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:
ld a,0
ld (MASK_LO),a
ld (MASK_HI),a
ld (CELL),a
ld (MODE),a
ld (FOUND),a
ld (VALID),a
ld (T0),a
ld (T1),a
ld (T2),a
ld (T3),a
ret
dirty_cells_step:
; FOUND = 0
ld a,0
ld (FOUND),a
; VALID = 0
ld a,0
ld (VALID),a
; if MODE == 2:
ld a,(MODE)
cp 2
jp z,dirty_cells_L1
jp dirty_cells_L2
dirty_cells_L1:
; MASK_LO = 255
ld a,255
ld (MASK_LO),a
; MASK_HI = 255
ld a,255
ld (MASK_HI),a
; VALID = 1
ld a,1
ld (VALID),a
; return
ret
jp dirty_cells_L3
dirty_cells_L2:
dirty_cells_L3:
; if MODE == 0:
ld a,(MODE)
cp 0
jp z,dirty_cells_L4
jp dirty_cells_L5
dirty_cells_L4:
; if CELL >= 16:
ld a,(CELL)
cp 16
jp nc,dirty_cells_L7
jp dirty_cells_L8
dirty_cells_L7:
; return
ret
jp dirty_cells_L9
dirty_cells_L8:
dirty_cells_L9:
; T0 = 1
ld a,1
ld (T0),a
; T1 = CELL & 7
ld a,(CELL)
and 7
ld (T1),a
; while T1 != 0:
dirty_cells_L10:
ld a,(T1)
cp 0
jp nz,dirty_cells_L11
jp dirty_cells_L12
dirty_cells_L11:
; T0 = T0 << 1
ld a,(T0)
sla a
ld (T0),a
; T1 = T1 - 1
ld a,(T1)
sub 1
ld (T1),a
jp dirty_cells_L10
dirty_cells_L12:
; if CELL < 8:
ld a,(CELL)
cp 8
jp c,dirty_cells_L13
jp dirty_cells_L14
dirty_cells_L13:
; MASK_LO = MASK_LO | T0
ld a,(MASK_LO)
ld b,a
ld a,(T0)
ld c,a
ld a,b
or c
ld (MASK_LO),a
jp dirty_cells_L15
dirty_cells_L14:
; MASK_HI = MASK_HI | T0
ld a,(MASK_HI)
ld b,a
ld a,(T0)
ld c,a
ld a,b
or c
ld (MASK_HI),a
dirty_cells_L15:
; VALID = 1
ld a,1
ld (VALID),a
; return
ret
jp dirty_cells_L6
dirty_cells_L5:
dirty_cells_L6:
; if MODE != 1:
ld a,(MODE)
cp 1
jp nz,dirty_cells_L16
jp dirty_cells_L17
dirty_cells_L16:
; return
ret
jp dirty_cells_L18
dirty_cells_L17:
dirty_cells_L18:
; VALID = 1
ld a,1
ld (VALID),a
; CELL = 255
ld a,255
ld (CELL),a
; if MASK_LO == 0:
ld a,(MASK_LO)
cp 0
jp z,dirty_cells_L19
jp dirty_cells_L20
dirty_cells_L19:
; if MASK_HI == 0:
ld a,(MASK_HI)
cp 0
jp z,dirty_cells_L22
jp dirty_cells_L23
dirty_cells_L22:
; return
ret
jp dirty_cells_L24
dirty_cells_L23:
dirty_cells_L24:
jp dirty_cells_L21
dirty_cells_L20:
dirty_cells_L21:
; T0 = MASK_LO
ld a,(MASK_LO)
ld (T0),a
; T1 = 0
ld a,0
ld (T1),a
; if T0 == 0:
ld a,(T0)
cp 0
jp z,dirty_cells_L25
jp dirty_cells_L26
dirty_cells_L25:
; T0 = MASK_HI
ld a,(MASK_HI)
ld (T0),a
; T1 = 8
ld a,8
ld (T1),a
jp dirty_cells_L27
dirty_cells_L26:
dirty_cells_L27:
; T2 = 1
ld a,1
ld (T2),a
; T3 = T0 & T2
ld a,(T0)
ld b,a
ld a,(T2)
ld c,a
ld a,b
and c
ld (T3),a
; while T3 == 0:
dirty_cells_L28:
ld a,(T3)
cp 0
jp z,dirty_cells_L29
jp dirty_cells_L30
dirty_cells_L29:
; T2 = T2 << 1
ld a,(T2)
sla a
ld (T2),a
; T1 = T1 + 1
ld a,(T1)
add a,1
ld (T1),a
; T3 = T0 & T2
ld a,(T0)
ld b,a
ld a,(T2)
ld c,a
ld a,b
and c
ld (T3),a
jp dirty_cells_L28
dirty_cells_L30:
; CELL = T1
ld a,(T1)
