Code Like It's 198x

Parse Two Hexadecimal Digits on Amiga

A short hexadecimal input still needs validation before it becomes an address, colour or seed.

A short hexadecimal input still needs validation before it becomes an address, colour or seed.

Amiga shows the working diagnostic row AB 01 41 62 00 00 00 00.
A completed native caller displays AB 01 41 62 00 00 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

FIRST and SECOND are ASCII bytes. Accept exactly two digits 0–9, A–F or a–f. Return VALUE and VALID=1; invalid input returns VALUE=0 and VALID=0. Inputs are preserved.

Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: VALUE, VALID, FIRST, SECOND. They occupy 4 contiguous bytes. Also reserve the private workspace: T0, T1, T2 (3 bytes). Array names in the explanation refer to the numbered consecutive bindings, such as C0, C1 and so on.

Call hexadecimal_parser_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 hexadecimal_parser_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 FIRST and SECOND
Read after service VALUE and VALID

Validate each ASCII interval before converting it to a nibble. Shifting the accumulated high nibble by four leaves room for the second digit.

Reusable initialisation and service

component.inc
; SPDX-License-Identifier: MIT
; Parse Two Hexadecimal Digits. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
hexadecimal_parser_init:
    move.b #0,d0
    move.b d0,VALUE
    move.b d0,VALID
    move.b d0,FIRST
    move.b d0,SECOND
    move.b d0,T0
    move.b d0,T1
    move.b d0,T2
    rts
hexadecimal_parser_step:
; VALUE = 0
    move.b #0,d0
    move.b d0,VALUE
; VALID = 0
    move.b #0,d0
    move.b d0,VALID
; T0 = 0
    move.b #0,d0
    move.b d0,T0
; while T0 < 2:
hexadecimal_parser_L1:
    move.b T0,d0
    cmp.b #2,d0
    bcs hexadecimal_parser_L2
    jmp hexadecimal_parser_L3
hexadecimal_parser_L2:
; T1 = FIRST
    move.b FIRST,d0
    move.b d0,T1
; if T0 != 0:
    move.b T0,d0
    cmp.b #0,d0
    bne hexadecimal_parser_L4
    jmp hexadecimal_parser_L5
hexadecimal_parser_L4:
; T1 = SECOND
    move.b SECOND,d0
    move.b d0,T1
    jmp hexadecimal_parser_L6
hexadecimal_parser_L5:
hexadecimal_parser_L6:
; T2 = 255
    move.b #255,d0
    move.b d0,T2
; if T1 >= 48:
    move.b T1,d0
    cmp.b #48,d0
    bcc hexadecimal_parser_L7
    jmp hexadecimal_parser_L8
hexadecimal_parser_L7:
; if T1 <= 57:
    move.b T1,d0
    cmp.b #57,d0
    bls hexadecimal_parser_L10
    jmp hexadecimal_parser_L11
hexadecimal_parser_L10:
; T2 = T1 - 48
    move.b T1,d0
    sub.b #48,d0
    move.b d0,T2
    jmp hexadecimal_parser_L12
hexadecimal_parser_L11:
hexadecimal_parser_L12:
    jmp hexadecimal_parser_L9
hexadecimal_parser_L8:
hexadecimal_parser_L9:
; if T1 >= 65:
    move.b T1,d0
    cmp.b #65,d0
    bcc hexadecimal_parser_L13
    jmp hexadecimal_parser_L14
hexadecimal_parser_L13:
; if T1 <= 70:
    move.b T1,d0
    cmp.b #70,d0
    bls hexadecimal_parser_L16
    jmp hexadecimal_parser_L17
hexadecimal_parser_L16:
; T2 = T1 - 55
    move.b T1,d0
    sub.b #55,d0
    move.b d0,T2
    jmp hexadecimal_parser_L18
hexadecimal_parser_L17:
hexadecimal_parser_L18:
    jmp hexadecimal_parser_L15
hexadecimal_parser_L14:
hexadecimal_parser_L15:
; if T1 >= 97:
