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.
; AF/BC/DE/HL may change; IX/IY survive.
event_queue_init:
xor a
ld (HEAD),a
ld (TAIL),a
ld (COUNT),a
ld (VALUE),a
ld (MODE),a
ld (OK),a
ld (B0),a
ld (B1),a
ld (B2),a
ld (B3),a
ret
event_queue_step:
xor a
ld (OK),a
ld a,(MODE)
cp 1
jr z,queue_push
cp 2
ret nz
ld a,(COUNT)
or a
ret z
dec a
ld (COUNT),a
ld a,(HEAD)
ld e,a
ld d,0
ld hl,B0
add hl,de
ld a,(hl)
ld (VALUE),a
ld a,e
inc a
cp QUEUE_CAP
jr c,queue_pop_index
xor a
queue_pop_index:
ld (HEAD),a
jr queue_success
queue_push:
ld a,(COUNT)
cp QUEUE_CAP
ret nc
inc a
ld (COUNT),a
ld a,(TAIL)
ld e,a
ld d,0
ld hl,B0
add hl,de
ld a,(VALUE)
ld (hl),a
ld a,e
inc a
cp QUEUE_CAP
jr c,queue_push_index
xor a
queue_push_index:
ld (TAIL),a
queue_success:
ld a,1
ld (OK),a
ret
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.
AF/BC/DE/HL may change; IX/IY survive. A direct CALL uses two return-address stack bytes.
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 tape program starts at $8000, disables interrupts and owns bitmap/attribute RAM. It does not return to BASIC. The helper itself never disables interrupts or touches the display.
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
org $8000
CUSTOM equ 0
BROKEN equ 0
SCORE_CAP equ 65535
QUEUE_CAP equ 4
REPEAT_DELAY equ 4
REPEAT_RATE equ 2
include "caller.inc"
include "exercise.inc"
component_start:
include "component.inc"
include "cases.inc"
include "../../physics/rectangle-overlap/display.inc"
code_end:
STATE:
HEAD: defb 0
TAIL: defb 0
COUNT: defb 0
VALUE: defb 0
MODE: defb 0
OK: defb 0
B0: defb 0
B1: defb 0
B2: defb 0
B3: 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 separate client retaining state between calls
preview_rules:
if CUSTOM
ld a,17
ld (VALUE),a
ld a,1
ld (MODE),a
if BROKEN
ld a,(MODE)
cp 1
jr nz,trace_skip_reset_0_custom
ld a,0
ld (HEAD),a
ld a,0
ld (TAIL),a
ld a,0
ld (COUNT),a
trace_skip_reset_0_custom:
call checked_step
else
call checked_step
endif
call emit_state
ld a,34
ld (VALUE),a
ld a,1
ld (MODE),a
if BROKEN
ld a,(MODE)
cp 1
jr nz,trace_skip_reset_1_custom
ld a,0
ld (HEAD),a
ld a,0
ld (TAIL),a
ld a,0
ld (COUNT),a
trace_skip_reset_1_custom:
call checked_step
else
call checked_step
endif
call emit_state
ld a,51
ld (VALUE),a
ld a,1
ld (MODE),a
if BROKEN
ld a,(MODE)
cp 1
jr nz,trace_skip_reset_2_custom
ld a,0
ld (HEAD),a
ld a,0
ld (TAIL),a
ld a,0
ld (COUNT),a
trace_skip_reset_2_custom:
call checked_step
else
call checked_step
endif
call emit_state
ld a,2
ld (MODE),a
if BROKEN
ld a,(MODE)
cp 1
jr nz,trace_skip_reset_3_custom
ld a,0
ld (HEAD),a
ld a,0
ld (TAIL),a
ld a,0
ld (COUNT),a
trace_skip_reset_3_custom:
call checked_step
else
call checked_step
endif
call emit_state
ld a,2
ld (MODE),a
if BROKEN
ld a,(MODE)
cp 1
jr nz,trace_skip_reset_4_custom
ld a,0
ld (HEAD),a
ld a,0
ld (TAIL),a
ld a,0
ld (COUNT),a
trace_skip_reset_4_custom:
