Write Packed Bits with Bounds on Spectrum | Pattern Library

Flags and short fields can share a compact record without overwriting their neighbours.

Spectrum shows the working diagnostic row 01 0B 05 15 FE BF 00 00. — A completed native caller displays 01 0B 05 15 FE BF 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

Write COUNT 0–8 low bits of VALUE into a four-byte buffer, most-significant selected bit first. CURSOR counts bits from the most significant bit of B0. Require CURSOR≤32 and CURSOR+COUNT≤32. Success advances CURSOR; failure leaves cursor and buffer unchanged and returns VALID=0.

Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: VALID, CURSOR, COUNT, VALUE, B0, B1, B2, B3. They occupy 8 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 bounded_bit_writer_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 bounded_bit_writer_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	CURSOR and B0..B3
Supply before service	COUNT and VALUE
Read after service	VALID and updated CURSOR/buffer

Validate the full span first. Each bit clears only its destination mask, then conditionally sets it, preserving the surrounding packed fields.

Reusable initialisation and service

component.inc

; SPDX-License-Identifier: MIT
; Write Packed Bits with Bounds. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
bounded_bit_writer_init:
    ld a,0
    ld (VALID),a
    ld (CURSOR),a
    ld (COUNT),a
    ld (VALUE),a
    ld (B0),a
    ld (B1),a
    ld (B2),a
    ld (B3),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
bounded_bit_writer_step:
; VALID = 0
    ld a,0
    ld (VALID),a
; if COUNT > 8:
    ld a,(COUNT)
    cp 8
    jp z,bounded_bit_writer_L2
    jp nc,bounded_bit_writer_L1
    jp bounded_bit_writer_L2
bounded_bit_writer_L1:
; return
    ret
    jp bounded_bit_writer_L3
bounded_bit_writer_L2:
bounded_bit_writer_L3:
; if CURSOR > 32:
    ld a,(CURSOR)
    cp 32
    jp z,bounded_bit_writer_L5
    jp nc,bounded_bit_writer_L4
    jp bounded_bit_writer_L5
bounded_bit_writer_L4:
; return
    ret
    jp bounded_bit_writer_L6
bounded_bit_writer_L5:
bounded_bit_writer_L6:
; T0 = CURSOR + COUNT
    ld a,(CURSOR)
    ld b,a
    ld a,(COUNT)
    ld c,a
    ld a,b
    add a,c
    ld (T0),a
; if T0 > 32:
    ld a,(T0)
    cp 32
    jp z,bounded_bit_writer_L8
    jp nc,bounded_bit_writer_L7
    jp bounded_bit_writer_L8
bounded_bit_writer_L7:
; return
    ret
    jp bounded_bit_writer_L9
bounded_bit_writer_L8:
bounded_bit_writer_L9:
; VALID = 1
    ld a,1
    ld (VALID),a
; T0 = COUNT
    ld a,(COUNT)
    ld (T0),a
; while T0 != 0:
bounded_bit_writer_L10:
    ld a,(T0)
    cp 0
    jp nz,bounded_bit_writer_L11
    jp bounded_bit_writer_L12
bounded_bit_writer_L11:
; T0 = T0 - 1
    ld a,(T0)
    sub 1
    ld (T0),a
; T1 = VALUE
    ld a,(VALUE)
    ld (T1),a
; T2 = T0
    ld a,(T0)
    ld (T2),a
; while T2 != 0:
bounded_bit_writer_L13:
    ld a,(T2)
    cp 0
    jp nz,bounded_bit_writer_L14
    jp bounded_bit_writer_L15
bounded_bit_writer_L14:
; T1 = T1 >> 1
    ld a,(T1)
    srl a
    ld (T1),a
; T2 = T2 - 1
    ld a,(T2)
    sub 1
    ld (T2),a
    jp bounded_bit_writer_L13
bounded_bit_writer_L15:
; T1 = T1 & 1
    ld a,(T1)
    and 1
    ld (T1),a
; T2 = CURSOR >> 3
    ld a,(CURSOR)
    srl a
    srl a
    srl a
    ld (T2),a
; T3 = CURSOR & 7
    ld a,(CURSOR)
    and 7
    ld (T3),a
; T4 = 128
    ld a,128
    ld (T4),a
; while T3 != 0:
bounded_bit_writer_L16:
    ld a,(T3)
    cp 0
    jp nz,bounded_bit_writer_L17
    jp bounded_bit_writer_L18
bounded_bit_writer_L17:
; T4 = T4 >> 1
    ld a,(T4)
    srl a
    ld (T4),a
; T3 = T3 - 1
    ld a,(T3)
    sub 1
    ld (T3),a
    jp bounded_bit_writer_L16
bounded_bit_writer_L18:
; T5 = T4 ^ 255
    ld a,(T4)
    xor 255
    ld (T5),a
; T6 = B[T2]
    ld a,(T2)
    ld e,a
    ld d,0
    ld hl,B0
    add hl,de
    ld a,(hl)
    ld (T6),a
; T6 = T6 & T5
    ld a,(T6)
    ld b,a
    ld a,(T5)
    ld c,a
    ld a,b
    and c
    ld (T6),a
; if T1 != 0:
    ld a,(T1)
    cp 0
    jp nz,bounded_bit_writer_L19
    jp bounded_bit_writer_L20
bounded_bit_writer_L19:
; T6 = T6 | T4
    ld a,(T6)
    ld b,a
    ld a,(T4)
    ld c,a
    ld a,b
    or c
    ld (T6),a
    jp bounded_bit_writer_L21
bounded_bit_writer_L20:
bounded_bit_writer_L21:
; B[T2] = T6
    ld a,(T6)
    ld c,a
    ld a,(T2)
    ld e,a
    ld d,0
    ld hl,B0
    add hl,de
    ld (hl),c
; CURSOR = CURSOR + 1
    ld a,(CURSOR)
    add a,1
    ld (CURSOR),a
    jp bounded_bit_writer_L10
bounded_bit_writer_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:
VALID: defb 0
CURSOR: defb 0
COUNT: defb 0
VALUE: defb 0
B0: defb 0
B1: defb 0
B2: defb 0
B3: 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

