An urgent event may need service before an older routine event, while equal priorities should remain fair.
Use it in a project
A queue holds at most four event/priority pairs. MODE=0 inserts; MODE=1 removes the highest priority, with earlier insertion winning ties. COUNT must be 0–4. Return RESULT=1 on a change, VALID=0 for invalid count/mode. Full insertion and empty removal are valid no-ops. Failed operations return EVENT=255, PRIORITY=0.
Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: EVENT, PRIORITY, RESULT, VALID, COUNT, MODE, NEW_EVENT, NEW_PRIORITY, E0, E1, E2, E3, P0, P1, P2, P3. They occupy 16 contiguous bytes. Also reserve the private workspace: T0, T1, T2, EXPR_SAVE (4 bytes). Array names in the explanation refer to the numbered consecutive bindings, such as C0, C1 and so on.
Call priority_event_queue_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 priority_event_queue_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 | COUNT, E0..E3 and P0..P3 |
| Supply before service | MODE, NEW_EVENT and NEW_PRIORITY |
| Read after service | EVENT, PRIORITY, RESULT and VALID |
Insertion shifts only lower-priority entries. Stopping at an equal priority keeps arrival order stable; removal takes the first pair and closes the gap.
Reusable initialisation and service
; SPDX-License-Identifier: MIT
; Keep a Stable Priority Event Queue. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
priority_event_queue_init:
ld a,0
ld (EVENT),a
ld (PRIORITY),a
ld (RESULT),a
ld (VALID),a
ld (COUNT),a
ld (MODE),a
ld (NEW_EVENT),a
ld (NEW_PRIORITY),a
ld (E0),a
ld (E1),a
ld (E2),a
ld (E3),a
ld (P0),a
ld (P1),a
ld (P2),a
ld (P3),a
ld (T0),a
ld (T1),a
ld (T2),a
ld (EXPR_SAVE),a
ret
priority_event_queue_step:
; EVENT = 255
ld a,255
ld (EVENT),a
; PRIORITY = 0
ld a,0
ld (PRIORITY),a
; RESULT = 0
ld a,0
ld (RESULT),a
; VALID = 0
ld a,0
ld (VALID),a
; if COUNT > 4:
ld a,(COUNT)
cp 4
jp z,priority_event_queue_L2
jp nc,priority_event_queue_L1
jp priority_event_queue_L2
priority_event_queue_L1:
; return
ret
jp priority_event_queue_L3
priority_event_queue_L2:
priority_event_queue_L3:
; if MODE > 1:
ld a,(MODE)
cp 1
jp z,priority_event_queue_L5
jp nc,priority_event_queue_L4
jp priority_event_queue_L5
priority_event_queue_L4:
; return
ret
jp priority_event_queue_L6
priority_event_queue_L5:
priority_event_queue_L6:
; VALID = 1
ld a,1
ld (VALID),a
; if MODE == 0:
ld a,(MODE)
cp 0
jp z,priority_event_queue_L7
jp priority_event_queue_L8
priority_event_queue_L7:
; if COUNT == 4:
ld a,(COUNT)
cp 4
jp z,priority_event_queue_L10
jp priority_event_queue_L11
priority_event_queue_L10:
; return
ret
jp priority_event_queue_L12
priority_event_queue_L11:
priority_event_queue_L12:
; T0 = COUNT
ld a,(COUNT)
ld (T0),a
; while T0 != 0:
priority_event_queue_L13:
ld a,(T0)
cp 0
jp nz,priority_event_queue_L14
jp priority_event_queue_L15
priority_event_queue_L14:
; T1 = T0 - 1
