A particle update can walk a dense live prefix instead of checking holes forever.
Use it in a project
COUNT 0–8 selects paired flag/ID arrays F0–F7 and I0–I7. Keep nonzero-flag items in original order at the start, update COUNT, return REMOVED and clear the vacated portion of the original prefix. Entries beyond the original COUNT are untouched. Invalid COUNT returns VALID=0 and REMOVED=0 without changing arrays or count.
Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: COUNT, REMOVED, VALID, F0, F1, F2, F3, F4, F5, F6, F7, I0, I1, I2, I3, I4, I5, I6, I7. They occupy 19 contiguous bytes. Also reserve the private workspace: T0, T1, T2, T3, T4, EXPR_SAVE (6 bytes). Array names in the explanation refer to the numbered consecutive bindings, such as C0, C1 and so on.
Call stable_compaction_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 stable_compaction_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 and the owned array prefix |
| Supply before service | F0–F7 and I0–I7 before compaction |
| Read after service | COUNT, REMOVED and VALID |
A read cursor scans the old prefix while a write cursor advances only for retained items. The write cursor never overtakes the read cursor, making in-place stable filtering safe.
Reusable initialisation and service
; SPDX-License-Identifier: MIT
; Compact a Live Object Prefix. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
stable_compaction_init:
lda #0
sta COUNT
sta REMOVED
sta VALID
sta F0
sta F1
sta F2
sta F3
sta F4
sta F5
sta F6
sta F7
sta I0
sta I1
sta I2
sta I3
sta I4
sta I5
sta I6
sta I7
sta T0
sta T1
sta T2
sta T3
sta T4
sta EXPR_SAVE
rts
stable_compaction_step:
; REMOVED = 0
lda #0
sta REMOVED
; VALID = 0
lda #0
sta VALID
; if COUNT > 8:
lda COUNT
cmp #8
beq stable_compaction_L5
bcs stable_compaction_L4
stable_compaction_L5:
jmp stable_compaction_L2
stable_compaction_L4:
jmp stable_compaction_L1
stable_compaction_L1:
; return
rts
jmp stable_compaction_L3
stable_compaction_L2:
stable_compaction_L3:
; VALID = 1
lda #1
sta VALID
; T0 = 0
lda #0
sta T0
; T1 = 0
lda #0
sta T1
; T4 = COUNT
lda COUNT
sta T4
; while T0 < T4:
stable_compaction_L6:
lda T0
cmp T4
bcc stable_compaction_L9
stable_compaction_L10:
jmp stable_compaction_L8
stable_compaction_L9:
jmp stable_compaction_L7
stable_compaction_L7:
; T2 = F[T0]
ldx T0
lda F0,x
sta T2
; if T2 != 0:
lda T2
cmp #0
bne stable_compaction_L14
stable_compaction_L15:
jmp stable_compaction_L12
stable_compaction_L14:
jmp stable_compaction_L11
stable_compaction_L11:
; T3 = I[T0]
ldx T0
lda I0,x
sta T3
; F[T1] = T2
lda T2
sta EXPR_SAVE
ldx T1
lda EXPR_SAVE
sta F0,x
; I[T1] = T3
lda T3
sta EXPR_SAVE
ldx T1
lda EXPR_SAVE
sta I0,x
; T1 = T1 + 1
lda T1
clc
adc #1
sta T1
jmp stable_compaction_L13
stable_compaction_L12:
stable_compaction_L13:
; T0 = T0 + 1
lda T0
clc
adc #1
sta T0
jmp stable_compaction_L6
stable_compaction_L8:
; COUNT = T1
lda T1
sta COUNT
; REMOVED = T4 - T1
lda T4
sec
sbc T1
sta REMOVED
; while T1 < T4:
stable_compaction_L16:
lda T1
cmp T4
bcc stable_compaction_L19
stable_compaction_L20:
jmp stable_compaction_L18
stable_compaction_L19:
jmp stable_compaction_L17
stable_compaction_L17:
; F[T1] = 0
