Eight occupied flags can fit in one byte when objects already live in fixed slots.
Use it in a project
USED is an eight-bit occupied mask. MODE=0 allocates the lowest free slot; MODE=1 releases REQUEST 0–7. Return RESULT=1 only for a successful allocation or release, SLOT for that slot, otherwise SLOT=255. Invalid mode or release index returns VALID=0 without changing USED.
Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: SLOT, RESULT, VALID, USED, MODE, REQUEST. They occupy 6 contiguous bytes. Also reserve the private workspace: T0, T1, T2 (3 bytes). Array names in the explanation refer to the numbered consecutive bindings, such as C0, C1 and so on.
Call bitset_slot_allocation_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 bitset_slot_allocation_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 | USED |
| Supply before service | MODE and REQUEST |
| Read after service | SLOT, RESULT, VALID and updated USED |
Walk a one-bit mask from the lowest slot upwards. Release clears one occupied bit; freeing an already free slot reports no change.
Reusable initialisation and service
; SPDX-License-Identifier: MIT
; Allocate Slots with a Bitset. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
bitset_slot_allocation_init:
move.b #0,d0
move.b d0,SLOT
move.b d0,RESULT
move.b d0,VALID
move.b d0,USED
move.b d0,MODE
move.b d0,REQUEST
move.b d0,T0
move.b d0,T1
move.b d0,T2
rts
bitset_slot_allocation_step:
; SLOT = 255
move.b #255,d0
move.b d0,SLOT
; RESULT = 0
move.b #0,d0
move.b d0,RESULT
; VALID = 0
move.b #0,d0
move.b d0,VALID
; if MODE > 1:
move.b MODE,d0
cmp.b #1,d0
bhi bitset_slot_allocation_L1
jmp bitset_slot_allocation_L2
bitset_slot_allocation_L1:
; return
rts
jmp bitset_slot_allocation_L3
bitset_slot_allocation_L2:
bitset_slot_allocation_L3:
; if MODE == 1:
move.b MODE,d0
cmp.b #1,d0
beq bitset_slot_allocation_L4
jmp bitset_slot_allocation_L5
bitset_slot_allocation_L4:
; if REQUEST >= 8:
move.b REQUEST,d0
cmp.b #8,d0
bcc bitset_slot_allocation_L7
jmp bitset_slot_allocation_L8
bitset_slot_allocation_L7:
; return
rts
jmp bitset_slot_allocation_L9
bitset_slot_allocation_L8:
bitset_slot_allocation_L9:
jmp bitset_slot_allocation_L6
bitset_slot_allocation_L5:
bitset_slot_allocation_L6:
; VALID = 1
move.b #1,d0
move.b d0,VALID
; T0 = 0
move.b #0,d0
move.b d0,T0
; T1 = 1
move.b #1,d0
move.b d0,T1
; if MODE == 0:
move.b MODE,d0
cmp.b #0,d0
beq bitset_slot_allocation_L10
jmp bitset_slot_allocation_L11
bitset_slot_allocation_L10:
; while T0 < 8:
bitset_slot_allocation_L13:
move.b T0,d0
cmp.b #8,d0
bcs bitset_slot_allocation_L14
jmp bitset_slot_allocation_L15
bitset_slot_allocation_L14:
; T2 = USED & T1
move.b USED,d0
move.b T1,d1
and.b d1,d0
move.b d0,T2
; if T2 == 0:
move.b T2,d0
cmp.b #0,d0
beq bitset_slot_allocation_L16
jmp bitset_slot_allocation_L17
bitset_slot_allocation_L16:
; USED = USED | T1
move.b USED,d0
move.b T1,d1
or.b d1,d0
move.b d0,USED
; SLOT = T0
move.b T0,d0
move.b d0,SLOT
; RESULT = 1
move.b #1,d0
move.b d0,RESULT
; return
rts
jmp bitset_slot_allocation_L18
bitset_slot_allocation_L17:
bitset_slot_allocation_L18:
; T0 = T0 + 1
move.b T0,d0
add.b #1,d0
move.b d0,T0
; T1 = T1 << 1
move.b T1,d0
lsl.b #1,d0
move.b d0,T1
jmp bitset_slot_allocation_L13
bitset_slot_allocation_L15:
jmp bitset_slot_allocation_L12
bitset_slot_allocation_L11:
; T0 = REQUEST
move.b REQUEST,d0
move.b d0,T0
; while T0 != 0:
bitset_slot_allocation_L19:
move.b T0,d0
cmp.b #0,d0
bne bitset_slot_allocation_L20
jmp bitset_slot_allocation_L21
bitset_slot_allocation_L20:
; T1 = T1 << 1
move.b T1,d0
lsl.b #1,d0
move.b d0,T1
; T0 = T0 - 1
move.b T0,d0
sub.b #1,d0
move.b d0,T0
jmp bitset_slot_allocation_L19
bitset_slot_allocation_L21:
; T2 = USED & T1
move.b USED,d0
move.b T1,d1
and.b d1,d0
move.b d0,T2
; if T2 != 0:
move.b T2,d0
cmp.b #0,d0
bne bitset_slot_allocation_L22
jmp bitset_slot_allocation_L23
bitset_slot_allocation_L22:
; T1 = T1 ^ 255
move.b T1,d0
eor.b #255,d0
move.b d0,T1
; USED = USED & T1
move.b USED,d0
move.b T1,d1
and.b d1,d0
move.b d0,USED
; SLOT = REQUEST
move.b REQUEST,d0
move.b d0,SLOT
; RESULT = 1
move.b #1,d0
move.b d0,RESULT
jmp bitset_slot_allocation_L24
bitset_slot_allocation_L23:
bitset_slot_allocation_L24:
bitset_slot_allocation_L12:
rts
component_end:Machine contract
D0–D3/A0–A1/CCR may change; D4–D7/A2–A6 are preserved. A direct BSR/JSR needs four 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 A500 OCS diagnostic cold-boots an OFS disk, owns the Copper and bitplane and disables interrupt/DMA sources. The helper does not take over hardware or allocate Chip RAM.
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 and BUILD198X=/path/to/build198x. The Makefile produces normal, custom, deliberately wrong and repaired programs. Tools must already be installed; the sample downloads nothing.
Bindings and include order
section code,code_c
CUSTOM = 0
BROKEN = 0
COYOTE_STEPS = 4
BUFFER_STEPS = 4
TEXT_CAP = 8
PROPERTY_COUNT = 16
STREAM_BYTES = 13
CRC_POLY = 7
include "caller.inc"
include "exercise.inc"
component_start:
include "component.inc"
even
include "data.inc"
include "cases.inc"
even
include "display.inc"
include "../../framework/turn-deadline/font.inc"
code_end:
STATE:
SLOT: dc.b 0
RESULT: dc.b 0
VALID: dc.b 0
USED: dc.b 0
MODE: dc.b 0
REQUEST: dc.b 0
T0: dc.b 0
T1: dc.b 0
T2: dc.b 0
even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
even
signature: dc.b "198XTOOLKIT0000",0
even
copperlist:
dc.w $08e,$2c81,$090,$2cc1,$092,$0038,$094,$00d0
dc.w $100,$1200,$102,0,$104,0,$108,0,$10a,0
dc.w $180,$012,$182,$fff
copbpl:
dc.w $0e0,0,$0e2,0
dc.w $ffff,$fffe
plane: dcb.b 10240,0A client retaining state between calls
preview_rules:
ifne CUSTOM
; USED = 254
move.b #254,d0
move.b d0,USED
ifne BROKEN
; USED = 0
move.b #0,d0
move.b d0,USED
endif
bsr checked_step
bsr emit_state
ifne BROKEN
; USED = 0
move.b #0,d0
move.b d0,USED
endif
bsr checked_step
bsr emit_state
; MODE = 1
move.b #1,d0
move.b d0,MODE
; REQUEST = 7
move.b #7,d0
move.b d0,REQUEST
ifne BROKEN
; USED = 0
move.b #0,d0
move.b d0,USED
endif
bsr checked_step
bsr emit_state
rts
else
; USED = 3
move.b #3,d0
move.b d0,USED