ld (CELL),a
; T2 = T2 ^ 255
ld a,(T2)
xor 255
ld (T2),a
; if CELL < 8:
ld a,(CELL)
cp 8
jp c,dirty_cells_L31
jp dirty_cells_L32
dirty_cells_L31:
; MASK_LO = MASK_LO & T2
ld a,(MASK_LO)
ld b,a
ld a,(T2)
ld c,a
ld a,b
and c
ld (MASK_LO),a
jp dirty_cells_L33
dirty_cells_L32:
; MASK_HI = MASK_HI & T2
ld a,(MASK_HI)
ld b,a
ld a,(T2)
ld c,a
ld a,b
and c
ld (MASK_HI),a
dirty_cells_L33:
; FOUND = 1
ld a,1
ld (FOUND),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. Scope or rename routine and field labels and allocate a separate block for each independent instance. 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:
MASK_LO: defb 0
MASK_HI: defb 0
CELL: defb 0
MODE: defb 0
FOUND: defb 0
VALID: defb 0
T0: defb 0
T1: defb 0
T2: defb 0
T3: 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
; MODE = 2
ld a,2
ld (MODE),a
if BROKEN
; MASK_LO = 0
ld a,0
ld (MASK_LO),a
; MASK_HI = 0
ld a,0
ld (MASK_HI),a
endif
call checked_step
call emit_state
; MODE = 1
ld a,1
ld (MODE),a
if BROKEN
; MASK_LO = 0
ld a,0
ld (MASK_LO),a
; MASK_HI = 0
ld a,0
ld (MASK_HI),a
endif
call checked_step
call emit_state
if BROKEN
; MASK_LO = 0
ld a,0
ld (MASK_LO),a
; MASK_HI = 0
ld a,0
ld (MASK_HI),a
endif
call checked_step
call emit_state
if BROKEN
; MASK_LO = 0
ld a,0
ld (MASK_LO),a
; MASK_HI = 0
ld a,0
ld (MASK_HI),a
endif
call checked_step
call emit_state
ret
else
; CELL = 3
ld a,3
ld (CELL),a
if BROKEN
; MASK_LO = 0
ld a,0
ld (MASK_LO),a
; MASK_HI = 0
ld a,0
ld (MASK_HI),a
endif
call checked_step
call emit_state
; CELL = 12
ld a,12
ld (CELL),a
if BROKEN
; MASK_LO = 0
ld a,0
ld (MASK_LO),a
; MASK_HI = 0
ld a,0
ld (MASK_HI),a
endif
call checked_step
call emit_state
; CELL = 3
ld a,3
ld (CELL),a
if BROKEN
; MASK_LO = 0
ld a,0
ld (MASK_LO),a
; MASK_HI = 0
ld a,0
ld (MASK_HI),a
endif
call checked_step
call emit_state
; MODE = 1
ld a,1
ld (MODE),a
if BROKEN
; MASK_LO = 0
ld a,0
ld (MASK_LO),a
; MASK_HI = 0
ld a,0
ld (MASK_HI),a
endif
call checked_step
call emit_state
endif
retCustomise 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
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:
MASK_LO: defb 0
MASK_HI: defb 0
CELL: defb 0
MODE: defb 0
FOUND: defb 0
VALID: defb 0
T0: defb 0
T1: defb 0
T2: defb 0
T3: 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 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
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:
MASK_LO: defb 0
MASK_HI: defb 0
CELL: defb 0
MODE: defb 0
FOUND: defb 0
VALID: defb 0
T0: defb 0
T1: defb 0
T2: defb 0
T3: 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 402 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 48K PAL ROM tape load and USR, 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.