When three things happen before the game gets around to handling them, one event byte can only remember the last one. A small FIFO keeps their order. This queue has an explicit full result, so a burst cannot silently overwrite an event that still matters.
Include, initialise and call
Bind HEAD, TAIL, COUNT, VALUE, MODE, OK, B0, B1, B2, B3 as 10 contiguous private bytes in that order. Include the helper once. Call event_queue_init at the deliberate reset boundary: it clears all named bytes, including inputs. Set your starting values afterwards. Call event_queue_step when this service is needed.
Initialise once, then offer each event and check OK. If a push fails, choose a game policy: retry, discard a dispensable effect, or stop producing work. Do not pretend the event was queued. Pop until OK becomes zero, with a per-update work budget if producers could keep adding events.
HEAD and TAIL are indices into contiguous B0–B3. COUNT is the number of occupied entries. MODE 1 pushes VALUE, MODE 2 pops into VALUE; other modes do nothing. OK is replaced with 1 on success or 0 on rejection. A full push and an empty pop preserve the queue and VALUE. Use only state produced by init/step: indices are below QUEUE_CAP, COUNT is 0–QUEUE_CAP and TAIL equals (HEAD + COUNT) modulo capacity.
Reusable initialisation and service
; SPDX-License-Identifier: MIT
; A Bounded Event Queue. Bind these contiguous private bytes: HEAD, TAIL, COUNT, VALUE, MODE, OK, B0, B1, B2, B3.
; init clears the whole block; step uses current inputs. No I/O, waits or IRQ owner.
; A/X/flags may change; Y and decimal mode survive. Binary arithmetic requires D=0.
event_queue_init:
lda #0
sta HEAD
sta TAIL
sta COUNT
sta VALUE
sta MODE
sta OK
sta B0
sta B1
sta B2
sta B3
rts
event_queue_step:
lda #0
sta OK
lda MODE
cmp #1
beq queue_push
cmp #2
bne queue_done
lda COUNT
beq queue_done
ldx HEAD
lda B0,x
sta VALUE
inx
cpx #QUEUE_CAP
bcc queue_pop_index
ldx #0
queue_pop_index:
stx HEAD
dec COUNT
jmp queue_success
queue_push:
lda COUNT
cmp #QUEUE_CAP
bcs queue_done
ldx TAIL
lda VALUE
sta B0,x
inx
cpx #QUEUE_CAP
bcc queue_push_index
ldx #0
queue_push_index:
stx TAIL
inc COUNT
queue_success:
inc OK
queue_done:
rts
component_end:State and ownership
Six control/input/output bytes precede four reserved buffer bytes. A queue stores byte event IDs, not payloads or pointers. Popped buffer cells retain stale data; COUNT decides which entries are live. Head equals tail can mean either empty or full, so COUNT must be retained.
A/X/flags may change; Y and decimal mode survive. Clear decimal mode before binary arithmetic; the standalone caller uses CLD. A direct call pushes a two-byte return address.
This implementation has fixed symbol bindings and one mutation owner. Independent users need distinct state blocks and scoped/renamed bindings; including another copy with the same global labels is not sufficient. Sequential queries may reuse a scratch block only after the previous result has been consumed. Do not let an interrupt and the main loop mutate the same block without a separately designed ownership policy.
The standalone cartridge owns its background, CHR font and RAM layout. Its diagnostic packet is bounded below the end of the NES’s 2 KiB internal RAM; the component owns no PPU or audio registers.
A small caller you can adapt
The standalone program includes the helper and a separate client. Its read-only table exercises 96 valid inputs, then this client initialises once and retains state between calls. Replace the supplied input changes with your own game values; the display and packet machinery are diagnostics rather than dependencies of the component.
