Spawning or deleting objects during a traversal can accidentally update a new object twice or skip a neighbour.
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
; 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:
ld a,0
ld (APPLIED),a
ld (REJECTED),a
ld (FULL),a
ld (VALID),a
ld (MODE),a
ld (SLOT),a
ld (GENERATION),a
ld (ACTIVE),a
ld (COUNT),a
ld (G0),a
ld (G1),a
ld (G2),a
ld (G3),a
ld (O0),a
ld (O1),a
ld (O2),a
ld (O3),a
ld (S0),a
ld (S1),a
ld (S2),a
ld (S3),a
ld (H0),a
ld (H1),a
ld (H2),a
ld (H3),a
ld (T0),a
ld (T1),a
ld (T2),a
ld (T3),a
ld (T4),a
ld (T5),a
ld (T6),a
ld (EXPR_SAVE),a
ret
object_command_buffer_step:
; APPLIED = 0
ld a,0
ld (APPLIED),a
; REJECTED = 0
ld a,0
ld (REJECTED),a
; FULL = 0
ld a,0
ld (FULL),a
; VALID = 0
ld a,0
ld (VALID),a
; if ACTIVE >= 16:
ld a,(ACTIVE)
cp 16
jp nc,object_command_buffer_L1
jp object_command_buffer_L2
object_command_buffer_L1:
; return
ret
jp object_command_buffer_L3
object_command_buffer_L2:
object_command_buffer_L3:
; if COUNT > 4:
ld a,(COUNT)
cp 4
jp z,object_command_buffer_L5
jp nc,object_command_buffer_L4
jp object_command_buffer_L5
object_command_buffer_L4:
; return
ret
jp object_command_buffer_L6
object_command_buffer_L5:
object_command_buffer_L6:
; if MODE > 3:
ld a,(MODE)
cp 3
jp z,object_command_buffer_L8
jp nc,object_command_buffer_L7
jp object_command_buffer_L8
object_command_buffer_L7:
; return
ret
jp object_command_buffer_L9
object_command_buffer_L8:
object_command_buffer_L9:
; if MODE == 0:
ld a,(MODE)
cp 0
jp z,object_command_buffer_L10
jp object_command_buffer_L11
object_command_buffer_L10:
; COUNT = 0
ld a,0
ld (COUNT),a
; VALID = 1
ld a,1
ld (VALID),a
; return
ret
jp object_command_buffer_L12
object_command_buffer_L11:
object_command_buffer_L12:
; if MODE < 3:
ld a,(MODE)
cp 3
jp c,object_command_buffer_L13
jp object_command_buffer_L14
object_command_buffer_L13:
; if SLOT >= 4:
ld a,(SLOT)
cp 4
jp nc,object_command_buffer_L16
jp object_command_buffer_L17
object_command_buffer_L16:
; return
ret
jp object_command_buffer_L18
object_command_buffer_L17:
object_command_buffer_L18:
; VALID = 1
ld a,1
ld (VALID),a
; if COUNT == 4:
ld a,(COUNT)
cp 4
jp z,object_command_buffer_L19
jp object_command_buffer_L20
object_command_buffer_L19:
; FULL = 1
ld a,1
ld (FULL),a
; return
ret
jp object_command_buffer_L21
object_command_buffer_L20:
object_command_buffer_L21:
; O[COUNT] = MODE
ld a,(MODE)
ld c,a
ld a,(COUNT)
ld e,a
ld d,0
ld hl,O0
add hl,de
ld (hl),c
; S[COUNT] = SLOT
ld a,(SLOT)
ld c,a
ld a,(COUNT)
ld e,a
ld d,0
ld hl,S0
add hl,de
ld (hl),c
; H[COUNT] = GENERATION
ld a,(GENERATION)
ld c,a
ld a,(COUNT)
ld e,a
ld d,0
ld hl,H0
add hl,de
ld (hl),c
; COUNT = COUNT + 1
ld a,(COUNT)
add a,1
ld (COUNT),a
; return
ret
jp object_command_buffer_L15
object_command_buffer_L14:
object_command_buffer_L15:
; T0 = 0
ld a,0
ld (T0),a
; while T0 < COUNT:
object_command_buffer_L22:
ld a,(T0)
ld b,a
ld a,(COUNT)
ld c,a
ld a,b
cp c
jp c,object_command_buffer_L23
jp object_command_buffer_L24
object_command_buffer_L23:
; T1 = O[T0]
ld a,(T0)
ld e,a
ld d,0
ld hl,O0
add hl,de
ld a,(hl)
ld (T1),a
; if T1 == 0:
ld a,(T1)
