Defer Object Changes Until After Traversal on C64 | Pattern Library

Spawning or deleting objects during a traversal can accidentally update a new object twice or skip a neighbour.

C64 shows the working diagnostic row 00 01 00 01 03 01 00 02. — A completed native caller displays 00 01 00 01 03 01 00 02. 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

Four object slots use ACTIVE bits0..3 and generations G0..G3. MODE=1 queues a spawn in SLOT; MODE=2 queues a removal of SLOT with GENERATION; MODE=3 flushes. MODE=0 clears pending commands without changing objects. COUNT is0..4. Queueing never changes the object table. A full queue returns FULL=1 and preserves its commands. Flush handles commands in insertion order: spawn only a free slot and increment its generation modulo256; remove only a live matching-generation slot. APPLIED/REJECTED count results and COUNT becomes zero. Invalid global state or queued command structure sets VALID=0 and leaves objects and queue intact. Slot payload initialisation is caller-owned.

Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: APPLIED, REJECTED, FULL, VALID, MODE, SLOT, GENERATION, ACTIVE, COUNT, G0, G1, G2, G3, O0, O1, O2, O3, S0, S1, S2, S3, H0, H1, H2, H3. They occupy 25 contiguous bytes. Also reserve the private workspace: T0, T1, T2, T3, T4, T5, T6, EXPR_SAVE (8 bytes). Array names in the explanation refer to the numbered consecutive bindings, such as C0, C1 and so on.

Call object_command_buffer_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 object_command_buffer_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	ACTIVE, G0..G3, COUNT and queued O/S/H arrays
Supply before service	MODE, SLOT and GENERATION
Read after service	APPLIED, REJECTED, FULL, VALID and updated table/queue

A validation pass checks every queued operation before mutation. The application pass then honours insertion order and current generations, so a stale removal cannot delete a newly reused slot. Queueing and flushing are distinct mutation boundaries.