ld a,(T0)
sub 1
ld (T1),a
; T2 = P[T1]
ld a,(T1)
ld e,a
ld d,0
ld hl,P0
add hl,de
ld a,(hl)
ld (T2),a
; if NEW_PRIORITY <= T2:
ld a,(NEW_PRIORITY)
ld b,a
ld a,(T2)
ld c,a
ld a,b
cp c
jp c,priority_event_queue_L16
jp z,priority_event_queue_L16
jp priority_event_queue_L17
priority_event_queue_L16:
; break
jp priority_event_queue_L15
jp priority_event_queue_L18
priority_event_queue_L17:
priority_event_queue_L18:
; P[T0] = T2
ld a,(T2)
ld c,a
ld a,(T0)
ld e,a
ld d,0
ld hl,P0
add hl,de
ld (hl),c
; T2 = E[T1]
ld a,(T1)
ld e,a
ld d,0
ld hl,E0
add hl,de
ld a,(hl)
ld (T2),a
; E[T0] = T2
ld a,(T2)
ld c,a
ld a,(T0)
ld e,a
ld d,0
ld hl,E0
add hl,de
ld (hl),c
; T0 = T0 - 1
ld a,(T0)
sub 1
ld (T0),a
jp priority_event_queue_L13
priority_event_queue_L15:
; E[T0] = NEW_EVENT
ld a,(NEW_EVENT)
ld c,a
ld a,(T0)
ld e,a
ld d,0
ld hl,E0
add hl,de
ld (hl),c
; P[T0] = NEW_PRIORITY
ld a,(NEW_PRIORITY)
ld c,a
ld a,(T0)
ld e,a
ld d,0
ld hl,P0
add hl,de
ld (hl),c
; COUNT = COUNT + 1
ld a,(COUNT)
add a,1
ld (COUNT),a
; EVENT = NEW_EVENT
ld a,(NEW_EVENT)
ld (EVENT),a
; PRIORITY = NEW_PRIORITY
ld a,(NEW_PRIORITY)
ld (PRIORITY),a
jp priority_event_queue_L9
priority_event_queue_L8:
; if COUNT == 0:
ld a,(COUNT)
cp 0
jp z,priority_event_queue_L19
jp priority_event_queue_L20
priority_event_queue_L19:
; return
ret
jp priority_event_queue_L21
priority_event_queue_L20:
priority_event_queue_L21:
; EVENT = E0
ld a,(E0)
ld (EVENT),a
; PRIORITY = P0
ld a,(P0)
ld (PRIORITY),a
; COUNT = COUNT - 1
ld a,(COUNT)
sub 1
ld (COUNT),a
; T0 = 0
ld a,0
ld (T0),a
; while T0 < COUNT:
priority_event_queue_L22:
ld a,(T0)
ld b,a
ld a,(COUNT)
ld c,a
ld a,b
cp c
jp c,priority_event_queue_L23
jp priority_event_queue_L24
priority_event_queue_L23:
; T1 = T0 + 1
ld a,(T0)
add a,1
ld (T1),a
; T2 = E[T1]
ld a,(T1)
ld e,a
ld d,0
ld hl,E0
add hl,de
ld a,(hl)
ld (T2),a
; E[T0] = T2
ld a,(T2)
ld c,a
ld a,(T0)
ld e,a
ld d,0
ld hl,E0
add hl,de
ld (hl),c
; T2 = P[T1]
ld a,(T1)
ld e,a
ld d,0
ld hl,P0
add hl,de
ld a,(hl)
ld (T2),a
; P[T0] = T2
ld a,(T2)
ld c,a
ld a,(T0)
ld e,a
ld d,0
ld hl,P0
add hl,de
ld (hl),c
; T0 = T0 + 1
ld a,(T0)
add a,1
ld (T0),a
jp priority_event_queue_L22
priority_event_queue_L24:
; E[T0] = 0
ld a,0
ld c,a
ld a,(T0)
ld e,a
ld d,0
ld hl,E0
add hl,de
ld (hl),c
; P[T0] = 0
ld a,0
ld c,a
ld a,(T0)
ld e,a
ld d,0
ld hl,P0
add hl,de
ld (hl),c
priority_event_queue_L9:
; RESULT = 1
ld a,1
ld (RESULT),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. 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 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:
EVENT: defb 0
PRIORITY: defb 0
RESULT: defb 0
VALID: defb 0
COUNT: defb 0
MODE: defb 0
NEW_EVENT: defb 0
NEW_PRIORITY: defb 0
E0: defb 0
E1: defb 0
E2: defb 0
E3: defb 0
P0: defb 0
P1: defb 0
P2: defb 0
P3: defb 0
T0: defb 0
T1: defb 0
T2: defb 0
EXPR_SAVE: 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
; NEW_EVENT = 7
ld a,7
