A short hexadecimal input still needs validation before it becomes an address, colour or seed.
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
; 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:
ld a,0
ld (VALUE),a
ld (VALID),a
ld (FIRST),a
ld (SECOND),a
ld (T0),a
ld (T1),a
ld (T2),a
ret
hexadecimal_parser_step:
; VALUE = 0
ld a,0
ld (VALUE),a
; VALID = 0
ld a,0
ld (VALID),a
; T0 = 0
ld a,0
ld (T0),a
; while T0 < 2:
hexadecimal_parser_L1:
ld a,(T0)
cp 2
jp c,hexadecimal_parser_L2
jp hexadecimal_parser_L3
hexadecimal_parser_L2:
; T1 = FIRST
ld a,(FIRST)
ld (T1),a
; if T0 != 0:
ld a,(T0)
cp 0
jp nz,hexadecimal_parser_L4
jp hexadecimal_parser_L5
hexadecimal_parser_L4:
; T1 = SECOND
ld a,(SECOND)
ld (T1),a
jp hexadecimal_parser_L6
hexadecimal_parser_L5:
hexadecimal_parser_L6:
; T2 = 255
ld a,255
ld (T2),a
; if T1 >= 48:
ld a,(T1)
cp 48
jp nc,hexadecimal_parser_L7
jp hexadecimal_parser_L8
hexadecimal_parser_L7:
; if T1 <= 57:
ld a,(T1)
cp 57
jp c,hexadecimal_parser_L10
jp z,hexadecimal_parser_L10
jp hexadecimal_parser_L11
hexadecimal_parser_L10:
; T2 = T1 - 48
ld a,(T1)
sub 48
ld (T2),a
jp hexadecimal_parser_L12
hexadecimal_parser_L11:
hexadecimal_parser_L12:
jp hexadecimal_parser_L9
hexadecimal_parser_L8:
hexadecimal_parser_L9:
; if T1 >= 65:
ld a,(T1)
cp 65
jp nc,hexadecimal_parser_L13
jp hexadecimal_parser_L14
hexadecimal_parser_L13:
; if T1 <= 70:
ld a,(T1)
cp 70
jp c,hexadecimal_parser_L16
jp z,hexadecimal_parser_L16
jp hexadecimal_parser_L17
hexadecimal_parser_L16:
; T2 = T1 - 55
ld a,(T1)
sub 55
ld (T2),a
jp hexadecimal_parser_L18
hexadecimal_parser_L17:
hexadecimal_parser_L18:
jp hexadecimal_parser_L15
hexadecimal_parser_L14:
hexadecimal_parser_L15:
; if T1 >= 97:
ld a,(T1)
cp 97
jp nc,hexadecimal_parser_L19
jp hexadecimal_parser_L20
hexadecimal_parser_L19:
; if T1 <= 102:
ld a,(T1)
cp 102
jp c,hexadecimal_parser_L22
jp z,hexadecimal_parser_L22
jp hexadecimal_parser_L23
hexadecimal_parser_L22:
; T2 = T1 - 87
ld a,(T1)
sub 87
ld (T2),a
jp hexadecimal_parser_L24
hexadecimal_parser_L23:
hexadecimal_parser_L24:
jp hexadecimal_parser_L21
hexadecimal_parser_L20:
hexadecimal_parser_L21:
; if T2 == 255:
ld a,(T2)
cp 255
jp z,hexadecimal_parser_L25
jp hexadecimal_parser_L26
hexadecimal_parser_L25:
; VALUE = 0
ld a,0
ld (VALUE),a
; return
ret
jp hexadecimal_parser_L27
hexadecimal_parser_L26:
hexadecimal_parser_L27:
; VALUE = VALUE << 4
ld a,(VALUE)
sla a
sla a
sla a
sla a
ld (VALUE),a
; VALUE = VALUE | T2
ld a,(VALUE)
ld b,a
ld a,(T2)
ld c,a
ld a,b
or c
ld (VALUE),a
; T0 = T0 + 1
ld a,(T0)
add a,1
ld (T0),a
jp hexadecimal_parser_L1
hexadecimal_parser_L3:
; VALID = 1
ld a,1
ld (VALID),a
ret
component_end:Machine contract
AF/BC/DE/HL may change; IX/IY are preserved. A direct CALL 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 48K diagnostic boots through ROM tape loading and USR, disables interrupts and owns the screen. The helper performs no port or screen writes.
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
org $8000
CUSTOM equ 0
BROKEN equ 0
COYOTE_STEPS equ 4
BUFFER_STEPS equ 4
TEXT_CAP equ 8
PROPERTY_COUNT equ 16
STREAM_BYTES equ 13
CRC_POLY equ 7
include "caller.inc"
include "exercise.inc"
component_start:
include "component.inc"
include "data.inc"
include "cases.inc"
include "../../physics/rectangle-overlap/display.inc"
code_end:
STATE:
VALUE: defb 0
VALID: defb 0
FIRST: defb 0
SECOND: defb 0
T0: defb 0
T1: defb 0
T2: defb 0
input_ptr: defw 0
output_ptr: defw 0
cases_left: defb 0
ready: defb 0
preview: defs 8,0
OUTPUT: defs 1300,0
end startA client retaining state between calls
preview_rules:
if CUSTOM
; FIRST = 102
ld a,102
ld (FIRST),a
; SECOND = 70
ld a,70
ld (SECOND),a
if BROKEN
; SECOND = 48
ld a,48
ld (SECOND),a
endif
call checked_step
call emit_state
ret
else
; FIRST = 65
ld a,65
ld (FIRST),a
; SECOND = 98
ld a,98
ld (SECOND),a
if BROKEN
; SECOND = 48
ld a,48
ld (SECOND),a
endif
call checked_step
call emit_state
endif
retCustomise 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
org $8000
CUSTOM equ 1
BROKEN equ 0
COYOTE_STEPS equ 2
BUFFER_STEPS equ 2
TEXT_CAP equ 4
PROPERTY_COUNT equ 16
STREAM_BYTES equ 13
CRC_POLY equ 49
include "caller.inc"
include "exercise.inc"
component_start:
include "component.inc"
include "data.inc"
include "cases.inc"
include "../../physics/rectangle-overlap/display.inc"
code_end:
STATE:
VALUE: defb 0
VALID: defb 0
FIRST: defb 0
SECOND: defb 0
T0: defb 0
T1: defb 0
T2: defb 0
input_ptr: defw 0
output_ptr: defw 0
cases_left: defb 0
ready: defb 0
preview: defs 8,0
OUTPUT: defs 1300,0
end startThe 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
org $8000
CUSTOM equ 0
BROKEN equ 1
COYOTE_STEPS equ 4
BUFFER_STEPS equ 4
TEXT_CAP equ 8
PROPERTY_COUNT equ 16
STREAM_BYTES equ 13
CRC_POLY equ 7
include "caller.inc"
include "exercise.inc"
component_start:
include "component.inc"
include "data.inc"
include "cases.inc"
include "../../physics/rectangle-overlap/display.inc"
code_end:
STATE:
VALUE: defb 0
VALID: defb 0
FIRST: defb 0
SECOND: defb 0
T0: defb 0
T1: defb 0
T2: defb 0
input_ptr: defw 0
output_ptr: defw 0
cases_left: defb 0
ready: defb 0
preview: defs 8,0
OUTPUT: defs 1300,0
end start
Cost and alternatives
The measured helper extent is 262 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 48K PAL ROM tape load and USR, 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
Hexadecimal formatting: Formatting and parsing can round-trip every byte when the caller preserves the two characters.