call checked_step
else
call checked_step
endif
call emit_state
ld a,2
ld (MODE),a
if BROKEN
ld a,(MODE)
cp 1
jr nz,trace_skip_reset_5_custom
ld a,0
ld (HEAD),a
ld a,0
ld (TAIL),a
ld a,0
ld (COUNT),a
trace_skip_reset_5_custom:
call checked_step
else
call checked_step
endif
call emit_state
else
ld a,17
ld (VALUE),a
ld a,1
ld (MODE),a
if BROKEN
ld a,(MODE)
cp 1
jr nz,trace_skip_reset_0
ld a,0
ld (HEAD),a
ld a,0
ld (TAIL),a
ld a,0
ld (COUNT),a
trace_skip_reset_0:
call checked_step
else
call checked_step
endif
call emit_state
ld a,34
ld (VALUE),a
ld a,1
ld (MODE),a
if BROKEN
ld a,(MODE)
cp 1
jr nz,trace_skip_reset_1
ld a,0
ld (HEAD),a
ld a,0
ld (TAIL),a
ld a,0
ld (COUNT),a
trace_skip_reset_1:
call checked_step
else
call checked_step
endif
call emit_state
ld a,51
ld (VALUE),a
ld a,1
ld (MODE),a
if BROKEN
ld a,(MODE)
cp 1
jr nz,trace_skip_reset_2
ld a,0
ld (HEAD),a
ld a,0
ld (TAIL),a
ld a,0
ld (COUNT),a
trace_skip_reset_2:
call checked_step
else
call checked_step
endif
call emit_state
ld a,2
ld (MODE),a
if BROKEN
ld a,(MODE)
cp 1
jr nz,trace_skip_reset_3
ld a,0
ld (HEAD),a
ld a,0
ld (TAIL),a
ld a,0
ld (COUNT),a
trace_skip_reset_3:
call checked_step
else
call checked_step
endif
call emit_state
ld a,2
ld (MODE),a
if BROKEN
ld a,(MODE)
cp 1
jr nz,trace_skip_reset_4
ld a,0
ld (HEAD),a
ld a,0
ld (TAIL),a
ld a,0
ld (COUNT),a
trace_skip_reset_4:
call checked_step
else
call checked_step
endif
call emit_state
ld a,2
ld (MODE),a
if BROKEN
ld a,(MODE)
cp 1
jr nz,trace_skip_reset_5
ld a,0
ld (HEAD),a
ld a,0
ld (TAIL),a
ld a,0
ld (COUNT),a
trace_skip_reset_5:
call checked_step
else
call checked_step
endif
call emit_state
endif
retThe 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
org $8000
CUSTOM equ 1
BROKEN equ 0
SCORE_CAP equ 999
QUEUE_CAP equ 2
REPEAT_DELAY equ 6
REPEAT_RATE equ 3
include "caller.inc"
include "exercise.inc"
component_start:
include "component.inc"
include "cases.inc"
include "../../physics/rectangle-overlap/display.inc"
code_end:
STATE:
HEAD: defb 0
TAIL: defb 0
COUNT: defb 0
VALUE: defb 0
MODE: defb 0
OK: defb 0
B0: defb 0
B1: defb 0
B2: defb 0
B3: 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 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
org $8000
CUSTOM equ 0
BROKEN equ 1
SCORE_CAP equ 65535
QUEUE_CAP equ 4
REPEAT_DELAY equ 4
REPEAT_RATE equ 2
include "caller.inc"
include "exercise.inc"
component_start:
include "component.inc"
include "cases.inc"
include "../../physics/rectangle-overlap/display.inc"
code_end:
STATE:
HEAD: defb 0
TAIL: defb 0
COUNT: defb 0
VALUE: defb 0
MODE: defb 0
OK: defb 0
B0: defb 0
B1: defb 0
B2: defb 0
B3: 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
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 121 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 Spectrum 48K with stock ROM tape loading and USR entry; 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.