Buildable caller, bindings and include order
.segment "HEADER"
.byte "NES",$1a,2,1,0,0,0,0,0,0,0,0,0,0
HEAD=$80
TAIL=$81
COUNT=$82
VALUE=$83
MODE=$84
OK=$85
B0=$86
B1=$87
B2=$88
B3=$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
SCORE_CAP=65535
QUEUE_CAP=4
REPEAT_DELAY=4
REPEAT_RATE=2
.include "caller.inc"
.include "exercise.inc"
component_start:
.include "component.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 separate client retaining state between calls
preview_rules:
.if CUSTOM
lda #17
sta VALUE
lda #1
sta MODE
.if BROKEN
lda MODE
cmp #1
bne trace_skip_reset_0_custom
lda #0
sta HEAD
lda #0
sta TAIL
lda #0
sta COUNT
trace_skip_reset_0_custom:
jsr checked_step
.else
jsr checked_step
.endif
jsr emit_state
lda #34
sta VALUE
lda #1
sta MODE
.if BROKEN
lda MODE
cmp #1
bne trace_skip_reset_1_custom
lda #0
sta HEAD
lda #0
sta TAIL
lda #0
sta COUNT
trace_skip_reset_1_custom:
jsr checked_step
.else
jsr checked_step
.endif
jsr emit_state
lda #51
sta VALUE
lda #1
sta MODE
.if BROKEN
lda MODE
cmp #1
bne trace_skip_reset_2_custom
lda #0
sta HEAD
lda #0
sta TAIL
lda #0
sta COUNT
trace_skip_reset_2_custom:
jsr checked_step
.else
jsr checked_step
.endif
jsr emit_state
lda #2
sta MODE
.if BROKEN
lda MODE
cmp #1
bne trace_skip_reset_3_custom
lda #0
sta HEAD
lda #0
sta TAIL
lda #0
sta COUNT
trace_skip_reset_3_custom:
jsr checked_step
.else
jsr checked_step
.endif
jsr emit_state
lda #2
sta MODE
.if BROKEN
lda MODE
cmp #1
bne trace_skip_reset_4_custom
lda #0
sta HEAD
lda #0
sta TAIL
lda #0
sta COUNT
trace_skip_reset_4_custom:
jsr checked_step
.else
jsr checked_step
.endif
jsr emit_state
lda #2
sta MODE
.if BROKEN
lda MODE
cmp #1
bne trace_skip_reset_5_custom
lda #0
sta HEAD
lda #0
sta TAIL
lda #0
sta COUNT
trace_skip_reset_5_custom:
jsr checked_step
.else
jsr checked_step
.endif
jsr emit_state
.else
lda #17
sta VALUE
lda #1
sta MODE
.if BROKEN
lda MODE
cmp #1
bne trace_skip_reset_0
lda #0
sta HEAD
lda #0
sta TAIL
lda #0
sta COUNT
trace_skip_reset_0:
jsr checked_step
.else
jsr checked_step
.endif
jsr emit_state
lda #34
sta VALUE
lda #1
sta MODE
.if BROKEN
lda MODE
cmp #1
bne trace_skip_reset_1
lda #0
sta HEAD
lda #0
sta TAIL
lda #0
sta COUNT
trace_skip_reset_1:
jsr checked_step
.else
jsr checked_step
.endif
jsr emit_state
lda #51
sta VALUE
lda #1
sta MODE
.if BROKEN
lda MODE
cmp #1
bne trace_skip_reset_2
lda #0
sta HEAD
lda #0
sta TAIL
lda #0
sta COUNT
trace_skip_reset_2:
jsr checked_step
.else
jsr checked_step
.endif
jsr emit_state
lda #2
sta MODE
.if BROKEN
lda MODE
cmp #1
bne trace_skip_reset_3
lda #0
sta HEAD
lda #0
sta TAIL
lda #0
sta COUNT
trace_skip_reset_3:
jsr checked_step
.else
jsr checked_step
.endif
jsr emit_state
lda #2
sta MODE
.if BROKEN
lda MODE
cmp #1
bne trace_skip_reset_4
lda #0
sta HEAD
lda #0
sta TAIL
lda #0
sta COUNT
trace_skip_reset_4:
jsr checked_step
.else
jsr checked_step
.endif
jsr emit_state
lda #2
sta MODE
.if BROKEN
lda MODE
cmp #1
bne trace_skip_reset_5
lda #0
sta HEAD
lda #0
sta TAIL
lda #0
sta COUNT
trace_skip_reset_5:
jsr checked_step
.else
jsr checked_step
.endif
jsr emit_state
.endif
rtsThe eight display fields follow HEAD, TAIL, COUNT, VALUE, MODE, OK, B0, B1; the full packet also retains B2, B3. Each field is a hexadecimal byte. The working row is 03 03 00 33 02 01 11 22. The output is a completed diagnostic state, not a recording of movement, input or sound hardware.
