Packed flags and small fields can share a byte, provided the reader knows exactly how many bits remain.
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
; 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:
move.b #0,d0
move.b d0,VALUE
move.b d0,VALID
move.b d0,CURSOR
move.b d0,COUNT
move.b d0,B0
move.b d0,B1
move.b d0,B2
move.b d0,B3
move.b d0,T0
move.b d0,T1
move.b d0,T2
move.b d0,T3
move.b d0,T4
rts
bounded_bit_reader_step:
; VALUE = 0
move.b #0,d0
move.b d0,VALUE
; VALID = 0
move.b #0,d0
move.b d0,VALID
; if CURSOR > 32:
move.b CURSOR,d0
cmp.b #32,d0
bhi bounded_bit_reader_L1
jmp bounded_bit_reader_L2
bounded_bit_reader_L1:
; return
rts
jmp bounded_bit_reader_L3
bounded_bit_reader_L2:
bounded_bit_reader_L3:
; if COUNT > 8:
move.b COUNT,d0
cmp.b #8,d0
bhi bounded_bit_reader_L4
jmp bounded_bit_reader_L5
bounded_bit_reader_L4:
; return
rts
jmp bounded_bit_reader_L6
bounded_bit_reader_L5:
bounded_bit_reader_L6:
; T0 = 32 - CURSOR
move.b #32,d0
move.b CURSOR,d1
sub.b d1,d0
move.b d0,T0
; if COUNT > T0:
move.b COUNT,d0
move.b T0,d1
cmp.b d1,d0
bhi bounded_bit_reader_L7
jmp bounded_bit_reader_L8
bounded_bit_reader_L7:
; return
rts
jmp bounded_bit_reader_L9
bounded_bit_reader_L8:
bounded_bit_reader_L9:
; VALID = 1
move.b #1,d0
move.b d0,VALID
; T0 = COUNT
move.b COUNT,d0
move.b d0,T0
; while T0 != 0:
bounded_bit_reader_L10:
move.b T0,d0
cmp.b #0,d0
bne bounded_bit_reader_L11
jmp bounded_bit_reader_L12
bounded_bit_reader_L11:
; T1 = CURSOR >> 3
move.b CURSOR,d0
lsr.b #3,d0
move.b d0,T1
; T2 = CURSOR & 7
move.b CURSOR,d0
and.b #7,d0
move.b d0,T2
; T3 = 128
move.b #128,d0
move.b d0,T3
; while T2 != 0:
bounded_bit_reader_L13:
move.b T2,d0
cmp.b #0,d0
bne bounded_bit_reader_L14
jmp bounded_bit_reader_L15
bounded_bit_reader_L14:
; T3 = T3 >> 1
move.b T3,d0
lsr.b #1,d0
move.b d0,T3
; T2 = T2 - 1
move.b T2,d0
sub.b #1,d0
move.b d0,T2
jmp bounded_bit_reader_L13
bounded_bit_reader_L15:
; T4 = B[T1]
moveq #0,d1
move.b T1,d1
lea B0,a0
move.b 0(a0,d1.w),d0
move.b d0,T4
; T4 = T4 & T3
move.b T4,d0
move.b T3,d1
and.b d1,d0
move.b d0,T4
; VALUE = VALUE << 1
move.b VALUE,d0
lsl.b #1,d0
move.b d0,VALUE
; if T4 != 0:
move.b T4,d0
cmp.b #0,d0
bne bounded_bit_reader_L16
jmp bounded_bit_reader_L17
bounded_bit_reader_L16:
; VALUE = VALUE | 1
move.b VALUE,d0
or.b #1,d0
move.b d0,VALUE
jmp bounded_bit_reader_L18
bounded_bit_reader_L17:
bounded_bit_reader_L18:
; CURSOR = CURSOR + 1
move.b CURSOR,d0
add.b #1,d0
move.b d0,CURSOR
; T0 = T0 - 1
move.b T0,d0
sub.b #1,d0
move.b d0,T0
jmp bounded_bit_reader_L10
bounded_bit_reader_L12:
rts
component_end:Machine contract
D0–D3/A0–A1/CCR may change; D4–D7/A2–A6 are preserved. A direct BSR/JSR needs four 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 A500 OCS diagnostic cold-boots an OFS disk, owns the Copper and bitplane and disables interrupt/DMA sources. The helper does not take over hardware or allocate Chip RAM.
