Allocate Slots with a Bitset on Spectrum | Pattern Library

Eight occupied flags can fit in one byte when objects already live in fixed slots.

Spectrum shows the working diagnostic row 01 01 01 0D 01 01 00 00. — A completed native caller displays 01 01 01 0D 01 01 00 00. The public field order below identifies these bytes; the capture is a state diagnostic, not a recording of a finished game.

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

component.inc

; 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:
    ld a,0
    ld (SLOT),a
    ld (RESULT),a
    ld (VALID),a
    ld (USED),a
    ld (MODE),a
    ld (REQUEST),a
    ld (T0),a
    ld (T1),a
    ld (T2),a
    ret
bitset_slot_allocation_step:
; SLOT = 255
    ld a,255
    ld (SLOT),a
; RESULT = 0
    ld a,0
    ld (RESULT),a
; VALID = 0
    ld a,0
    ld (VALID),a
; if MODE > 1:
    ld a,(MODE)
    cp 1
    jp z,bitset_slot_allocation_L2
    jp nc,bitset_slot_allocation_L1
    jp bitset_slot_allocation_L2
bitset_slot_allocation_L1:
; return
    ret
    jp bitset_slot_allocation_L3
bitset_slot_allocation_L2:
bitset_slot_allocation_L3:
; if MODE == 1:
    ld a,(MODE)
    cp 1
    jp z,bitset_slot_allocation_L4
    jp bitset_slot_allocation_L5
bitset_slot_allocation_L4:
; if REQUEST >= 8:
    ld a,(REQUEST)
    cp 8
    jp nc,bitset_slot_allocation_L7
    jp bitset_slot_allocation_L8
bitset_slot_allocation_L7:
; return
    ret
    jp bitset_slot_allocation_L9
bitset_slot_allocation_L8:
bitset_slot_allocation_L9:
    jp bitset_slot_allocation_L6
bitset_slot_allocation_L5:
bitset_slot_allocation_L6:
; VALID = 1
    ld a,1
    ld (VALID),a
; T0 = 0
    ld a,0
    ld (T0),a
; T1 = 1
    ld a,1
    ld (T1),a
; if MODE == 0:
    ld a,(MODE)
    cp 0
    jp z,bitset_slot_allocation_L10
    jp bitset_slot_allocation_L11
bitset_slot_allocation_L10:
; while T0 < 8:
bitset_slot_allocation_L13:
    ld a,(T0)
    cp 8
    jp c,bitset_slot_allocation_L14
    jp bitset_slot_allocation_L15
bitset_slot_allocation_L14:
; T2 = USED & T1
    ld a,(USED)
    ld b,a
    ld a,(T1)
    ld c,a
    ld a,b
    and c
    ld (T2),a
; if T2 == 0:
    ld a,(T2)
    cp 0
    jp z,bitset_slot_allocation_L16
    jp bitset_slot_allocation_L17
bitset_slot_allocation_L16:
; USED = USED | T1
    ld a,(USED)
    ld b,a
    ld a,(T1)
    ld c,a
    ld a,b
    or c
    ld (USED),a
; SLOT = T0
    ld a,(T0)
    ld (SLOT),a
; RESULT = 1
    ld a,1
    ld (RESULT),a
; return
    ret
    jp bitset_slot_allocation_L18
bitset_slot_allocation_L17:
bitset_slot_allocation_L18:
; T0 = T0 + 1
    ld a,(T0)
    add a,1
    ld (T0),a
; T1 = T1 << 1
    ld a,(T1)
    sla a
    ld (T1),a
    jp bitset_slot_allocation_L13
bitset_slot_allocation_L15:
    jp bitset_slot_allocation_L12
bitset_slot_allocation_L11:
; T0 = REQUEST
    ld a,(REQUEST)
    ld (T0),a
; while T0 != 0:
bitset_slot_allocation_L19:
    ld a,(T0)
    cp 0
    jp nz,bitset_slot_allocation_L20
    jp bitset_slot_allocation_L21
bitset_slot_allocation_L20:
; T1 = T1 << 1
    ld a,(T1)
    sla a
    ld (T1),a
; T0 = T0 - 1
    ld a,(T0)
    sub 1
    ld (T0),a
    jp bitset_slot_allocation_L19
bitset_slot_allocation_L21:
; T2 = USED & T1
    ld a,(USED)
    ld b,a
    ld a,(T1)
    ld c,a
    ld a,b
    and c
    ld (T2),a
; if T2 != 0:
    ld a,(T2)
    cp 0
    jp nz,bitset_slot_allocation_L22
    jp bitset_slot_allocation_L23
bitset_slot_allocation_L22:
; T1 = T1 ^ 255
    ld a,(T1)
    xor 255
    ld (T1),a
; USED = USED & T1
    ld a,(USED)
    ld b,a
    ld a,(T1)
    ld c,a
    ld a,b
    and c
    ld (USED),a
; SLOT = REQUEST
    ld a,(REQUEST)
    ld (SLOT),a
; RESULT = 1
    ld a,1
    ld (RESULT),a
    jp bitset_slot_allocation_L24
bitset_slot_allocation_L23:
bitset_slot_allocation_L24:
bitset_slot_allocation_L12:
    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

demo.asm

    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:
SLOT: defb 0
RESULT: defb 0
VALID: defb 0
USED: defb 0
MODE: defb 0
REQUEST: defb 0
T0: defb 0
T1: defb 0
T2: 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

A client retaining state between calls

exercise.inc, excerpt

preview_rules:
    if CUSTOM
; USED = 254
    ld a,254
    ld (USED),a
    if BROKEN
; USED = 0
    ld a,0
    ld (USED),a
    endif
    call checked_step
    call emit_state

    if BROKEN
; USED = 0
    ld a,0
    ld (USED),a
    endif
    call checked_step
    call emit_state
; MODE = 1
    ld a,1
    ld (MODE),a
; REQUEST = 7
    ld a,7
    ld (REQUEST),a
    if BROKEN
; USED = 0
    ld a,0
    ld (USED),a
    endif
    call checked_step
    call emit_state
    ret
    else
; USED = 3
    ld a,3
    ld (USED),a
    if BROKEN
; USED = 0
    ld a,0
    ld (USED),a
    endif
    call checked_step
    call emit_state

    if BROKEN
; USED = 0
    ld a,0
    ld (USED),a
    endif
    call checked_step
    call emit_state
; MODE = 1
    ld a,1
    ld (MODE),a
; REQUEST = 1
    ld a,1
    ld (REQUEST),a
    if BROKEN
; USED = 0
    ld a,0
    ld (USED),a
    endif
    call checked_step
    call emit_state
    endif
    ret

Customise 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

custom.asm

    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:
SLOT: defb 0
RESULT: defb 0
VALID: defb 0
USED: defb 0
MODE: defb 0
REQUEST: defb 0
T0: defb 0
T1: defb 0
T2: 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

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

wrong.asm

    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:
SLOT: defb 0
RESULT: defb 0
VALID: defb 0
USED: defb 0
MODE: defb 0
REQUEST: defb 0
T0: defb 0
T1: defb 0
T2: 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

Spectrum shows the deliberate error row FF 00 01 00 01 01 00 00. — The wrong client completes with FF 00 01 00 01 01 00 00; its changed inputs and state explain the result.

Cost and alternatives

The measured helper extent is 299 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 48K PAL ROM tape load and USR, 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.

The Craft · Maths for Games · Source and native evidence.