Write Packed Bits with Bounds on NES | Pattern Library

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

NES 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

6502.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:
    lda #0
    sta VALID
    sta CURSOR
    sta COUNT
    sta VALUE
    sta B0
    sta B1
    sta B2
    sta B3
    sta T0
    sta T1
    sta T2
    sta T3
    sta T4
    sta T5
    sta T6
    sta EXPR_SAVE
    rts
bounded_bit_writer_step:
; VALID = 0
    lda #0
    sta VALID
; if COUNT > 8:
    lda COUNT
    cmp #8
    beq bounded_bit_writer_L5
    bcs bounded_bit_writer_L4
bounded_bit_writer_L5:
    jmp bounded_bit_writer_L2
bounded_bit_writer_L4:
    jmp bounded_bit_writer_L1
bounded_bit_writer_L1:
; return
    rts
    jmp bounded_bit_writer_L3
bounded_bit_writer_L2:
bounded_bit_writer_L3:
; if CURSOR > 32:
    lda CURSOR
    cmp #32
    beq bounded_bit_writer_L10
    bcs bounded_bit_writer_L9
bounded_bit_writer_L10:
    jmp bounded_bit_writer_L7
bounded_bit_writer_L9:
    jmp bounded_bit_writer_L6
bounded_bit_writer_L6:
; return
    rts
    jmp bounded_bit_writer_L8
bounded_bit_writer_L7:
bounded_bit_writer_L8:
; T0 = CURSOR + COUNT
    lda CURSOR
    clc
    adc COUNT
    sta T0
; if T0 > 32:
    lda T0
    cmp #32
    beq bounded_bit_writer_L15
    bcs bounded_bit_writer_L14
bounded_bit_writer_L15:
    jmp bounded_bit_writer_L12
bounded_bit_writer_L14:
    jmp bounded_bit_writer_L11
bounded_bit_writer_L11:
; return
    rts
    jmp bounded_bit_writer_L13
bounded_bit_writer_L12:
bounded_bit_writer_L13:
; VALID = 1
    lda #1
    sta VALID
; T0 = COUNT
    lda COUNT
    sta T0
; while T0 != 0:
bounded_bit_writer_L16:
    lda T0
    cmp #0
    bne bounded_bit_writer_L19
bounded_bit_writer_L20:
    jmp bounded_bit_writer_L18
bounded_bit_writer_L19:
    jmp bounded_bit_writer_L17
bounded_bit_writer_L17:
; T0 = T0 - 1
    lda T0
    sec
    sbc #1
    sta T0
; T1 = VALUE
    lda VALUE
    sta T1
; T2 = T0
    lda T0
    sta T2
; while T2 != 0:
bounded_bit_writer_L21:
    lda T2
    cmp #0
    bne bounded_bit_writer_L24
bounded_bit_writer_L25:
    jmp bounded_bit_writer_L23
bounded_bit_writer_L24:
    jmp bounded_bit_writer_L22
bounded_bit_writer_L22:
; T1 = T1 >> 1
    lda T1
    lsr
    sta T1
; T2 = T2 - 1
    lda T2
    sec
    sbc #1
    sta T2
    jmp bounded_bit_writer_L21
bounded_bit_writer_L23:
; T1 = T1 & 1
    lda T1
    and #1
    sta T1
; T2 = CURSOR >> 3
    lda CURSOR
    lsr
    lsr
    lsr
    sta T2
; T3 = CURSOR & 7
    lda CURSOR
    and #7
    sta T3
; T4 = 128
    lda #128
    sta T4
; while T3 != 0:
bounded_bit_writer_L26:
    lda T3
    cmp #0
    bne bounded_bit_writer_L29
bounded_bit_writer_L30:
    jmp bounded_bit_writer_L28
bounded_bit_writer_L29:
    jmp bounded_bit_writer_L27
bounded_bit_writer_L27:
; T4 = T4 >> 1
    lda T4
    lsr
    sta T4
; T3 = T3 - 1
    lda T3
    sec
    sbc #1
    sta T3
    jmp bounded_bit_writer_L26
bounded_bit_writer_L28:
; T5 = T4 ^ 255
    lda T4
    eor #255
    sta T5
; T6 = B[T2]
    ldx T2
    lda B0,x
    sta T6
; T6 = T6 & T5
    lda T6
    and T5
    sta T6
; if T1 != 0:
    lda T1
    cmp #0
    bne bounded_bit_writer_L34
bounded_bit_writer_L35:
    jmp bounded_bit_writer_L32
bounded_bit_writer_L34:
    jmp bounded_bit_writer_L31
bounded_bit_writer_L31:
; T6 = T6 | T4
    lda T6
    ora T4
    sta T6
    jmp bounded_bit_writer_L33
bounded_bit_writer_L32:
bounded_bit_writer_L33:
; B[T2] = T6
    lda T6
    sta EXPR_SAVE
    ldx T2
    lda EXPR_SAVE
    sta B0,x
; CURSOR = CURSOR + 1
    lda CURSOR
    clc
    adc #1
    sta CURSOR
    jmp bounded_bit_writer_L16
bounded_bit_writer_L18:
    rts
component_end:

