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:
lda #0
sta MASK_LO
sta MASK_HI
sta CELL
sta MODE
sta FOUND
sta VALID
sta T0
sta T1
sta T2
sta T3
rts
dirty_cells_step:
; FOUND = 0
lda #0
sta FOUND
; VALID = 0
lda #0
sta VALID
; if MODE == 2:
lda MODE
cmp #2
beq dirty_cells_L4
dirty_cells_L5:
jmp dirty_cells_L2
dirty_cells_L4:
jmp dirty_cells_L1
dirty_cells_L1:
; MASK_LO = 255
lda #255
sta MASK_LO
; MASK_HI = 255
lda #255
sta MASK_HI
; VALID = 1
lda #1
sta VALID
; return
rts
jmp dirty_cells_L3
dirty_cells_L2:
dirty_cells_L3:
; if MODE == 0:
lda MODE
cmp #0
beq dirty_cells_L9
dirty_cells_L10:
jmp dirty_cells_L7
dirty_cells_L9:
jmp dirty_cells_L6
dirty_cells_L6:
; if CELL >= 16:
lda CELL
cmp #16
bcs dirty_cells_L14
dirty_cells_L15:
jmp dirty_cells_L12
dirty_cells_L14:
jmp dirty_cells_L11
dirty_cells_L11:
; return
rts
jmp dirty_cells_L13
dirty_cells_L12:
dirty_cells_L13:
; T0 = 1
lda #1
sta T0
; T1 = CELL & 7
lda CELL
and #7
sta T1
; while T1 != 0:
dirty_cells_L16:
lda T1
cmp #0
bne dirty_cells_L19
dirty_cells_L20:
jmp dirty_cells_L18
dirty_cells_L19:
jmp dirty_cells_L17
dirty_cells_L17:
; T0 = T0 << 1
lda T0
asl
sta T0
; T1 = T1 - 1
lda T1
sec
sbc #1
sta T1
jmp dirty_cells_L16
dirty_cells_L18:
; if CELL < 8:
lda CELL
cmp #8
bcc dirty_cells_L24
dirty_cells_L25:
jmp dirty_cells_L22
dirty_cells_L24:
jmp dirty_cells_L21
dirty_cells_L21:
; MASK_LO = MASK_LO | T0
lda MASK_LO
ora T0
sta MASK_LO
jmp dirty_cells_L23
dirty_cells_L22:
; MASK_HI = MASK_HI | T0
lda MASK_HI
ora T0
sta MASK_HI
dirty_cells_L23:
; VALID = 1
lda #1
sta VALID
; return
rts
jmp dirty_cells_L8
dirty_cells_L7:
dirty_cells_L8:
; if MODE != 1:
lda MODE
cmp #1
bne dirty_cells_L29
dirty_cells_L30:
jmp dirty_cells_L27
dirty_cells_L29:
jmp dirty_cells_L26
dirty_cells_L26:
; return
rts
jmp dirty_cells_L28
dirty_cells_L27:
dirty_cells_L28:
; VALID = 1
lda #1
sta VALID
; CELL = 255
lda #255
sta CELL
; if MASK_LO == 0:
lda MASK_LO
cmp #0
beq dirty_cells_L34
dirty_cells_L35:
jmp dirty_cells_L32
dirty_cells_L34:
jmp dirty_cells_L31
dirty_cells_L31:
; if MASK_HI == 0:
lda MASK_HI
cmp #0
beq dirty_cells_L39
dirty_cells_L40:
jmp dirty_cells_L37
dirty_cells_L39:
jmp dirty_cells_L36
dirty_cells_L36:
; return
rts
jmp dirty_cells_L38
dirty_cells_L37:
dirty_cells_L38:
jmp dirty_cells_L33
dirty_cells_L32:
dirty_cells_L33:
; T0 = MASK_LO
lda MASK_LO
sta T0
; T1 = 0
lda #0
sta T1
; if T0 == 0:
lda T0
cmp #0
beq dirty_cells_L44
dirty_cells_L45:
jmp dirty_cells_L42
dirty_cells_L44:
jmp dirty_cells_L41
dirty_cells_L41:
; T0 = MASK_HI
lda MASK_HI
sta T0
; T1 = 8
lda #8
sta T1
jmp dirty_cells_L43
dirty_cells_L42:
dirty_cells_L43:
; T2 = 1
lda #1
sta T2
; T3 = T0 & T2
lda T0
and T2
sta T3
; while T3 == 0:
dirty_cells_L46:
lda T3
cmp #0
beq dirty_cells_L49
dirty_cells_L50:
jmp dirty_cells_L48
dirty_cells_L49:
jmp dirty_cells_L47
dirty_cells_L47:
; T2 = T2 << 1
lda T2
asl
sta T2
; T1 = T1 + 1
lda T1
clc
adc #1
sta T1
; T3 = T0 & T2
lda T0
and T2
sta T3
jmp dirty_cells_L46
dirty_cells_L48:
; CELL = T1
lda T1
sta CELL
; T2 = T2 ^ 255
lda T2
eor #255
sta T2
; if CELL < 8:
lda CELL
cmp #8
bcc dirty_cells_L54
dirty_cells_L55:
jmp dirty_cells_L52
dirty_cells_L54:
jmp dirty_cells_L51
dirty_cells_L51:
; MASK_LO = MASK_LO & T2
lda MASK_LO
and T2
sta MASK_LO
jmp dirty_cells_L53
dirty_cells_L52:
; MASK_HI = MASK_HI & T2
lda MASK_HI
and T2
sta MASK_HI
dirty_cells_L53:
; FOUND = 1
lda #1
sta FOUND
rts
component_end:Machine contract
A/X/status may change; Y is preserved. The 2A03 has no decimal arithmetic mode. A direct JSR 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. Scope or rename routine and field labels and allocate a separate block for each independent instance. Give each block one mutation owner.