Reusable initialisation and service

6502.inc

; SPDX-License-Identifier: MIT
; Defer Object Changes Until After Traversal. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
object_command_buffer_init:
    lda #0
    sta APPLIED
    sta REJECTED
    sta FULL
    sta VALID
    sta MODE
    sta SLOT
    sta GENERATION
    sta ACTIVE
    sta COUNT
    sta G0
    sta G1
    sta G2
    sta G3
    sta O0
    sta O1
    sta O2
    sta O3
    sta S0
    sta S1
    sta S2
    sta S3
    sta H0
    sta H1
    sta H2
    sta H3
    sta T0
    sta T1
    sta T2
    sta T3
    sta T4
    sta T5
    sta T6
    sta EXPR_SAVE
    rts
object_command_buffer_step:
; APPLIED = 0
    lda #0
    sta APPLIED
; REJECTED = 0
    lda #0
    sta REJECTED
; FULL = 0
    lda #0
    sta FULL
; VALID = 0
    lda #0
    sta VALID
; if ACTIVE >= 16:
    lda ACTIVE
    cmp #16
    bcs object_command_buffer_L4
object_command_buffer_L5:
    jmp object_command_buffer_L2
object_command_buffer_L4:
    jmp object_command_buffer_L1
object_command_buffer_L1:
; return
    rts
    jmp object_command_buffer_L3
object_command_buffer_L2:
object_command_buffer_L3:
; if COUNT > 4:
    lda COUNT
    cmp #4
    beq object_command_buffer_L10
    bcs object_command_buffer_L9
object_command_buffer_L10:
    jmp object_command_buffer_L7
object_command_buffer_L9:
    jmp object_command_buffer_L6
object_command_buffer_L6:
; return
    rts
    jmp object_command_buffer_L8
object_command_buffer_L7:
object_command_buffer_L8:
; if MODE > 3:
    lda MODE
    cmp #3
    beq object_command_buffer_L15
    bcs object_command_buffer_L14
object_command_buffer_L15:
    jmp object_command_buffer_L12
object_command_buffer_L14:
    jmp object_command_buffer_L11
object_command_buffer_L11:
; return
    rts
    jmp object_command_buffer_L13
object_command_buffer_L12:
object_command_buffer_L13:
; if MODE == 0:
    lda MODE
    cmp #0
    beq object_command_buffer_L19
object_command_buffer_L20:
    jmp object_command_buffer_L17
object_command_buffer_L19:
    jmp object_command_buffer_L16
object_command_buffer_L16:
; COUNT = 0
    lda #0
    sta COUNT
; VALID = 1
    lda #1
    sta VALID
; return
    rts
    jmp object_command_buffer_L18
object_command_buffer_L17:
object_command_buffer_L18:
; if MODE < 3:
    lda MODE
    cmp #3
    bcc object_command_buffer_L24
object_command_buffer_L25:
    jmp object_command_buffer_L22
object_command_buffer_L24:
    jmp object_command_buffer_L21
object_command_buffer_L21:
; if SLOT >= 4:
    lda SLOT
    cmp #4
    bcs object_command_buffer_L29
object_command_buffer_L30:
    jmp object_command_buffer_L27
object_command_buffer_L29:
    jmp object_command_buffer_L26
object_command_buffer_L26:
; return
    rts
    jmp object_command_buffer_L28
object_command_buffer_L27:
object_command_buffer_L28:
; VALID = 1
    lda #1
    sta VALID
; if COUNT == 4:
    lda COUNT
    cmp #4
    beq object_command_buffer_L34
object_command_buffer_L35:
    jmp object_command_buffer_L32
object_command_buffer_L34:
    jmp object_command_buffer_L31
object_command_buffer_L31:
; FULL = 1
    lda #1
    sta FULL
; return
    rts
    jmp object_command_buffer_L33
object_command_buffer_L32:
object_command_buffer_L33:
; O[COUNT] = MODE
    lda MODE
    sta EXPR_SAVE
    ldx COUNT
    lda EXPR_SAVE
    sta O0,x
; S[COUNT] = SLOT
    lda SLOT
    sta EXPR_SAVE
    ldx COUNT
    lda EXPR_SAVE
    sta S0,x
; H[COUNT] = GENERATION
    lda GENERATION
    sta EXPR_SAVE
    ldx COUNT
    lda EXPR_SAVE
    sta H0,x
; COUNT = COUNT + 1
    lda COUNT
    clc
    adc #1
    sta COUNT
; return
    rts
    jmp object_command_buffer_L23
object_command_buffer_L22:
object_command_buffer_L23:
; T0 = 0
    lda #0
    sta T0
; while T0 < COUNT:
object_command_buffer_L36:
    lda T0
    cmp COUNT
    bcc object_command_buffer_L39
object_command_buffer_L40:
    jmp object_command_buffer_L38
object_command_buffer_L39:
    jmp object_command_buffer_L37
object_command_buffer_L37:
; T1 = O[T0]
    ldx T0
    lda O0,x
    sta T1
; if T1 == 0:
    lda T1
    cmp #0
    beq object_command_buffer_L44
object_command_buffer_L45:
    jmp object_command_buffer_L42
object_command_buffer_L44:
    jmp object_command_buffer_L41
object_command_buffer_L41:
; return
    rts
