Code Like It's 198x

Decode Bounded Byte Deltas on Amiga

A path or table whose values change slowly can be stored as a starting value followed by small signed differences.

A path or table whose values change slowly can be stored as a starting value followed by small signed differences.

Amiga shows the working diagnostic row 04 01 04 0A 01 FF 02 FE.
A completed native caller displays 04 01 04 0A 01 FF 02 FE. 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

COUNT 0–8 deltas are two’s-complement bytes. Starting at BASE, add each delta modulo 256 and write successive values to R0..R7. Return WRITTEN=COUNT and VALID=1. Invalid count leaves all output bytes unchanged and returns WRITTEN=0, VALID=0. BASE is preserved.

Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: WRITTEN, VALID, COUNT, BASE, Q0, Q1, Q2, Q3, Q4, Q5, Q6, Q7, R0, R1, R2, R3, R4, R5, R6, R7. They occupy 20 contiguous bytes. Also reserve the private workspace: T0, T1, T2, EXPR_SAVE (4 bytes). Array names in the explanation refer to the numbered consecutive bindings, such as C0, C1 and so on.

Call bounded_delta_decoder_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_delta_decoder_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 none
Supply before service COUNT, BASE and Q0..Q7
Read after service WRITTEN, VALID and R0..R7

Keep a running byte total. Two’s-complement negative deltas work through modulo addition, so no sign branch is needed.

Reusable initialisation and service

component.inc
; SPDX-License-Identifier: MIT
; Decode Bounded Byte Deltas. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
bounded_delta_decoder_init:
    move.b #0,d0
    move.b d0,WRITTEN
    move.b d0,VALID
    move.b d0,COUNT
    move.b d0,BASE
    move.b d0,Q0
    move.b d0,Q1
    move.b d0,Q2
    move.b d0,Q3
    move.b d0,Q4
    move.b d0,Q5
    move.b d0,Q6
    move.b d0,Q7
    move.b d0,R0
    move.b d0,R1
    move.b d0,R2
    move.b d0,R3
    move.b d0,R4
    move.b d0,R5
    move.b d0,R6
    move.b d0,R7
    move.b d0,T0
    move.b d0,T1
    move.b d0,T2
    move.b d0,EXPR_SAVE
    rts
bounded_delta_decoder_step:
; WRITTEN = 0
    move.b #0,d0
    move.b d0,WRITTEN
; VALID = 0
    move.b #0,d0
    move.b d0,VALID
; if COUNT > 8:
    move.b COUNT,d0
    cmp.b #8,d0
    bhi bounded_delta_decoder_L1
    jmp bounded_delta_decoder_L2
bounded_delta_decoder_L1:
; return
    rts
    jmp bounded_delta_decoder_L3
bounded_delta_decoder_L2:
bounded_delta_decoder_L3:
; VALID = 1
    move.b #1,d0
    move.b d0,VALID
; T0 = BASE
    move.b BASE,d0
    move.b d0,T0
; T1 = 0
    move.b #0,d0
    move.b d0,T1
; while T1 < COUNT:
bounded_delta_decoder_L4:
    move.b T1,d0
    move.b COUNT,d1
    cmp.b d1,d0
    bcs bounded_delta_decoder_L5
    jmp bounded_delta_decoder_L6
bounded_delta_decoder_L5:
; T2 = Q[T1]
    moveq #0,d1
    move.b T1,d1
    lea Q0,a0
    move.b 0(a0,d1.w),d0
    move.b d0,T2
; T0 = T0 + T2
    move.b T0,d0
    move.b T2,d1
    add.b d1,d0
    move.b d0,T0
; R[T1] = T0
    move.b T0,d0
    move.b d0,d2
    moveq #0,d1
    move.b T1,d1
    lea R0,a0
    move.b d2,0(a0,d1.w)
; T1 = T1 + 1
    move.b T1,d0
    add.b #1,d0
    move.b d0,T1
    jmp bounded_delta_decoder_L4
bounded_delta_decoder_L6:
; WRITTEN = COUNT
    move.b COUNT,d0
    move.b d0,WRITTEN
    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

demo.asm
    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:
WRITTEN: dc.b 0
VALID: dc.b 0
COUNT: dc.b 0
BASE: dc.b 0
Q0: dc.b 0
Q1: dc.b 0
Q2: dc.b 0
Q3: dc.b 0
Q4: dc.b 0
Q5: dc.b 0
Q6: dc.b 0
Q7: dc.b 0
R0: dc.b 0
R1: dc.b 0
R2: dc.b 0
R3: dc.b 0
R4: dc.b 0
R5: dc.b 0
R6: dc.b 0
R7: dc.b 0
T0: dc.b 0
T1: dc.b 0
T2: 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