The NROM caller owns PPU setup and background display, with NMI and DMC disabled. Table and packet sizes are bounded to fit the cartridge caller and internal RAM; the helper owns no PPU registers.
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
.segment "HEADER"
.byte "NES",$1a,2,1,0,0,0,0,0,0,0,0,0,0
MASK_LO=$80
MASK_HI=$81
CELL=$82
MODE=$83
FOUND=$84
VALID=$85
T0=$86
T1=$87
T2=$88
T3=$89
STATE=$80
input_ptr=$70
cases_left=$72
ptr=$74
saved_y=$76
OUTPUT=$0200
preview=OUTPUT+1400
ready=OUTPUT+1408
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"
.include "data.inc"
.include "cases.inc"
code_end:
.segment "VECTORS"
.word nmi,reset,irq
.segment "CHARS"
.include "../../input/controller-reading/edges-font.inc"
.res 8192-272,0A client retaining state between calls
preview_rules:
.if CUSTOM
; MODE = 2
lda #2
sta MODE
.if BROKEN
; MASK_LO = 0
lda #0
sta MASK_LO
; MASK_HI = 0
lda #0
sta MASK_HI
.endif
jsr checked_step
jsr emit_state
; MODE = 1
lda #1
sta MODE
.if BROKEN
; MASK_LO = 0
lda #0
sta MASK_LO
; MASK_HI = 0
lda #0
sta MASK_HI
.endif
jsr checked_step
jsr emit_state
.if BROKEN
; MASK_LO = 0
lda #0
sta MASK_LO
; MASK_HI = 0
lda #0
sta MASK_HI
.endif
jsr checked_step
jsr emit_state
.if BROKEN
; MASK_LO = 0
lda #0
sta MASK_LO
; MASK_HI = 0
lda #0
sta MASK_HI
.endif
jsr checked_step
jsr emit_state
rts
.else
; CELL = 3
lda #3
sta CELL
.if BROKEN
; MASK_LO = 0
lda #0
sta MASK_LO
; MASK_HI = 0
lda #0
sta MASK_HI
.endif
jsr checked_step
jsr emit_state
; CELL = 12
lda #12
sta CELL
.if BROKEN
; MASK_LO = 0
lda #0
sta MASK_LO
; MASK_HI = 0
lda #0
sta MASK_HI
.endif
jsr checked_step
jsr emit_state
; CELL = 3
lda #3
sta CELL
.if BROKEN
; MASK_LO = 0
lda #0
sta MASK_LO
; MASK_HI = 0
lda #0
sta MASK_HI
.endif
jsr checked_step
jsr emit_state
; MODE = 1
lda #1
sta MODE
.if BROKEN
; MASK_LO = 0
lda #0
sta MASK_LO
; MASK_HI = 0
lda #0
sta MASK_HI
.endif
jsr checked_step
jsr 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
.segment "HEADER"
.byte "NES",$1a,2,1,0,0,0,0,0,0,0,0,0,0
MASK_LO=$80
MASK_HI=$81
CELL=$82
MODE=$83
FOUND=$84
VALID=$85
T0=$86
T1=$87
T2=$88
T3=$89
STATE=$80
input_ptr=$70
cases_left=$72
ptr=$74
saved_y=$76
OUTPUT=$0200
preview=OUTPUT+1400
ready=OUTPUT+1408
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"
.include "data.inc"
.include "cases.inc"
code_end:
.segment "VECTORS"
.word nmi,reset,irq
.segment "CHARS"
.include "../../input/controller-reading/edges-font.inc"
.res 8192-272,0The 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
.segment "HEADER"
.byte "NES",$1a,2,1,0,0,0,0,0,0,0,0,0,0
MASK_LO=$80
MASK_HI=$81
CELL=$82
MODE=$83
FOUND=$84
VALID=$85
T0=$86
T1=$87
T2=$88
T3=$89
STATE=$80
input_ptr=$70
cases_left=$72
ptr=$74
saved_y=$76
OUTPUT=$0200
preview=OUTPUT+1400
ready=OUTPUT+1408
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"
.include "data.inc"
.include "cases.inc"
code_end:
.segment "VECTORS"
.word nmi,reset,irq
.segment "CHARS"
.include "../../input/controller-reading/edges-font.inc"
.res 8192-272,0
Cost and alternatives
The measured helper extent is 330 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 NTSC NROM, background only, DMC/NMI disabled, using Asm198x 0.0.72, Emu198x 0.44.2. 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.