lda #0
sta EXPR_SAVE
ldx T1
lda EXPR_SAVE
sta F0,x
; I[T1] = 0
lda #0
sta EXPR_SAVE
ldx T1
lda EXPR_SAVE
sta I0,x
; T1 = T1 + 1
lda T1
clc
adc #1
sta T1
jmp stable_compaction_L16
stable_compaction_L18:
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
COUNT=$80
REMOVED=$81
VALID=$82
F0=$83
F1=$84
F2=$85
F3=$86
F4=$87
F5=$88
F6=$89
F7=$8a
I0=$8b
I1=$8c
I2=$8d
I3=$8e
I4=$8f
I5=$90
I6=$91
I7=$92
T0=$93
T1=$94
T2=$95
T3=$96
T4=$97
EXPR_SAVE=$98
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
preview_rules:
!if CUSTOM {
; COUNT = 4
lda #4
sta COUNT
; F1 = 1
lda #1
sta F1
; F3 = 1
lda #1
sta F3
; I0 = 20
lda #20
sta I0
; I1 = 21
lda #21
sta I1
; I2 = 22
lda #22
sta I2
; I3 = 23
lda #23
sta I3
; I4 = 24
lda #24
sta I4
; I5 = 25
lda #25
sta I5
; I6 = 26
lda #26
sta I6
; I7 = 27
lda #27
sta I7
!if BROKEN {
; COUNT = 8
lda #8
sta COUNT
}
jsr checked_step
jsr emit_state
rts
} else {
; COUNT = 6
lda #6
sta COUNT
; F0 = 1
lda #1
sta F0
; F2 = 1
lda #1
sta F2
; F4 = 1
lda #1
sta F4
; F7 = 1
lda #1
sta F7
; I0 = 10
lda #10
sta I0
; I1 = 11
lda #11
sta I1
; I2 = 12
lda #12
sta I2
; I3 = 13
lda #13
sta I3
; I4 = 14
lda #14
sta I4
; I5 = 15
lda #15
sta I5
; I6 = 16
lda #16
sta I6
; I7 = 99
lda #99
sta I7
!if BROKEN {
; COUNT = 8
lda #8
sta COUNT
}
jsr checked_step
jsr emit_state
}
rtsCustomise it
Compact a shorter prefix with a different live/dead arrangement. 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
COUNT=$80
REMOVED=$81
VALID=$82
F0=$83
F1=$84
F2=$85
F3=$86
F4=$87
F5=$88
F6=$89
F7=$8a
I0=$8b
I1=$8c
I2=$8d
I3=$8e
I4=$8f
I5=$90
I6=$91
I7=$92
T0=$93
T1=$94
T2=$95
T3=$96
T4=$97
EXPR_SAVE=$98
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 02 01 01 01 00 00 00. The full client trace in the source explains how it gets there.
Mistake and repair
The wrong caller uses capacity eight as the live count. A flagged item beyond the owned live prefix is pulled into the compacted set. Pass the actual live count, and preserve the meaning of unused capacity.
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
COUNT=$80
REMOVED=$81
VALID=$82
F0=$83
F1=$84
F2=$85
F3=$86
F4=$87
F5=$88
F6=$89
F7=$8a
I0=$8b
I1=$8c
I2=$8d
I3=$8e
I4=$8f
I5=$90
I6=$91
I7=$92
T0=$93
T1=$94
T2=$95
T3=$96
T4=$97
EXPR_SAVE=$98
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:
Cost and alternatives
The measured helper extent is 225 code bytes; public state is 19 bytes and private scratch is 6 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 eight visits and eight paired moves, plus clearing vacated entries. A sparse fixed-slot pool avoids relocation but makes consumers scan or maintain a separate active list.
Compaction changes storage indices. External index references, sorted draw lists and parallel arrays must be updated or replaced with stable IDs/handles. Do not silently compact a pool whose public contract promises stable slot indices.
Execution evidence
Four variants run on PAL stock-ROM PRG, using Asm198x 0.0.72, Emu198x 0.44.2. Each executes 52 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: Stable slot identity and dense storage solve different problems; choose how references survive before moving records.