ld (NEW_EVENT),a
; NEW_PRIORITY = 0
ld a,0
ld (NEW_PRIORITY),a
if BROKEN
; NEW_PRIORITY = 0
ld a,0
ld (NEW_PRIORITY),a
endif
call checked_step
call emit_state
; NEW_EVENT = 9
ld a,9
ld (NEW_EVENT),a
; NEW_PRIORITY = 255
ld a,255
ld (NEW_PRIORITY),a
if BROKEN
; NEW_PRIORITY = 0
ld a,0
ld (NEW_PRIORITY),a
endif
call checked_step
call emit_state
; MODE = 1
ld a,1
ld (MODE),a
if BROKEN
; NEW_PRIORITY = 0
ld a,0
ld (NEW_PRIORITY),a
endif
call checked_step
call emit_state
ret
else
; NEW_EVENT = 11
ld a,11
ld (NEW_EVENT),a
; NEW_PRIORITY = 2
ld a,2
ld (NEW_PRIORITY),a
if BROKEN
; NEW_PRIORITY = 0
ld a,0
ld (NEW_PRIORITY),a
endif
call checked_step
call emit_state
; NEW_EVENT = 22
ld a,22
ld (NEW_EVENT),a
; NEW_PRIORITY = 8
ld a,8
ld (NEW_PRIORITY),a
if BROKEN
; NEW_PRIORITY = 0
ld a,0
ld (NEW_PRIORITY),a
endif
call checked_step
call emit_state
; NEW_EVENT = 33
ld a,33
ld (NEW_EVENT),a
; NEW_PRIORITY = 8
ld a,8
ld (NEW_PRIORITY),a
if BROKEN
; NEW_PRIORITY = 0
ld a,0
ld (NEW_PRIORITY),a
endif
call checked_step
call emit_state
; MODE = 1
ld a,1
ld (MODE),a
if BROKEN
; NEW_PRIORITY = 0
ld a,0
ld (NEW_PRIORITY),a
endif
call checked_step
call emit_state
endif
retCustomise it
Mix the minimum and maximum priorities. 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:
EVENT: defb 0
PRIORITY: defb 0
RESULT: defb 0
VALID: defb 0
COUNT: defb 0
MODE: defb 0
NEW_EVENT: defb 0
NEW_PRIORITY: defb 0
E0: defb 0
E1: defb 0
E2: defb 0
E3: defb 0
P0: defb 0
P1: defb 0
P2: defb 0
P3: defb 0
T0: defb 0
T1: defb 0
T2: defb 0
EXPR_SAVE: 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 09 FF 01 01 01 01 09 FF. The full client trace in the source explains how it gets there.
Mistake and repair
The wrong caller discards insertion priority. The queue then behaves like an ordinary FIFO and returns event 11 before urgent event 22. Preserve the requested priority.
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:
EVENT: defb 0
PRIORITY: defb 0
RESULT: defb 0
VALID: defb 0
COUNT: defb 0
MODE: defb 0
NEW_EVENT: defb 0
NEW_PRIORITY: defb 0
E0: defb 0
E1: defb 0
E2: defb 0
E3: defb 0
P0: defb 0
P1: defb 0
P2: defb 0
P3: defb 0
T0: defb 0
T1: defb 0
T2: defb 0
EXPR_SAVE: 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 489 code bytes; public state is 16 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.
At most four pair moves. A binary heap costs more machinery but suits larger queues; a plain FIFO gives simpler fairness when urgency is unnecessary.
The retained queue must already be sorted by descending priority. This is not an interrupt-safe producer/consumer queue and can starve low-priority events.
Execution evidence
Four variants run on 48K PAL ROM tape load and USR, using Asm198x 0.0.72, Emu198x 0.44.2. Each executes 62 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
Event queues: Choose FIFO or priority order according to the rules of the game.