Changing one cell should not force a whole board to be redrawn.
Use it in a project
A 16-cell board uses two dirty-mask bytes. MODE=0 marks CELL 0–15; repeated marks coalesce. MODE=1 takes and clears the lowest pending cell, returning FOUND=1 and its CELL; empty returns FOUND=0 and CELL=255. MODE=2 marks all cells. Other modes or an invalid mark index reject without changing masks.
Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: MASK_LO, MASK_HI, CELL, MODE, FOUND, VALID. They occupy 6 contiguous bytes. Also reserve the private workspace: T0, T1, T2, T3 (4 bytes). Array names in the explanation refer to the numbered consecutive bindings, such as C0, C1 and so on.
Call dirty_cells_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 dirty_cells_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 | MASK_LO and MASK_HI |
| Supply before service | MODE and CELL when marking |
| Read after service | CELL when taking, FOUND and VALID |
A bit represents pending work rather than how many times the cell changed. Marking it repeatedly is cheap and idempotent. Taking the lowest bit provides a predictable order for a renderer with a small per-frame budget.
Reusable initialisation and service
; SPDX-License-Identifier: MIT
; Remember Which Cells Need Redrawing. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
dirty_cells_init:
move.b #0,d0
move.b d0,MASK_LO
move.b d0,MASK_HI
move.b d0,CELL
move.b d0,MODE
move.b d0,FOUND
move.b d0,VALID
move.b d0,T0
move.b d0,T1
move.b d0,T2
move.b d0,T3
rts
dirty_cells_step:
; FOUND = 0
move.b #0,d0
move.b d0,FOUND
; VALID = 0
move.b #0,d0
move.b d0,VALID
; if MODE == 2:
move.b MODE,d0
cmp.b #2,d0
beq dirty_cells_L1
jmp dirty_cells_L2
dirty_cells_L1:
; MASK_LO = 255
move.b #255,d0
move.b d0,MASK_LO
; MASK_HI = 255
move.b #255,d0
move.b d0,MASK_HI
; VALID = 1
move.b #1,d0
move.b d0,VALID
; return
rts
jmp dirty_cells_L3
dirty_cells_L2:
dirty_cells_L3:
; if MODE == 0:
move.b MODE,d0
cmp.b #0,d0
beq dirty_cells_L4
jmp dirty_cells_L5
dirty_cells_L4:
; if CELL >= 16:
move.b CELL,d0
cmp.b #16,d0
bcc dirty_cells_L7
jmp dirty_cells_L8
dirty_cells_L7:
; return
rts
jmp dirty_cells_L9
dirty_cells_L8:
dirty_cells_L9:
; T0 = 1
move.b #1,d0
move.b d0,T0
; T1 = CELL & 7
move.b CELL,d0
and.b #7,d0
move.b d0,T1
; while T1 != 0:
dirty_cells_L10:
move.b T1,d0
cmp.b #0,d0
bne dirty_cells_L11
jmp dirty_cells_L12
dirty_cells_L11:
; T0 = T0 << 1
move.b T0,d0
lsl.b #1,d0
move.b d0,T0
; T1 = T1 - 1
move.b T1,d0
sub.b #1,d0
move.b d0,T1
jmp dirty_cells_L10
dirty_cells_L12:
; if CELL < 8:
move.b CELL,d0
cmp.b #8,d0
bcs dirty_cells_L13
jmp dirty_cells_L14
dirty_cells_L13:
; MASK_LO = MASK_LO | T0
move.b MASK_LO,d0
move.b T0,d1
or.b d1,d0
move.b d0,MASK_LO
jmp dirty_cells_L15
dirty_cells_L14:
; MASK_HI = MASK_HI | T0
move.b MASK_HI,d0
move.b T0,d1
or.b d1,d0
move.b d0,MASK_HI
dirty_cells_L15:
; VALID = 1
move.b #1,d0
move.b d0,VALID
; return
rts
jmp dirty_cells_L6