ifne BROKEN
; USED = 0
move.b #0,d0
move.b d0,USED
endif
bsr checked_step
bsr emit_state
ifne BROKEN
; USED = 0
move.b #0,d0
move.b d0,USED
endif
bsr checked_step
bsr emit_state
; MODE = 1
move.b #1,d0
move.b d0,MODE
; REQUEST = 1
move.b #1,d0
move.b d0,REQUEST
ifne BROKEN
; USED = 0
move.b #0,d0
move.b d0,USED
endif
bsr checked_step
bsr emit_state
endif
rtsCustomise it
Start with only slot zero free, then test full allocation and release. 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
section code,code_c
CUSTOM = 1
BROKEN = 0
COYOTE_STEPS = 2
BUFFER_STEPS = 2
TEXT_CAP = 4
PROPERTY_COUNT = 16
STREAM_BYTES = 13
CRC_POLY = 49
include "caller.inc"
include "exercise.inc"
component_start:
include "component.inc"
even
include "data.inc"
include "cases.inc"
even
include "display.inc"
include "../../framework/turn-deadline/font.inc"
code_end:
STATE:
SLOT: dc.b 0
RESULT: dc.b 0
VALID: dc.b 0
USED: dc.b 0
MODE: dc.b 0
REQUEST: dc.b 0
T0: dc.b 0
T1: dc.b 0
T2: dc.b 0
even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
even
signature: dc.b "198XTOOLKIT0000",0
even
copperlist:
dc.w $08e,$2c81,$090,$2cc1,$092,$0038,$094,$00d0
dc.w $100,$1200,$102,0,$104,0,$108,0,$10a,0
dc.w $180,$012,$182,$fff
copbpl:
dc.w $0e0,0,$0e2,0
dc.w $ffff,$fffe
plane: dcb.b 10240,0The custom diagnostic row is 07 01 01 7F 01 07 00 00. The full client trace in the source explains how it gets there.
Mistake and repair
The wrong caller clears occupancy before service and can allocate the same live slot twice. Initialise once; retain USED until the object is actually released.
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
section code,code_c
CUSTOM = 0
BROKEN = 1
COYOTE_STEPS = 4
BUFFER_STEPS = 4
TEXT_CAP = 8
PROPERTY_COUNT = 16
STREAM_BYTES = 13
CRC_POLY = 7
include "caller.inc"
include "exercise.inc"
component_start:
include "component.inc"
even
include "data.inc"
include "cases.inc"
even
include "display.inc"
include "../../framework/turn-deadline/font.inc"
code_end:
STATE:
SLOT: dc.b 0
RESULT: dc.b 0
VALID: dc.b 0
USED: dc.b 0
MODE: dc.b 0
REQUEST: dc.b 0
T0: dc.b 0
T1: dc.b 0
T2: dc.b 0
even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
even
signature: dc.b "198XTOOLKIT0000",0
even
copperlist:
dc.w $08e,$2c81,$090,$2cc1,$092,$0038,$094,$00d0
dc.w $100,$1200,$102,0,$104,0,$108,0,$10a,0
dc.w $180,$012,$182,$fff
copbpl:
dc.w $0e0,0,$0e2,0
dc.w $ffff,$fffe
plane: dcb.b 10240,0
Cost and alternatives
The measured helper extent is 440 code bytes; public state is 6 bytes and private scratch is 3 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 mask tests and one persistent byte. A free-list scales better for a much larger pool.
This stores occupancy only. Generation handles are still needed to detect stale references after slot reuse.
Execution evidence
Four variants run on A500 OCS PAL, Kickstart1.3 OFS cold boot, using Asm198x 0.0.72, Emu198x 0.44.2 and Build198x 0.2.9. Each executes 64 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: Occupancy says a slot is live; a generation distinguishes successive occupants.