    jmp object_command_buffer_L43
object_command_buffer_L42:
object_command_buffer_L43:
; if T1 > 2:
    lda T1
    cmp #2
    beq object_command_buffer_L50
    bcs object_command_buffer_L49
object_command_buffer_L50:
    jmp object_command_buffer_L47
object_command_buffer_L49:
    jmp object_command_buffer_L46
object_command_buffer_L46:
; return
    rts
    jmp object_command_buffer_L48
object_command_buffer_L47:
object_command_buffer_L48:
; T1 = S[T0]
    ldx T0
    lda S0,x
    sta T1
; if T1 >= 4:
    lda T1
    cmp #4
    bcs object_command_buffer_L54
object_command_buffer_L55:
    jmp object_command_buffer_L52
object_command_buffer_L54:
    jmp object_command_buffer_L51
object_command_buffer_L51:
; return
    rts
    jmp object_command_buffer_L53
object_command_buffer_L52:
object_command_buffer_L53:
; T0 = T0 + 1
    lda T0
    clc
    adc #1
    sta T0
    jmp object_command_buffer_L36
object_command_buffer_L38:
; VALID = 1
    lda #1
    sta VALID
; T0 = 0
    lda #0
    sta T0
; while T0 < COUNT:
object_command_buffer_L56:
    lda T0
    cmp COUNT
    bcc object_command_buffer_L59
object_command_buffer_L60:
    jmp object_command_buffer_L58
object_command_buffer_L59:
    jmp object_command_buffer_L57
object_command_buffer_L57:
; T1 = S[T0]
    ldx T0
    lda S0,x
    sta T1
; T2 = 1
    lda #1
    sta T2
; T3 = T1
    lda T1
    sta T3
; while T3 != 0:
object_command_buffer_L61:
    lda T3
    cmp #0
    bne object_command_buffer_L64
object_command_buffer_L65:
    jmp object_command_buffer_L63
object_command_buffer_L64:
    jmp object_command_buffer_L62
object_command_buffer_L62:
; T2 = T2 << 1
    lda T2
    asl
    sta T2
; T3 = T3 - 1
    lda T3
    sec
    sbc #1
    sta T3
    jmp object_command_buffer_L61
object_command_buffer_L63:
; T3 = ACTIVE & T2
    lda ACTIVE
    and T2
    sta T3
; T4 = O[T0]
    ldx T0
    lda O0,x
    sta T4
; T5 = 0
    lda #0
    sta T5
; if T4 == 1:
    lda T4
    cmp #1
    beq object_command_buffer_L69
object_command_buffer_L70:
    jmp object_command_buffer_L67
object_command_buffer_L69:
    jmp object_command_buffer_L66
object_command_buffer_L66:
; if T3 == 0:
    lda T3
    cmp #0
    beq object_command_buffer_L74
object_command_buffer_L75:
    jmp object_command_buffer_L72
object_command_buffer_L74:
    jmp object_command_buffer_L71
object_command_buffer_L71:
; ACTIVE = ACTIVE | T2
    lda ACTIVE
    ora T2
    sta ACTIVE
; T4 = G[T1]
    ldx T1
    lda G0,x
    sta T4
; T4 = T4 + 1
    lda T4
    clc
    adc #1
    sta T4
; G[T1] = T4
    lda T4
    sta EXPR_SAVE
    ldx T1
    lda EXPR_SAVE
    sta G0,x
; T5 = 1
    lda #1
    sta T5
    jmp object_command_buffer_L73
object_command_buffer_L72:
object_command_buffer_L73:
    jmp object_command_buffer_L68
object_command_buffer_L67:
; if T3 != 0:
    lda T3
    cmp #0
    bne object_command_buffer_L79
object_command_buffer_L80:
    jmp object_command_buffer_L77
object_command_buffer_L79:
    jmp object_command_buffer_L76
object_command_buffer_L76:
; T4 = G[T1]
    ldx T1
    lda G0,x
    sta T4
; T6 = H[T0]
    ldx T0
    lda H0,x
    sta T6
; if T4 == T6:
    lda T4
    cmp T6
    beq object_command_buffer_L84
object_command_buffer_L85:
    jmp object_command_buffer_L82
object_command_buffer_L84:
    jmp object_command_buffer_L81
object_command_buffer_L81:
; ACTIVE = ACTIVE ^ T2
    lda ACTIVE
    eor T2
    sta ACTIVE
; T5 = 1
    lda #1
    sta T5
    jmp object_command_buffer_L83
object_command_buffer_L82:
object_command_buffer_L83:
    jmp object_command_buffer_L78
object_command_buffer_L77:
object_command_buffer_L78:
object_command_buffer_L68:
; if T5 != 0:
    lda T5
    cmp #0
    bne object_command_buffer_L89
object_command_buffer_L90:
    jmp object_command_buffer_L87
object_command_buffer_L89:
    jmp object_command_buffer_L86
object_command_buffer_L86:
; APPLIED = APPLIED + 1
    lda APPLIED
    clc
    adc #1
    sta APPLIED
    jmp object_command_buffer_L88
object_command_buffer_L87:
; REJECTED = REJECTED + 1
    lda REJECTED
    clc
    adc #1
    sta REJECTED
object_command_buffer_L88:
; T0 = T0 + 1
    lda T0
    clc
    adc #1
    sta T0
    jmp object_command_buffer_L56
object_command_buffer_L58:
; COUNT = 0
    lda #0
    sta COUNT
    rts
component_end:

Machine contract

A/X/status may change; Y and decimal mode are preserved. Use CLD for binary arithmetic. 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. 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 diagnostic PRG owns screen, font and colour RAM and disables interrupts; it does not return to BASIC. The reusable helper does none of those things.

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

demo.asm

APPLIED=$80
REJECTED=$81
FULL=$82
VALID=$83
MODE=$84
SLOT=$85
GENERATION=$86
ACTIVE=$87
COUNT=$88
G0=$89
G1=$8a
G2=$8b
G3=$8c
O0=$8d
O1=$8e
O2=$8f
O3=$90
S0=$91
S1=$92
S2=$93
S3=$94
H0=$95
H1=$96
H2=$97
H3=$98
T0=$99
T1=$9a
T2=$9b
T3=$9c
T4=$9d
T5=$9e
T6=$9f
EXPR_SAVE=$a0
STATE=$80
input_ptr=$70
cases_left=$72
ptr=$74
saved_y=$76
OUTPUT=$c000
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
!source "caller.inc"
!source "exercise.inc"
component_start:
!source "component.inc"
!source "data.inc"
!source "cases.inc"
hex_font:
    !byte 0,0,0,0,0,0,0,0,0,56,40,40,40,56,0,0,0,16,48,16,16,56,0,0,0,56,8,56,32,56,0,0,0,56,8,56,8,56,0,0,0,40,40,56,8,8,0,0,0,56,32,56,8,56,0,0,0,56,32,56,40,56,0,0,0,56,8,8,8,8,0,0,0,56,40,56,40,56,0,0,0,56,40,56,8,56,0,0,0,16,40,56,40,40,0,0,0,48,40,48,40,48,0,0,0,56,32,32,32,56,0,0,0,48,40,40,40,48,0,0,0,56,32,48,32,56,0,0,0,56,32,48,32,32,0,0
code_end:

A client retaining state between calls

exercise.inc, excerpt

preview_rules:
!if CUSTOM {
; MODE = 1
    lda #1
    sta MODE
; SLOT = 2
    lda #2
    sta SLOT
!if BROKEN {
; COUNT = 0
    lda #0
    sta COUNT
}
    jsr checked_step
    jsr emit_state
; MODE = 2
    lda #2
    sta MODE
; GENERATION = 1
    lda #1
    sta GENERATION
!if BROKEN {
; COUNT = 0
    lda #0
    sta COUNT
}
    jsr checked_step
    jsr emit_state
; MODE = 3
    lda #3
    sta MODE
!if BROKEN {
; COUNT = 0
    lda #0
    sta COUNT
}
    jsr checked_step
    jsr emit_state
    rts
} else {
; MODE = 1
    lda #1
    sta MODE
; SLOT = 1
    lda #1
    sta SLOT
!if BROKEN {
; COUNT = 0
    lda #0
    sta COUNT
}
    jsr checked_step
    jsr emit_state
; MODE = 3
    lda #3
    sta MODE
!if BROKEN {
; COUNT = 0
    lda #0
    sta COUNT
}
    jsr checked_step
    jsr emit_state
; MODE = 2
    lda #2
    sta MODE
; SLOT = 1
    lda #1
    sta SLOT
; GENERATION = 0
    lda #0
    sta GENERATION
!if BROKEN {
; COUNT = 0
    lda #0
    sta COUNT
}
    jsr checked_step
    jsr emit_state
; MODE = 3
    lda #3
    sta MODE
!if BROKEN {
; COUNT = 0
    lda #0
    sta COUNT
}
    jsr checked_step
    jsr emit_state
}
    rts

Customise it

Queue a spawn then a matching removal of the same slot; insertion order leaves it free after one flush. 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

