Read Bits from a Bounded Data Block on NES | Pattern Library

Packed flags and small fields can share a byte, provided the reader knows exactly how many bits remain.

NES shows the working diagnostic row 0C 01 0C 04 AB C0 00 00. — A completed native caller displays 0C 01 0C 04 AB C0 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

B0–B3 are a four-byte, most-significant-bit-first block. Retain bit CURSOR 0–32 and request COUNT 0–8 bits. Return the field right-aligned in VALUE and advance CURSOR only when the complete request fits. Zero bits at the end is valid. Bad count, cursor or remaining length returns VALUE=0, VALID=0 and leaves the cursor unchanged.

Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: VALUE, VALID, CURSOR, COUNT, B0, B1, B2, B3. They occupy 8 contiguous bytes. Also reserve the private workspace: T0, T1, T2, T3, T4 (5 bytes). Array names in the explanation refer to the numbered consecutive bindings, such as C0, C1 and so on.

Call bounded_bit_reader_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_reader_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
Supply before service	COUNT and B0–B3
Read after service	VALUE and VALID

Preflight the whole field, then locate each source byte and bit. Shifting the result before inserting the next bit preserves the field’s written order.

Reusable initialisation and service

6502.inc

; SPDX-License-Identifier: MIT
; Read Bits from a Bounded Data Block. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
bounded_bit_reader_init:
    lda #0
    sta VALUE
    sta VALID
    sta CURSOR
    sta COUNT
    sta B0
    sta B1
    sta B2
    sta B3
    sta T0
    sta T1
    sta T2
    sta T3
    sta T4
    rts
bounded_bit_reader_step:
; VALUE = 0
    lda #0
    sta VALUE
; VALID = 0
    lda #0
    sta VALID
; if CURSOR > 32:
    lda CURSOR
    cmp #32
    beq bounded_bit_reader_L5
    bcs bounded_bit_reader_L4
bounded_bit_reader_L5:
    jmp bounded_bit_reader_L2
bounded_bit_reader_L4:
    jmp bounded_bit_reader_L1
bounded_bit_reader_L1:
; return
    rts
    jmp bounded_bit_reader_L3
bounded_bit_reader_L2:
bounded_bit_reader_L3:
; if COUNT > 8:
    lda COUNT
    cmp #8
    beq bounded_bit_reader_L10
    bcs bounded_bit_reader_L9
bounded_bit_reader_L10:
    jmp bounded_bit_reader_L7
bounded_bit_reader_L9:
    jmp bounded_bit_reader_L6
bounded_bit_reader_L6:
; return
    rts
    jmp bounded_bit_reader_L8
bounded_bit_reader_L7:
bounded_bit_reader_L8:
; T0 = 32 - CURSOR
    lda #32
    sec
    sbc CURSOR
    sta T0
; if COUNT > T0:
    lda COUNT
    cmp T0
    beq bounded_bit_reader_L15
    bcs bounded_bit_reader_L14
bounded_bit_reader_L15:
    jmp bounded_bit_reader_L12
bounded_bit_reader_L14:
    jmp bounded_bit_reader_L11
bounded_bit_reader_L11:
; return
    rts
    jmp bounded_bit_reader_L13
bounded_bit_reader_L12:
bounded_bit_reader_L13:
; VALID = 1
    lda #1
    sta VALID
; T0 = COUNT
    lda COUNT
    sta T0
; while T0 != 0:
bounded_bit_reader_L16:
    lda T0
    cmp #0
    bne bounded_bit_reader_L19
bounded_bit_reader_L20:
    jmp bounded_bit_reader_L18
bounded_bit_reader_L19:
    jmp bounded_bit_reader_L17
bounded_bit_reader_L17:
; T1 = CURSOR >> 3
    lda CURSOR
    lsr
    lsr
    lsr
    sta T1
; T2 = CURSOR & 7
    lda CURSOR
    and #7
    sta T2
; T3 = 128
    lda #128
    sta T3
; while T2 != 0:
bounded_bit_reader_L21:
    lda T2
    cmp #0
    bne bounded_bit_reader_L24
bounded_bit_reader_L25:
    jmp bounded_bit_reader_L23
bounded_bit_reader_L24:
    jmp bounded_bit_reader_L22
bounded_bit_reader_L22:
; T3 = T3 >> 1
    lda T3
    lsr
    sta T3
; T2 = T2 - 1
    lda T2
    sec
    sbc #1
    sta T2
    jmp bounded_bit_reader_L21
bounded_bit_reader_L23:
; T4 = B[T1]
    ldx T1
    lda B0,x
    sta T4
; T4 = T4 & T3
    lda T4
    and T3
    sta T4
; VALUE = VALUE << 1
    lda VALUE
    asl
    sta VALUE
; if T4 != 0:
    lda T4
    cmp #0
    bne bounded_bit_reader_L29
bounded_bit_reader_L30:
    jmp bounded_bit_reader_L27
bounded_bit_reader_L29:
    jmp bounded_bit_reader_L26
bounded_bit_reader_L26:
; VALUE = VALUE | 1
    lda VALUE
    ora #1
    sta VALUE
    jmp bounded_bit_reader_L28
bounded_bit_reader_L27:
bounded_bit_reader_L28:
; CURSOR = CURSOR + 1
    lda CURSOR
    clc
    adc #1
    sta CURSOR
; T0 = T0 - 1
    lda T0
    sec
    sbc #1
    sta T0
    jmp bounded_bit_reader_L16
bounded_bit_reader_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
VALUE=$80
VALID=$81
CURSOR=$82
COUNT=$83
B0=$84
B1=$85
B2=$86
B3=$87
T0=$88
T1=$89
T2=$8a
T3=$8b
T4=$8c
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
; B0 = 171
    lda #171
    sta B0
; B1 = 192
    lda #192
    sta B1
; COUNT = 8
    lda #8
    sta COUNT
.if BROKEN
; CURSOR = 0
    lda #0
    sta CURSOR
.endif
    jsr checked_step
    jsr emit_state