A client retaining state between calls

exercise.inc, excerpt
preview_rules:
    ifne CUSTOM
; COUNT = 3
    move.b #3,d0
    move.b d0,COUNT
; BASE = 255
    move.b #255,d0
    move.b d0,BASE
; Q0 = 1
    move.b #1,d0
    move.b d0,Q0
; Q1 = 128
    move.b #128,d0
    move.b d0,Q1
; Q2 = 127
    move.b #127,d0
    move.b d0,Q2
    ifne BROKEN
; BASE = 0
    move.b #0,d0
    move.b d0,BASE
    endif
    bsr checked_step
    bsr emit_state
    rts
    else
; COUNT = 4
    move.b #4,d0
    move.b d0,COUNT
; BASE = 10
    move.b #10,d0
    move.b d0,BASE
; Q0 = 1
    move.b #1,d0
    move.b d0,Q0
; Q1 = 255
    move.b #255,d0
    move.b d0,Q1
; Q2 = 2
    move.b #2,d0
    move.b d0,Q2
; Q3 = 254
    move.b #254,d0
    move.b d0,Q3
    ifne BROKEN
; BASE = 0
    move.b #0,d0
    move.b d0,BASE
    endif
    bsr checked_step
    bsr emit_state
    endif
    rts

Customise it

Exercise wrapping and the −128 signed delta. 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
    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:
WRITTEN: dc.b 0
VALID: dc.b 0
COUNT: dc.b 0
BASE: dc.b 0
Q0: dc.b 0
Q1: dc.b 0
Q2: dc.b 0
Q3: dc.b 0
Q4: dc.b 0
Q5: dc.b 0
Q6: dc.b 0
Q7: dc.b 0
R0: dc.b 0
R1: dc.b 0
R2: dc.b 0
R3: dc.b 0
R4: dc.b 0
R5: dc.b 0
R6: dc.b 0
R7: dc.b 0
T0: dc.b 0
T1: dc.b 0
T2: 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

The custom diagnostic row is 03 01 03 FF 01 80 7F 00. The full client trace in the source explains how it gets there.

Mistake and repair

The wrong caller discards the starting value. Every decoded coordinate is shifted. Preserve BASE alongside the delta stream.

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
    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:
WRITTEN: dc.b 0
VALID: dc.b 0
COUNT: dc.b 0
BASE: dc.b 0
Q0: dc.b 0
Q1: dc.b 0
Q2: dc.b 0
Q3: dc.b 0
Q4: dc.b 0
Q5: dc.b 0
Q6: dc.b 0
Q7: dc.b 0
R0: dc.b 0
R1: dc.b 0
R2: dc.b 0
R3: dc.b 0
R4: dc.b 0
R5: dc.b 0
R6: dc.b 0
R7: dc.b 0
T0: dc.b 0
T1: dc.b 0
T2: 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
Amiga shows the deliberate error row 04 01 04 00 01 FF 02 FE.
The wrong client completes with 04 01 04 00 01 FF 02 FE; its changed inputs and state explain the result.

Cost and alternatives

The measured helper extent is 320 code bytes; public state is 20 bytes and private scratch is 4 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.

Linear in at most eight deltas. Random access needs a checkpoint or decoding the preceding deltas; absolute values cost the same number of bytes unless deltas are further packed.

This format is lossless modulo 256, with exactly one output per delta. It does not itself compress bytes; a narrower packed-delta format needs escape codes for large changes.

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 50 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 RLE decoding: Choose an encoding that matches repetition or small changes in the actual data.

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