APPLIED=$80
REJECTED=$81
FULL=$82
VALID=$83
MODE=$84
SLOT=$85
GENERATION=$86
ACTIVE=$87
COUNT=$88
G0=$89
G1=$8a
G2=$8b
G3=$8c
O0=$8d
O1=$8e
O2=$8f
O3=$90
S0=$91
S1=$92
S2=$93
S3=$94
H0=$95
H1=$96
H2=$97
H3=$98
T0=$99
T1=$9a
T2=$9b
T3=$9c
T4=$9d
T5=$9e
T6=$9f
EXPR_SAVE=$a0
STATE=$80
input_ptr=$70
cases_left=$72
ptr=$74
saved_y=$76
OUTPUT=$c000
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
!source "caller.inc"
!source "exercise.inc"
component_start:
!source "component.inc"
!source "data.inc"
!source "cases.inc"
hex_font:
    !byte 0,0,0,0,0,0,0,0,0,56,40,40,40,56,0,0,0,16,48,16,16,56,0,0,0,56,8,56,32,56,0,0,0,56,8,56,8,56,0,0,0,40,40,56,8,8,0,0,0,56,32,56,8,56,0,0,0,56,32,56,40,56,0,0,0,56,8,8,8,8,0,0,0,56,40,56,40,56,0,0,0,56,40,56,8,56,0,0,0,16,40,56,40,40,0,0,0,48,40,48,40,48,0,0,0,56,32,32,32,56,0,0,0,48,40,40,40,48,0,0,0,56,32,48,32,56,0,0,0,56,32,48,32,32,0,0
code_end:

The custom diagnostic row is 02 00 00 01 03 02 01 00. The full client trace in the source explains how it gets there.

Mistake and repair

The wrong client clears the pending count before each service. Flush sees an empty queue and the requested spawn never happens. Retain the command buffer until the traversal ends and a deliberate flush consumes it.

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

APPLIED=$80
REJECTED=$81
FULL=$82
VALID=$83
MODE=$84
SLOT=$85
GENERATION=$86
ACTIVE=$87
COUNT=$88
G0=$89
G1=$8a
G2=$8b
G3=$8c
O0=$8d
O1=$8e
O2=$8f
O3=$90
S0=$91
S1=$92
S2=$93
S3=$94
H0=$95
H1=$96
H2=$97
H3=$98
T0=$99
T1=$9a
T2=$9b
T3=$9c
T4=$9d
T5=$9e
T6=$9f
EXPR_SAVE=$a0
STATE=$80
input_ptr=$70
cases_left=$72
ptr=$74
saved_y=$76
OUTPUT=$c000
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
!source "caller.inc"
!source "exercise.inc"
component_start:
!source "component.inc"
!source "data.inc"
!source "cases.inc"
hex_font:
    !byte 0,0,0,0,0,0,0,0,0,56,40,40,40,56,0,0,0,16,48,16,16,56,0,0,0,56,8,56,32,56,0,0,0,56,8,56,8,56,0,0,0,40,40,56,8,8,0,0,0,56,32,56,8,56,0,0,0,56,32,56,40,56,0,0,0,56,8,8,8,8,0,0,0,56,40,56,40,56,0,0,0,56,40,56,8,56,0,0,0,16,40,56,40,40,0,0,0,48,40,48,40,48,0,0,0,56,32,32,32,56,0,0,0,48,40,40,40,48,0,0,0,56,32,48,32,56,0,0,0,56,32,48,32,32,0,0
code_end:

C64 shows the deliberate error row 00 00 00 01 03 01 00 00. — The wrong client completes with 00 00 00 01 03 01 00 00; its changed inputs and state explain the result.

Cost and alternatives

The measured helper extent is 578 code bytes; public state is 25 bytes and private scratch is 8 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.

At most four validation steps and four applications, each with at most three bit shifts. Compared with immediate mutation this needs queue storage and a flush phase, but traversal sees a stable object set. The existing event FIFO is smaller when only byte event IDs are needed.

This component owns occupancy and generations only, not a payload constructor or resource allocator. After a successful flush initialise newly live payloads before any object consumer runs; retain the queued commands externally if you need per-command payload data or outcomes. A generation byte wraps after256 reuses, so ancient handles require a stronger lifetime policy. Single update owner only; interrupt producers need separate synchronisation.

Execution evidence

Four variants run on PAL stock-ROM PRG, using Asm198x 0.0.72, Emu198x 0.44.2. Each executes 40 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

Generation handles: Use the same lifetime rule for references outside the command buffer and keep payload ownership explicit.

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