.if BROKEN
; CURSOR = 0
    lda #0
    sta CURSOR
.endif
    jsr checked_step
    jsr emit_state
    rts
.else
; B0 = 171
    lda #171
    sta B0
; B1 = 192
    lda #192
    sta B1
; COUNT = 4
    lda #4
    sta COUNT
.if BROKEN
; CURSOR = 0
    lda #0
    sta CURSOR
.endif
    jsr checked_step
    jsr emit_state

.if BROKEN
; CURSOR = 0
    lda #0
    sta CURSOR
.endif
    jsr checked_step
    jsr emit_state

.if BROKEN
; CURSOR = 0
    lda #0
    sta CURSOR
.endif
    jsr checked_step
    jsr emit_state
.endif
    rts

Customise it

Read complete bytes instead of nibbles. 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
VALUE=$80
VALID=$81
CURSOR=$82
COUNT=$83
B0=$84
B1=$85
B2=$86
B3=$87
T0=$88
T1=$89
T2=$8a
T3=$8b
T4=$8c
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 C0 01 10 08 AB C0 00 00. The full client trace in the source explains how it gets there.

Mistake and repair

The wrong caller resets CURSOR before every field. Success flags still look healthy, but each read returns the beginning of the block. Keep the accepted cursor between fields and preserve it on rejected requests.

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
VALUE=$80
VALID=$81
CURSOR=$82
COUNT=$83
B0=$84
B1=$85
B2=$86
B3=$87
T0=$88
T1=$89
T2=$8a
T3=$8b
T4=$8c
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 0A 01 04 04 AB C0 00 00. — The wrong client completes with 0A 01 04 04 AB C0 00 00; its changed inputs and state explain the result.

Cost and alternatives

The measured helper extent is 218 code bytes; public state is 8 bytes and private scratch is 5 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 visits and up to seven mask shifts per visit. Byte-aligned fields can use a much cheaper direct load; larger hot readers often use a wider reservoir and refill policy.

Bit order is part of the data contract; an LSB-first asset needs a different reader. This helper reads a fixed small block, not a new project-wide file format or a streaming decompressor.

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

Packed cells: Fixed nibble indexing is cheaper for regular four-bit maps; a bit reader handles fields that cross byte boundaries.

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