Flags and short fields can share a compact record without overwriting their neighbours.
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
; 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:
move.b #0,d0
move.b d0,VALID
move.b d0,CURSOR
move.b d0,COUNT
move.b d0,VALUE
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
move.b d0,T5
move.b d0,T6
move.b d0,EXPR_SAVE
rts
bounded_bit_writer_step:
; VALID = 0
move.b #0,d0
move.b d0,VALID
; if COUNT > 8:
move.b COUNT,d0
cmp.b #8,d0
bhi bounded_bit_writer_L1
jmp bounded_bit_writer_L2
bounded_bit_writer_L1:
; return
rts
jmp bounded_bit_writer_L3
bounded_bit_writer_L2:
bounded_bit_writer_L3:
; if CURSOR > 32:
move.b CURSOR,d0
cmp.b #32,d0
bhi bounded_bit_writer_L4
jmp bounded_bit_writer_L5
bounded_bit_writer_L4:
; return
rts
jmp bounded_bit_writer_L6
bounded_bit_writer_L5:
bounded_bit_writer_L6:
; T0 = CURSOR + COUNT
move.b CURSOR,d0
move.b COUNT,d1
add.b d1,d0
move.b d0,T0
; if T0 > 32:
move.b T0,d0
cmp.b #32,d0
bhi bounded_bit_writer_L7
jmp bounded_bit_writer_L8
bounded_bit_writer_L7:
; return
rts
jmp bounded_bit_writer_L9
bounded_bit_writer_L8:
bounded_bit_writer_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_writer_L10:
move.b T0,d0
cmp.b #0,d0
bne bounded_bit_writer_L11
jmp bounded_bit_writer_L12
bounded_bit_writer_L11:
; T0 = T0 - 1
move.b T0,d0
sub.b #1,d0
move.b d0,T0
; T1 = VALUE
move.b VALUE,d0
move.b d0,T1
; T2 = T0
move.b T0,d0
move.b d0,T2
; while T2 != 0:
bounded_bit_writer_L13:
move.b T2,d0
cmp.b #0,d0
bne bounded_bit_writer_L14
jmp bounded_bit_writer_L15
bounded_bit_writer_L14:
; T1 = T1 >> 1
move.b T1,d0
lsr.b #1,d0
move.b d0,T1
; T2 = T2 - 1
move.b T2,d0
sub.b #1,d0
move.b d0,T2
jmp bounded_bit_writer_L13
bounded_bit_writer_L15:
; T1 = T1 & 1
move.b T1,d0
and.b #1,d0
move.b d0,T1
; T2 = CURSOR >> 3
move.b CURSOR,d0
lsr.b #3,d0
move.b d0,T2
; T3 = CURSOR & 7
move.b CURSOR,d0
and.b #7,d0
move.b d0,T3
; T4 = 128
move.b #128,d0
move.b d0,T4
; while T3 != 0:
bounded_bit_writer_L16:
move.b T3,d0
cmp.b #0,d0
bne bounded_bit_writer_L17
jmp bounded_bit_writer_L18
bounded_bit_writer_L17:
; T4 = T4 >> 1
move.b T4,d0
lsr.b #1,d0
move.b d0,T4
; T3 = T3 - 1
move.b T3,d0
sub.b #1,d0
move.b d0,T3
jmp bounded_bit_writer_L16
bounded_bit_writer_L18:
; T5 = T4 ^ 255
move.b T4,d0
eor.b #255,d0
move.b d0,T5
; T6 = B[T2]
moveq #0,d1
move.b T2,d1
lea B0,a0
move.b 0(a0,d1.w),d0
move.b d0,T6
; T6 = T6 & T5
move.b T6,d0
move.b T5,d1
and.b d1,d0
move.b d0,T6
; if T1 != 0:
move.b T1,d0
cmp.b #0,d0
bne bounded_bit_writer_L19
jmp bounded_bit_writer_L20
bounded_bit_writer_L19:
; T6 = T6 | T4
move.b T6,d0
move.b T4,d1
or.b d1,d0
move.b d0,T6
jmp bounded_bit_writer_L21
bounded_bit_writer_L20:
bounded_bit_writer_L21:
; B[T2] = T6
move.b T6,d0
move.b d0,d2
moveq #0,d1
move.b T2,d1
lea B0,a0
move.b d2,0(a0,d1.w)
; CURSOR = CURSOR + 1
move.b CURSOR,d0
add.b #1,d0
move.b d0,CURSOR
jmp bounded_bit_writer_L10
bounded_bit_writer_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:
VALID: dc.b 0
CURSOR: dc.b 0
COUNT: dc.b 0
VALUE: 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
T5: dc.b 0
T6: dc.b 0
EXPR_SAVE: dc.b 0
even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
even
signature: dc.b "198XTOOLKIT0000",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
; COUNT = 8
move.b #8,d0
move.b d0,COUNT
; VALUE = 165
move.b #165,d0
move.b d0,VALUE
; CURSOR = 24
move.b #24,d0
move.b d0,CURSOR
ifne BROKEN
; CURSOR = 0
move.b #0,d0
move.b d0,CURSOR
endif
bsr checked_step
bsr emit_state
rts
else
; COUNT = 5
move.b #5,d0
move.b d0,COUNT
; VALUE = 21
move.b #21,d0
move.b d0,VALUE
; CURSOR = 6
move.b #6,d0
move.b d0,CURSOR
; B0 = 255
move.b #255,d0
move.b d0,B0
; B1 = 255
move.b #255,d0
move.b d0,B1
ifne BROKEN
; CURSOR = 0
move.b #0,d0
move.b d0,CURSOR
endif
bsr checked_step
bsr emit_state
endif
rtsCustomise 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
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:
VALID: dc.b 0
CURSOR: dc.b 0
COUNT: dc.b 0
VALUE: 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
T5: dc.b 0
T6: dc.b 0
EXPR_SAVE: dc.b 0
even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
even
signature: dc.b "198XTOOLKIT0000",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 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
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:
VALID: dc.b 0
CURSOR: dc.b 0
COUNT: dc.b 0
VALUE: 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
T5: dc.b 0
T6: dc.b 0
EXPR_SAVE: dc.b 0
even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
even
signature: dc.b "198XTOOLKIT0000",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 502 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 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
Bounded bit readers: The reader consumes the same most-significant-first bit stream.