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 and BUILD198X=/path/to/build198x. The Makefile produces normal, custom, deliberately wrong and repaired programs. Tools must already be installed; the sample downloads nothing.
Bindings and include order
section code,code_c
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"
even
include "data.inc"
include "cases.inc"
even
include "display.inc"
include "../../framework/turn-deadline/font.inc"
code_end:
STATE:
VALUE: dc.b 0
VALID: dc.b 0
CURSOR: dc.b 0
COUNT: dc.b 0
B0: dc.b 0
B1: dc.b 0
B2: dc.b 0
B3: dc.b 0
T0: dc.b 0
T1: dc.b 0
T2: dc.b 0
T3: dc.b 0
T4: dc.b 0
even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
even
signature: dc.b "198XWORLDKIT000",0
even
copperlist:
dc.w $08e,$2c81,$090,$2cc1,$092,$0038,$094,$00d0
dc.w $100,$1200,$102,0,$104,0,$108,0,$10a,0
dc.w $180,$012,$182,$fff
copbpl:
dc.w $0e0,0,$0e2,0
dc.w $ffff,$fffe
plane: dcb.b 10240,0A client retaining state between calls
preview_rules:
ifne CUSTOM
; B0 = 171
move.b #171,d0
move.b d0,B0
; B1 = 192
move.b #192,d0
move.b d0,B1
; COUNT = 8
move.b #8,d0
move.b d0,COUNT
ifne BROKEN
; CURSOR = 0
move.b #0,d0
move.b d0,CURSOR
endif
bsr checked_step
bsr emit_state
ifne BROKEN
; CURSOR = 0
move.b #0,d0
move.b d0,CURSOR
endif
bsr checked_step
bsr emit_state
rts
else
; B0 = 171
move.b #171,d0
move.b d0,B0
; B1 = 192
move.b #192,d0
move.b d0,B1
; COUNT = 4
move.b #4,d0
move.b d0,COUNT
ifne BROKEN
; CURSOR = 0
move.b #0,d0
move.b d0,CURSOR
endif
bsr checked_step
bsr emit_state
ifne BROKEN
; CURSOR = 0
move.b #0,d0
move.b d0,CURSOR
endif
bsr checked_step
bsr emit_state
ifne BROKEN
; CURSOR = 0
move.b #0,d0
move.b d0,CURSOR
endif
bsr checked_step
bsr emit_state
endif
rtsCustomise 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
section code,code_c
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"
even
include "data.inc"
include "cases.inc"
even
include "display.inc"
include "../../framework/turn-deadline/font.inc"
code_end:
STATE:
VALUE: dc.b 0
VALID: dc.b 0
CURSOR: dc.b 0
COUNT: dc.b 0
B0: dc.b 0
B1: dc.b 0
B2: dc.b 0
B3: dc.b 0
T0: dc.b 0
T1: dc.b 0
T2: dc.b 0
T3: dc.b 0
T4: dc.b 0
even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
even
signature: dc.b "198XWORLDKIT000",0
even
copperlist:
dc.w $08e,$2c81,$090,$2cc1,$092,$0038,$094,$00d0
dc.w $100,$1200,$102,0,$104,0,$108,0,$10a,0
dc.w $180,$012,$182,$fff
copbpl:
dc.w $0e0,0,$0e2,0
dc.w $ffff,$fffe
plane: dcb.b 10240,0The 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
section code,code_c
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"
even
include "data.inc"
include "cases.inc"
even
include "display.inc"
include "../../framework/turn-deadline/font.inc"
code_end:
STATE:
VALUE: dc.b 0
VALID: dc.b 0
CURSOR: dc.b 0
COUNT: dc.b 0
B0: dc.b 0
B1: dc.b 0
B2: dc.b 0
B3: dc.b 0
T0: dc.b 0
T1: dc.b 0
T2: dc.b 0
T3: dc.b 0
T4: dc.b 0
even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
even
signature: dc.b "198XWORLDKIT000",0
even
copperlist:
dc.w $08e,$2c81,$090,$2cc1,$092,$0038,$094,$00d0
dc.w $100,$1200,$102,0,$104,0,$108,0,$10a,0
dc.w $180,$012,$182,$fff
copbpl:
dc.w $0e0,0,$0e2,0
dc.w $ffff,$fffe
plane: dcb.b 10240,0
Cost and alternatives
The measured helper extent is 398 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 A500 OCS PAL, Kickstart1.3 OFS cold boot, using Asm198x 0.0.72, Emu198x 0.44.2 and Build198x 0.2.9. 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.