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:
lda #0
sta SLOT
sta RESULT
sta VALID
sta USED
sta MODE
sta REQUEST
sta T0
sta T1
sta T2
rts
bitset_slot_allocation_step:
; SLOT = 255
lda #255
sta SLOT
; RESULT = 0
lda #0
sta RESULT
; VALID = 0
lda #0
sta VALID
; if MODE > 1:
lda MODE
cmp #1
beq bitset_slot_allocation_L5
bcs bitset_slot_allocation_L4
bitset_slot_allocation_L5:
jmp bitset_slot_allocation_L2
bitset_slot_allocation_L4:
jmp bitset_slot_allocation_L1
bitset_slot_allocation_L1:
; return
rts
jmp bitset_slot_allocation_L3
bitset_slot_allocation_L2:
bitset_slot_allocation_L3:
; if MODE == 1:
lda MODE
cmp #1
beq bitset_slot_allocation_L9
bitset_slot_allocation_L10:
jmp bitset_slot_allocation_L7
bitset_slot_allocation_L9:
jmp bitset_slot_allocation_L6
bitset_slot_allocation_L6:
; if REQUEST >= 8:
lda REQUEST
cmp #8
bcs bitset_slot_allocation_L14
bitset_slot_allocation_L15:
jmp bitset_slot_allocation_L12
bitset_slot_allocation_L14:
jmp bitset_slot_allocation_L11
bitset_slot_allocation_L11:
; return
rts
jmp bitset_slot_allocation_L13
bitset_slot_allocation_L12:
bitset_slot_allocation_L13:
jmp bitset_slot_allocation_L8
bitset_slot_allocation_L7:
bitset_slot_allocation_L8:
; VALID = 1
lda #1
sta VALID
; T0 = 0
lda #0
sta T0
; T1 = 1
lda #1
sta T1
; if MODE == 0:
lda MODE
cmp #0
beq bitset_slot_allocation_L19
bitset_slot_allocation_L20:
jmp bitset_slot_allocation_L17
bitset_slot_allocation_L19:
jmp bitset_slot_allocation_L16
bitset_slot_allocation_L16:
; while T0 < 8:
bitset_slot_allocation_L21:
lda T0
cmp #8
bcc bitset_slot_allocation_L24
bitset_slot_allocation_L25:
jmp bitset_slot_allocation_L23
bitset_slot_allocation_L24:
jmp bitset_slot_allocation_L22
bitset_slot_allocation_L22:
; T2 = USED & T1
lda USED
and T1
sta T2
; if T2 == 0:
lda T2
cmp #0
beq bitset_slot_allocation_L29
bitset_slot_allocation_L30:
jmp bitset_slot_allocation_L27
bitset_slot_allocation_L29:
jmp bitset_slot_allocation_L26
bitset_slot_allocation_L26:
; USED = USED | T1
lda USED
ora T1
sta USED
; SLOT = T0
lda T0
sta SLOT
; RESULT = 1
lda #1
sta RESULT
; return
rts
jmp bitset_slot_allocation_L28
bitset_slot_allocation_L27:
bitset_slot_allocation_L28:
; T0 = T0 + 1
lda T0
clc
adc #1
sta T0
; T1 = T1 << 1
lda T1
asl
sta T1
jmp bitset_slot_allocation_L21
bitset_slot_allocation_L23:
jmp bitset_slot_allocation_L18
bitset_slot_allocation_L17:
; T0 = REQUEST
lda REQUEST
sta T0
; while T0 != 0:
bitset_slot_allocation_L31:
lda T0
cmp #0
bne bitset_slot_allocation_L34
bitset_slot_allocation_L35:
jmp bitset_slot_allocation_L33
bitset_slot_allocation_L34:
jmp bitset_slot_allocation_L32
bitset_slot_allocation_L32:
; T1 = T1 << 1
lda T1
asl
sta T1
; T0 = T0 - 1
lda T0
sec
sbc #1
sta T0
jmp bitset_slot_allocation_L31
bitset_slot_allocation_L33:
; T2 = USED & T1
lda USED
and T1
sta T2
; if T2 != 0:
lda T2
cmp #0
bne bitset_slot_allocation_L39
bitset_slot_allocation_L40:
jmp bitset_slot_allocation_L37
bitset_slot_allocation_L39:
jmp bitset_slot_allocation_L36
bitset_slot_allocation_L36:
; T1 = T1 ^ 255
lda T1
eor #255
sta T1
; USED = USED & T1
lda USED
and T1
sta USED
; SLOT = REQUEST
lda REQUEST
sta SLOT
; RESULT = 1
lda #1
sta RESULT
jmp bitset_slot_allocation_L38
bitset_slot_allocation_L37:
bitset_slot_allocation_L38:
bitset_slot_allocation_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
SLOT=$80
RESULT=$81
VALID=$82
USED=$83
MODE=$84
REQUEST=$85
T0=$86
T1=$87
T2=$88
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 {
; USED = 254
lda #254
sta USED
!if BROKEN {
; USED = 0
lda #0
sta USED
}
jsr checked_step
jsr emit_state
!if BROKEN {
; USED = 0
lda #0
sta USED
}
jsr checked_step
jsr emit_state
; MODE = 1
lda #1
sta MODE
; REQUEST = 7
lda #7
sta REQUEST
!if BROKEN {
; USED = 0
lda #0
sta USED
}
jsr checked_step
jsr emit_state
rts
} else {
; USED = 3
lda #3
sta USED
!if BROKEN {
; USED = 0
lda #0
sta USED
}
jsr checked_step
jsr emit_state
!if BROKEN {
; USED = 0
lda #0
sta USED
}
jsr checked_step
jsr emit_state
; MODE = 1
lda #1
sta MODE
; REQUEST = 1
lda #1
sta REQUEST
!if BROKEN {
; USED = 0
lda #0
sta USED
}
jsr checked_step
jsr emit_state
}
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
SLOT=$80
RESULT=$81
VALID=$82
USED=$83
MODE=$84
REQUEST=$85
T0=$86
T1=$87
T2=$88
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 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
SLOT=$80
RESULT=$81
VALID=$82
USED=$83
MODE=$84
REQUEST=$85
T0=$86
T1=$87
T2=$88
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 245 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 PAL stock-ROM PRG, using Asm198x 0.0.72, Emu198x 0.44.2. 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.