    move.b T1,d0
    cmp.b #97,d0
    bcc hexadecimal_parser_L19
    jmp hexadecimal_parser_L20
hexadecimal_parser_L19:
; if T1 <= 102:
    move.b T1,d0
    cmp.b #102,d0
    bls hexadecimal_parser_L22
    jmp hexadecimal_parser_L23
hexadecimal_parser_L22:
; T2 = T1 - 87
    move.b T1,d0
    sub.b #87,d0
    move.b d0,T2
    jmp hexadecimal_parser_L24
hexadecimal_parser_L23:
hexadecimal_parser_L24:
    jmp hexadecimal_parser_L21
hexadecimal_parser_L20:
hexadecimal_parser_L21:
; if T2 == 255:
    move.b T2,d0
    cmp.b #255,d0
    beq hexadecimal_parser_L25
    jmp hexadecimal_parser_L26
hexadecimal_parser_L25:
; VALUE = 0
    move.b #0,d0
    move.b d0,VALUE
; return
    rts
    jmp hexadecimal_parser_L27
hexadecimal_parser_L26:
hexadecimal_parser_L27:
; VALUE = VALUE << 4
    move.b VALUE,d0
    lsl.b #4,d0
    move.b d0,VALUE
; VALUE = VALUE | T2
    move.b VALUE,d0
    move.b T2,d1
    or.b d1,d0
    move.b d0,VALUE
; T0 = T0 + 1
    move.b T0,d0
    add.b #1,d0
    move.b d0,T0
    jmp hexadecimal_parser_L1
hexadecimal_parser_L3:
; VALID = 1
    move.b #1,d0
    move.b d0,VALID
    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:
VALUE: dc.b 0
VALID: dc.b 0
FIRST: dc.b 0
SECOND: dc.b 0
T0: dc.b 0
T1: dc.b 0
T2: 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
; FIRST = 102
    move.b #102,d0
    move.b d0,FIRST
; SECOND = 70
    move.b #70,d0
    move.b d0,SECOND
    ifne BROKEN
; SECOND = 48
    move.b #48,d0
    move.b d0,SECOND
    endif
    bsr checked_step
    bsr emit_state
    rts
    else
; FIRST = 65
    move.b #65,d0
    move.b d0,FIRST
; SECOND = 98
    move.b #98,d0
    move.b d0,SECOND
    ifne BROKEN
; SECOND = 48
    move.b #48,d0
    move.b d0,SECOND
    endif
    bsr checked_step
    bsr emit_state
    endif
    rts

Customise it

Accept mixed-case fF as the maximum byte. 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:
VALUE: dc.b 0
VALID: dc.b 0
FIRST: dc.b 0
SECOND: dc.b 0
T0: dc.b 0
T1: dc.b 0
T2: 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 FF 01 66 46 00 00 00 00. The full client trace in the source explains how it gets there.

Mistake and repair

The wrong caller replaces the second typed character with zero. Ab becomes A0. Keep both input bytes and inspect VALID before using VALUE.

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:
VALUE: dc.b 0
VALID: dc.b 0
FIRST: dc.b 0
SECOND: dc.b 0
T0: dc.b 0
T1: dc.b 0
T2: 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 A0 01 41 30 00 00 00 00.
The wrong client completes with A0 01 41 30 00 00 00 00; its changed inputs and state explain the result.

Cost and alternatives

The measured helper extent is 374 code bytes; public state is 4 bytes and private scratch is 3 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.

Two character stages and no table. A 256-byte classification table trades ROM for predictable lookup work.

This is a fixed two-character parser. Prefixes, whitespace, editing, variable lengths and memory-address permissions belong to the caller.

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

Hexadecimal formatting: Formatting and parsing can round-trip every byte when the caller preserves the two characters.

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