Machine contract

A/X/status may change; Y is preserved. The 2A03 has no decimal arithmetic mode. 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 NROM caller owns PPU setup and background display, with NMI and DMC disabled. Table and packet sizes are bounded to fit the cartridge caller and internal RAM; the helper owns no PPU registers.

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

.segment "HEADER"
.byte "NES",$1a,2,1,0,0,0,0,0,0,0,0,0,0
VALID=$80
CURSOR=$81
COUNT=$82
VALUE=$83
B0=$84
B1=$85
B2=$86
B3=$87
T0=$88
T1=$89
T2=$8a
T3=$8b
T4=$8c
T5=$8d
T6=$8e
EXPR_SAVE=$8f
STATE=$80
input_ptr=$70
cases_left=$72
ptr=$74
saved_y=$76
OUTPUT=$0200
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
.include "caller.inc"
.include "exercise.inc"
component_start:
.include "component.inc"
.include "data.inc"
.include "cases.inc"
code_end:
.segment "VECTORS"
.word nmi,reset,irq
.segment "CHARS"
.include "../../input/controller-reading/edges-font.inc"
.res 8192-272,0

A client retaining state between calls

exercise.inc, excerpt

preview_rules:
.if CUSTOM
; COUNT = 8
    lda #8
    sta COUNT
; VALUE = 165
    lda #165
    sta VALUE
; CURSOR = 24
    lda #24
    sta CURSOR
.if BROKEN
; CURSOR = 0
    lda #0
    sta CURSOR
.endif
    jsr checked_step
    jsr emit_state
    rts
.else
; COUNT = 5
    lda #5
    sta COUNT
; VALUE = 21
    lda #21
    sta VALUE
; CURSOR = 6
    lda #6
    sta CURSOR
; B0 = 255
    lda #255
    sta B0
; B1 = 255
    lda #255
    sta B1
.if BROKEN
; CURSOR = 0
    lda #0
    sta CURSOR
.endif
    jsr checked_step
    jsr emit_state
.endif
    rts

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

.segment "HEADER"
.byte "NES",$1a,2,1,0,0,0,0,0,0,0,0,0,0
VALID=$80
CURSOR=$81
COUNT=$82
VALUE=$83
B0=$84
B1=$85
B2=$86
B3=$87
T0=$88
T1=$89
T2=$8a
T3=$8b
T4=$8c
T5=$8d
T6=$8e
EXPR_SAVE=$8f
STATE=$80
input_ptr=$70
cases_left=$72
ptr=$74
saved_y=$76
OUTPUT=$0200
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
.include "caller.inc"
.include "exercise.inc"
component_start:
.include "component.inc"
.include "data.inc"
.include "cases.inc"
code_end:
.segment "VECTORS"
.word nmi,reset,irq
.segment "CHARS"
.include "../../input/controller-reading/edges-font.inc"
.res 8192-272,0

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

.segment "HEADER"
.byte "NES",$1a,2,1,0,0,0,0,0,0,0,0,0,0
VALID=$80
CURSOR=$81
COUNT=$82
VALUE=$83
B0=$84
B1=$85
B2=$86
B3=$87
T0=$88
T1=$89
T2=$8a
T3=$8b
T4=$8c
T5=$8d
T6=$8e
EXPR_SAVE=$8f
STATE=$80
input_ptr=$70
cases_left=$72
ptr=$74
saved_y=$76
OUTPUT=$0200
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
.include "caller.inc"
.include "exercise.inc"
component_start:
.include "component.inc"
.include "data.inc"
.include "cases.inc"
code_end:
.segment "VECTORS"
.word nmi,reset,irq
.segment "CHARS"
.include "../../input/controller-reading/edges-font.inc"
.res 8192-272,0

NES 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 272 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 NTSC NROM, background only, DMC/NMI disabled, 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.