Make it your own
The custom build lowers QUEUE_CAP from four to two while retaining the four-byte storage layout. A third push is rejected, and the two original events still pop in order. This demonstrates a capacity limit rather than a RAM-saving layout change.
Maintained custom configuration or caller
.segment "HEADER"
.byte "NES",$1a,2,1,0,0,0,0,0,0,0,0,0,0
HEAD=$80
TAIL=$81
COUNT=$82
VALUE=$83
MODE=$84
OK=$85
B0=$86
B1=$87
B2=$88
B3=$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
SCORE_CAP=999
QUEUE_CAP=2
REPEAT_DELAY=6
REPEAT_RATE=3
.include "caller.inc"
.include "exercise.inc"
component_start:
.include "component.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 observed custom row is 00 00 00 22 02 00 11 22. The retained records qualify these named configurations; a new input domain or larger layout needs its own bounds and fresh execution checks.
Common mistake and repair
The wrong client resets HEAD, TAIL and COUNT before every push. The later event replaces the history of earlier submissions. The first pop now returns 51 instead of 17. Initialise at session creation or an intentional flush, not when submitting an event.
Deliberately wrong client selection
.segment "HEADER"
.byte "NES",$1a,2,1,0,0,0,0,0,0,0,0,0,0
HEAD=$80
TAIL=$81
COUNT=$82
VALUE=$83
MODE=$84
OK=$85
B0=$86
B1=$87
B2=$88
B3=$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
SCORE_CAP=65535
QUEUE_CAP=4
REPEAT_DELAY=4
REPEAT_RATE=2
.include "caller.inc"
.include "exercise.inc"
component_start:
.include "component.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
The wrong branch is in the displayed client above, selected by BROKEN. repaired.asm selects the original client again. Its executable matches demo exactly; Spectrum tape filenames can differ while loaded CODE is identical. The bad caller is a labelled experiment, not an alternative recommended implementation.
Cost and verification
The measured helper occupies 87 code bytes plus 10 named state bytes in this layout. Return addresses, alignment, tables and display diagnostics are additional. The sizes cover init and step together, not the whole standalone program.
Each push/pop touches one buffer cell and a few counters, without moving the other entries. This buys constant work at the cost of indices, count and explicit overflow handling. Draining the entire queue is a caller loop and needs its own budget.
Larger queues need wider indices or a bounded capacity and updated storage. A power-of-two capacity can use a mask to wrap, but this version uses comparison so its policy is visible. An IRQ producer and foreground consumer need a separately designed single-producer/single-consumer protocol or critical section; this is a single-owner queue.
Four native runs use NTSC NES with a mapper-0 cartridge, CHR-ROM diagnostic font, NMI and DMC disabled; Asm198x 0.0.72 and Emu198x 0.44.2. Every run checks 96 table results, dirty initialisation and every state in a separate sequential trace against an independent integer model. This is boundary/varied-input qualification, not exhaustive coverage of every byte combination. Source/tool/firmware hashes, native startup and completion, preserved-register sentinels, immutable loaded code, actual helper extent, capture bytes and pixels, and exact repair are checked. Deliberately changed observations must fail. Firmware is hashed, never distributed.