include "exercise.inc"
component_start:
include "component.inc"
include "data.inc"
include "cases.inc"
include "../../physics/rectangle-overlap/display.inc"
code_end:
STATE:
RESULT: defb 0
VALID: defb 0
COUNT: defb 0
MODE: defb 0
CELL: defb 0
VALUE: defb 0
C0: defb 0
C1: defb 0
C2: defb 0
C3: defb 0
C4: defb 0
C5: defb 0
C6: defb 0
C7: defb 0
C8: defb 0
C9: defb 0
C10: defb 0
C11: defb 0
C12: defb 0
C13: defb 0
C14: defb 0
C15: defb 0
J0: defb 0
J1: defb 0
J2: defb 0
J3: defb 0
K0: defb 0
K1: defb 0
K2: defb 0
K3: defb 0
T0: defb 0
T1: defb 0
T2: defb 0
EXPR_SAVE: defb 0
input_ptr: defw 0
output_ptr: defw 0
cases_left: defb 0
ready: defb 0
preview: defs 8,0
OUTPUT: defs 1300,0
end startThe 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
org $8000
CUSTOM equ 0
BROKEN equ 1
COYOTE_STEPS equ 4
BUFFER_STEPS equ 4
TEXT_CAP equ 8
PROPERTY_COUNT equ 16
STREAM_BYTES equ 13
CRC_POLY equ 7
include "caller.inc"
include "exercise.inc"
component_start:
include "component.inc"
include "data.inc"
include "cases.inc"
include "../../physics/rectangle-overlap/display.inc"
code_end:
STATE:
RESULT: defb 0
VALID: defb 0
COUNT: defb 0
MODE: defb 0
CELL: defb 0
VALUE: defb 0
C0: defb 0
C1: defb 0
C2: defb 0
C3: defb 0
C4: defb 0
C5: defb 0
C6: defb 0
C7: defb 0
C8: defb 0
C9: defb 0
C10: defb 0
C11: defb 0
C12: defb 0
C13: defb 0
C14: defb 0
C15: defb 0
J0: defb 0
J1: defb 0
J2: defb 0
J3: defb 0
K0: defb 0
K1: defb 0
K2: defb 0
K3: defb 0
T0: defb 0
T1: defb 0
T2: defb 0
EXPR_SAVE: defb 0
input_ptr: defw 0
output_ptr: defw 0
cases_left: defb 0
ready: defb 0
preview: defs 8,0
OUTPUT: defs 1300,0
end start
Cost and alternatives
The measured helper extent is 403 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 48K PAL ROM tape load and USR, using Asm198x 0.0.72, Emu198x 0.44.2. 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.