A client retaining state between calls

exercise.inc, excerpt

preview_rules:
    if CUSTOM
; COUNT = 8
    ld a,8
    ld (COUNT),a
; VALUE = 165
    ld a,165
    ld (VALUE),a
; CURSOR = 24
    ld a,24
    ld (CURSOR),a
    if BROKEN
; CURSOR = 0
    ld a,0
    ld (CURSOR),a
    endif
    call checked_step
    call emit_state
    ret
    else
; COUNT = 5
    ld a,5
    ld (COUNT),a
; VALUE = 21
    ld a,21
    ld (VALUE),a
; CURSOR = 6
    ld a,6
    ld (CURSOR),a
; B0 = 255
    ld a,255
    ld (B0),a
; B1 = 255
    ld a,255
    ld (B1),a
    if BROKEN
; CURSOR = 0
    ld a,0
    ld (CURSOR),a
    endif
    call checked_step
    call emit_state
    endif
    ret

Customise it

Write a full byte at the final legal offset. 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:
VALID: defb 0
CURSOR: defb 0
COUNT: defb 0
VALUE: defb 0
B0: defb 0
B1: defb 0
B2: defb 0
B3: 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

The custom diagnostic row is 01 20 08 A5 00 00 00 A5. The full client trace in the source explains how it gets there.

Mistake and repair

The wrong caller resets the bit cursor, overwriting the start of the record. Retain the cursor across fields and check VALID after every write.

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:
VALID: defb 0
CURSOR: defb 0
COUNT: defb 0
VALUE: defb 0
B0: defb 0
B1: defb 0
B2: defb 0
B3: 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

Spectrum shows the deliberate error row 01 05 05 15 AF FF 00 00. — The wrong client completes with 01 05 05 15 AF FF 00 00; its changed inputs and state explain the result.

Cost and alternatives

The measured helper extent is 347 code bytes; public state is 8 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 eight bit writes. Aligned whole-byte writes are faster; this bit-at-a-time path supports fields crossing a byte boundary.

Only the low COUNT bits are encoded; higher VALUE bits are deliberately discarded. The helper is not a general compression format.

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

Bounded bit readers: The reader consumes the same most-significant-first bit stream.

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