dirty_cells_L5:
dirty_cells_L6:
; if MODE != 1:
move.b MODE,d0
cmp.b #1,d0
bne dirty_cells_L16
jmp dirty_cells_L17
dirty_cells_L16:
; return
rts
jmp dirty_cells_L18
dirty_cells_L17:
dirty_cells_L18:
; VALID = 1
move.b #1,d0
move.b d0,VALID
; CELL = 255
move.b #255,d0
move.b d0,CELL
; if MASK_LO == 0:
move.b MASK_LO,d0
cmp.b #0,d0
beq dirty_cells_L19
jmp dirty_cells_L20
dirty_cells_L19:
; if MASK_HI == 0:
move.b MASK_HI,d0
cmp.b #0,d0
beq dirty_cells_L22
jmp dirty_cells_L23
dirty_cells_L22:
; return
rts
jmp dirty_cells_L24
dirty_cells_L23:
dirty_cells_L24:
jmp dirty_cells_L21
dirty_cells_L20:
dirty_cells_L21:
; T0 = MASK_LO
move.b MASK_LO,d0
move.b d0,T0
; T1 = 0
move.b #0,d0
move.b d0,T1
; if T0 == 0:
move.b T0,d0
cmp.b #0,d0
beq dirty_cells_L25
jmp dirty_cells_L26
dirty_cells_L25:
; T0 = MASK_HI
move.b MASK_HI,d0
move.b d0,T0
; T1 = 8
move.b #8,d0
move.b d0,T1
jmp dirty_cells_L27
dirty_cells_L26:
dirty_cells_L27:
; T2 = 1
move.b #1,d0
move.b d0,T2
; T3 = T0 & T2
move.b T0,d0
move.b T2,d1
and.b d1,d0
move.b d0,T3
; while T3 == 0:
dirty_cells_L28:
move.b T3,d0
cmp.b #0,d0
beq dirty_cells_L29
jmp dirty_cells_L30
dirty_cells_L29:
; T2 = T2 << 1
move.b T2,d0
lsl.b #1,d0
move.b d0,T2
; T1 = T1 + 1
move.b T1,d0
add.b #1,d0
move.b d0,T1
; T3 = T0 & T2
move.b T0,d0
move.b T2,d1
and.b d1,d0
move.b d0,T3
jmp dirty_cells_L28
dirty_cells_L30:
; CELL = T1
move.b T1,d0
move.b d0,CELL
; T2 = T2 ^ 255
move.b T2,d0
eor.b #255,d0
move.b d0,T2
; if CELL < 8:
move.b CELL,d0
cmp.b #8,d0
bcs dirty_cells_L31
jmp dirty_cells_L32
dirty_cells_L31:
; MASK_LO = MASK_LO & T2
move.b MASK_LO,d0
move.b T2,d1
and.b d1,d0
move.b d0,MASK_LO
jmp dirty_cells_L33
dirty_cells_L32:
; MASK_HI = MASK_HI & T2
move.b MASK_HI,d0
move.b T2,d1
and.b d1,d0
move.b d0,MASK_HI
dirty_cells_L33:
; FOUND = 1
move.b #1,d0
move.b d0,FOUND
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. Scope or rename routine and field labels and allocate a separate block for each independent instance. 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
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:
MASK_LO: dc.b 0
MASK_HI: dc.b 0
CELL: dc.b 0
MODE: dc.b 0
FOUND: dc.b 0
VALID: dc.b 0
T0: dc.b 0
T1: dc.b 0
T2: dc.b 0
T3: dc.b 0
even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
even
signature: dc.b "198XGAMEKIT0000",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 10240,0A client retaining state between calls
preview_rules:
ifne CUSTOM
; MODE = 2
move.b #2,d0
move.b d0,MODE
ifne BROKEN
; MASK_LO = 0
move.b #0,d0
move.b d0,MASK_LO
; MASK_HI = 0
move.b #0,d0
move.b d0,MASK_HI
endif
bsr checked_step
bsr emit_state
; MODE = 1
move.b #1,d0
move.b d0,MODE
ifne BROKEN
; MASK_LO = 0
move.b #0,d0
move.b d0,MASK_LO
; MASK_HI = 0
move.b #0,d0
move.b d0,MASK_HI
endif
bsr checked_step
bsr emit_state
ifne BROKEN
; MASK_LO = 0
move.b #0,d0
move.b d0,MASK_LO
; MASK_HI = 0
move.b #0,d0
move.b d0,MASK_HI
endif
bsr checked_step