cp 0
jp z,object_command_buffer_L25
jp object_command_buffer_L26
object_command_buffer_L25:
; return
ret
jp object_command_buffer_L27
object_command_buffer_L26:
object_command_buffer_L27:
; if T1 > 2:
ld a,(T1)
cp 2
jp z,object_command_buffer_L29
jp nc,object_command_buffer_L28
jp object_command_buffer_L29
object_command_buffer_L28:
; return
ret
jp object_command_buffer_L30
object_command_buffer_L29:
object_command_buffer_L30:
; T1 = S[T0]
ld a,(T0)
ld e,a
ld d,0
ld hl,S0
add hl,de
ld a,(hl)
ld (T1),a
; if T1 >= 4:
ld a,(T1)
cp 4
jp nc,object_command_buffer_L31
jp object_command_buffer_L32
object_command_buffer_L31:
; return
ret
jp object_command_buffer_L33
object_command_buffer_L32:
object_command_buffer_L33:
; T0 = T0 + 1
ld a,(T0)
add a,1
ld (T0),a
jp object_command_buffer_L22
object_command_buffer_L24:
; VALID = 1
ld a,1
ld (VALID),a
; T0 = 0
ld a,0
ld (T0),a
; while T0 < COUNT:
object_command_buffer_L34:
ld a,(T0)
ld b,a
ld a,(COUNT)
ld c,a
ld a,b
cp c
jp c,object_command_buffer_L35
jp object_command_buffer_L36
object_command_buffer_L35:
; T1 = S[T0]
ld a,(T0)
ld e,a
ld d,0
ld hl,S0
add hl,de
ld a,(hl)
ld (T1),a
; T2 = 1
ld a,1
ld (T2),a
; T3 = T1
ld a,(T1)
ld (T3),a
; while T3 != 0:
object_command_buffer_L37:
ld a,(T3)
cp 0
jp nz,object_command_buffer_L38
jp object_command_buffer_L39
object_command_buffer_L38:
; T2 = T2 << 1
ld a,(T2)
sla a
ld (T2),a
; T3 = T3 - 1
ld a,(T3)
sub 1
ld (T3),a
jp object_command_buffer_L37
object_command_buffer_L39:
; T3 = ACTIVE & T2
ld a,(ACTIVE)
ld b,a
ld a,(T2)
ld c,a
ld a,b
and c
ld (T3),a
; T4 = O[T0]
ld a,(T0)
ld e,a
ld d,0
ld hl,O0
add hl,de
ld a,(hl)
ld (T4),a
; T5 = 0
ld a,0
ld (T5),a
; if T4 == 1:
ld a,(T4)
cp 1
jp z,object_command_buffer_L40
jp object_command_buffer_L41
object_command_buffer_L40:
; if T3 == 0:
ld a,(T3)
cp 0
jp z,object_command_buffer_L43
jp object_command_buffer_L44
object_command_buffer_L43:
; ACTIVE = ACTIVE | T2
ld a,(ACTIVE)
ld b,a
ld a,(T2)
ld c,a
ld a,b
or c
ld (ACTIVE),a
; T4 = G[T1]
ld a,(T1)
ld e,a
ld d,0
ld hl,G0
add hl,de
ld a,(hl)
ld (T4),a
; T4 = T4 + 1
ld a,(T4)
add a,1
ld (T4),a
; G[T1] = T4
ld a,(T4)
ld c,a
ld a,(T1)
ld e,a
ld d,0
ld hl,G0
add hl,de
ld (hl),c
; T5 = 1
ld a,1
ld (T5),a
jp object_command_buffer_L45
object_command_buffer_L44:
object_command_buffer_L45:
jp object_command_buffer_L42
object_command_buffer_L41:
; if T3 != 0:
ld a,(T3)
cp 0
jp nz,object_command_buffer_L46
jp object_command_buffer_L47
object_command_buffer_L46:
; T4 = G[T1]
ld a,(T1)
ld e,a
ld d,0
ld hl,G0
add hl,de
ld a,(hl)
ld (T4),a
; T6 = H[T0]
ld a,(T0)
ld e,a
ld d,0
ld hl,H0
add hl,de
ld a,(hl)
ld (T6),a
; if T4 == T6:
ld a,(T4)
ld b,a
ld a,(T6)
ld c,a
ld a,b
cp c
jp z,object_command_buffer_L49
jp object_command_buffer_L50
object_command_buffer_L49:
; ACTIVE = ACTIVE ^ T2
ld a,(ACTIVE)
ld b,a
ld a,(T2)
ld c,a
ld a,b
xor c
ld (ACTIVE),a
; T5 = 1
ld a,1
ld (T5),a
jp object_command_buffer_L51
object_command_buffer_L50:
object_command_buffer_L51:
jp object_command_buffer_L48
object_command_buffer_L47:
object_command_buffer_L48:
object_command_buffer_L42:
; if T5 != 0:
ld a,(T5)
cp 0
jp nz,object_command_buffer_L52