bsr emit_state
ifne BROKEN
; MASK_LO = 0
move.b #0,d0
move.b d0,MASK_LO
; MASK_HI = 0
move.b #0,d0
move.b d0,MASK_HI
endif
bsr checked_step
bsr emit_state
rts
else
; CELL = 3
move.b #3,d0
move.b d0,CELL
ifne BROKEN
; MASK_LO = 0
move.b #0,d0
move.b d0,MASK_LO
; MASK_HI = 0
move.b #0,d0
move.b d0,MASK_HI
endif
bsr checked_step
bsr emit_state
; CELL = 12
move.b #12,d0
move.b d0,CELL
ifne BROKEN
; MASK_LO = 0
move.b #0,d0
move.b d0,MASK_LO
; MASK_HI = 0
move.b #0,d0
move.b d0,MASK_HI
endif
bsr checked_step
bsr emit_state
; CELL = 3
move.b #3,d0
move.b d0,CELL
ifne BROKEN
; MASK_LO = 0
move.b #0,d0
move.b d0,MASK_LO
; MASK_HI = 0
move.b #0,d0
move.b d0,MASK_HI
endif
bsr checked_step
bsr emit_state
; MODE = 1
move.b #1,d0
move.b d0,MODE
ifne BROKEN
; MASK_LO = 0
move.b #0,d0
move.b d0,MASK_LO
; MASK_HI = 0
move.b #0,d0
move.b d0,MASK_HI
endif
bsr checked_step
bsr emit_state
endif
rtsCustomise it
Start with all sixteen cells dirty, then drain three. 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
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:
MASK_LO: dc.b 0
MASK_HI: dc.b 0
CELL: dc.b 0
MODE: dc.b 0
FOUND: dc.b 0
VALID: dc.b 0
T0: dc.b 0
T1: dc.b 0
T2: dc.b 0
T3: dc.b 0
even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
even
signature: dc.b "198XGAMEKIT0000",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 10240,0The custom diagnostic row is F8 FF 02 01 01 01 00 00. The full client trace in the source explains how it gets there.
Mistake and repair
The wrong caller clears both masks before each operation. Pending redraw work disappears before it can be drained. Retain marks across updates and re-mark a taken cell if its redraw cannot be completed.
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
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:
MASK_LO: dc.b 0
MASK_HI: dc.b 0
CELL: dc.b 0
MODE: dc.b 0
FOUND: dc.b 0
VALID: dc.b 0
T0: dc.b 0
T1: dc.b 0
T2: dc.b 0
T3: dc.b 0
even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
even
signature: dc.b "198XGAMEKIT0000",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
Cost and alternatives
The measured helper extent is 596 code bytes; public state is 6 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.
Marking or finding a bit takes at most seven shifts. Sixteen cells need two persistent bytes; a byte-per-cell list is easier to index but takes eight times the flag storage.
Dirty means work pending, not work completed. This take operation clears before drawing, so a failed or deferred draw must re-mark the cell. Interrupt writers need a policy protecting read-modify-write. Hardware tile maps, character grids and bitmap cells use different redraw backends.
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 64 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
Tile-map queries: A changed map cell can mark display work pending without making the map-storage routine own the renderer.