jp object_command_buffer_L53
object_command_buffer_L52:
; APPLIED = APPLIED + 1
ld a,(APPLIED)
add a,1
ld (APPLIED),a
jp object_command_buffer_L54
object_command_buffer_L53:
; REJECTED = REJECTED + 1
ld a,(REJECTED)
add a,1
ld (REJECTED),a
object_command_buffer_L54:
; T0 = T0 + 1
ld a,(T0)
add a,1
ld (T0),a
jp object_command_buffer_L34
object_command_buffer_L36:
; COUNT = 0
ld a,0
ld (COUNT),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:
APPLIED: defb 0
REJECTED: defb 0
FULL: defb 0
VALID: defb 0
MODE: defb 0
SLOT: defb 0
GENERATION: defb 0
ACTIVE: defb 0
COUNT: defb 0
G0: defb 0
G1: defb 0
G2: defb 0
G3: defb 0
O0: defb 0
O1: defb 0
O2: defb 0
O3: defb 0
S0: defb 0
S1: defb 0
S2: defb 0
S3: defb 0
H0: defb 0
H1: defb 0
H2: defb 0
H3: defb 0
T0: defb 0
T1: defb 0
T2: defb 0
T3: defb 0
T4: defb 0
T5: defb 0
T6: 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
; MODE = 1
ld a,1
ld (MODE),a
; SLOT = 2
ld a,2
ld (SLOT),a
if BROKEN
; COUNT = 0
ld a,0
ld (COUNT),a
endif
call checked_step
call emit_state
; MODE = 2
ld a,2
ld (MODE),a
; GENERATION = 1
ld a,1
ld (GENERATION),a
if BROKEN
; COUNT = 0
ld a,0
ld (COUNT),a
endif
call checked_step
call emit_state
; MODE = 3
ld a,3
ld (MODE),a
if BROKEN
; COUNT = 0
ld a,0
ld (COUNT),a
endif
call checked_step
call emit_state
ret
else
; MODE = 1
ld a,1
ld (MODE),a
; SLOT = 1
ld a,1
ld (SLOT),a
if BROKEN
; COUNT = 0
ld a,0
ld (COUNT),a
endif
call checked_step
call emit_state
; MODE = 3
ld a,3
ld (MODE),a
if BROKEN
; COUNT = 0
ld a,0
ld (COUNT),a
endif
call checked_step
call emit_state
; MODE = 2
ld a,2
ld (MODE),a
; SLOT = 1
ld a,1
ld (SLOT),a
; GENERATION = 0
ld a,0
ld (GENERATION),a
if BROKEN
; COUNT = 0
ld a,0
ld (COUNT),a
endif
call checked_step
call emit_state
; MODE = 3
ld a,3
ld (MODE),a
if BROKEN
; COUNT = 0
ld a,0
ld (COUNT),a
endif
call checked_step
call emit_state
endif
retCustomise 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
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:
APPLIED: defb 0
REJECTED: defb 0
FULL: defb 0
VALID: defb 0
MODE: defb 0
SLOT: defb 0
GENERATION: defb 0
ACTIVE: defb 0
COUNT: defb 0
G0: defb 0
G1: defb 0
G2: defb 0
G3: defb 0
O0: defb 0
O1: defb 0
O2: defb 0
O3: defb 0
S0: defb 0
S1: defb 0
S2: defb 0
S3: defb 0
H0: defb 0
H1: defb 0
H2: defb 0
H3: defb 0
T0: defb 0
T1: defb 0
T2: defb 0
T3: defb 0
T4: defb 0
T5: defb 0
T6: 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 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
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:
APPLIED: defb 0
REJECTED: defb 0
FULL: defb 0
VALID: defb 0
MODE: defb 0
SLOT: defb 0
GENERATION: defb 0
ACTIVE: defb 0
COUNT: defb 0
G0: defb 0
G1: defb 0
G2: defb 0
G3: defb 0
O0: defb 0
O1: defb 0
O2: defb 0
O3: defb 0
S0: defb 0
S1: defb 0
S2: defb 0
S3: defb 0
H0: defb 0
H1: defb 0
H2: defb 0
H3: defb 0
T0: defb 0
T1: defb 0
T2: defb 0
T3: defb 0
T4: defb 0
T5: defb 0
T6: 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 736 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 48K PAL ROM tape load and USR, 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.