Parse a Bounded Decimal Number on Spectrum

A score name box and a numeric prompt need different input rules. Reject a bad digit without damaging the value already typed.

Use it in a project

Feed one ASCII CHARACTER. Digits 0–9 extend VALUE if the mathematical result remains 0–255. STATUS=1 accepts, 2 rejects a non-digit, 3 rejects overflow. On either rejection VALUE is unchanged. Call init once to begin a number and keep the value between characters.

Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: VALUE, CHARACTER, STATUS. They occupy 3 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 decimal_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 decimal_parser_step whenever the described event occurs. The helper reads no device, waits for no frame and installs no interrupt handler.

Before multiplying by ten, compare the accepted value with 25. At exactly 25 only a final digit up to five fits. That rejects overflow before it can wrap into an apparently valid small number.

Reusable initialisation and service

; SPDX-License-Identifier: MIT
; Parse a Bounded Decimal Number. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
decimal_parser_init:
    ld a,0
    ld (VALUE),a
    ld (CHARACTER),a
    ld (STATUS),a
    ld (T0),a
    ld (T1),a
    ld (T2),a
    ret
decimal_parser_step:
; STATUS = 2
    ld a,2
    ld (STATUS),a
; if CHARACTER < 48:
    ld a,(CHARACTER)
    cp 48
    jp c,decimal_parser_L1
    jp decimal_parser_L2
decimal_parser_L1:
; return
    ret
    jp decimal_parser_L3
decimal_parser_L2:
decimal_parser_L3:
; if CHARACTER > 57:
    ld a,(CHARACTER)
    cp 57
    jp z,decimal_parser_L5
    jp nc,decimal_parser_L4
    jp decimal_parser_L5
decimal_parser_L4:
; return
    ret
    jp decimal_parser_L6
decimal_parser_L5:
decimal_parser_L6:
; T0 = CHARACTER - 48
    ld a,(CHARACTER)
    sub 48
    ld (T0),a
; STATUS = 3
    ld a,3
    ld (STATUS),a
; if VALUE > 25:
    ld a,(VALUE)
    cp 25
    jp z,decimal_parser_L8
    jp nc,decimal_parser_L7
    jp decimal_parser_L8
decimal_parser_L7:
; return
    ret
    jp decimal_parser_L9
decimal_parser_L8:
decimal_parser_L9:
; if VALUE == 25:
    ld a,(VALUE)
    cp 25
    jp z,decimal_parser_L10
    jp decimal_parser_L11
decimal_parser_L10:
; if T0 > 5:
    ld a,(T0)
    cp 5
    jp z,decimal_parser_L14
    jp nc,decimal_parser_L13
    jp decimal_parser_L14
decimal_parser_L13:
; return
    ret
    jp decimal_parser_L15
decimal_parser_L14:
decimal_parser_L15:
    jp decimal_parser_L12
decimal_parser_L11:
decimal_parser_L12:
; T1 = VALUE << 1
    ld a,(VALUE)
    sla a
    ld (T1),a
; T2 = VALUE << 3
    ld a,(VALUE)
    sla a
    sla a
    sla a
    ld (T2),a
; T1 = T1 + T2
    ld a,(T1)
    ld b,a
    ld a,(T2)
    ld c,a
    ld a,b
    add a,c
    ld (T1),a
; VALUE = T1 + T0
    ld a,(T1)
    ld b,a
    ld a,(T0)
    ld c,a
    ld a,b
    add a,c
    ld (VALUE),a
; STATUS = 1
    ld a,1
    ld (STATUS),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. Scope or rename routine and field labels and allocate a separate block for each independent instance. 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
CHARACTER: defb 0
STATUS: 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

A client retaining state between calls

preview_rules:
    if CUSTOM
; CHARACTER = 49
    ld a,49
    ld (CHARACTER),a
    if BROKEN
; VALUE = 0
    ld a,0
    ld (VALUE),a
    endif
    call checked_step
    call emit_state
; CHARACTER = 48
    ld a,48
    ld (CHARACTER),a
    if BROKEN
; VALUE = 0
    ld a,0
    ld (VALUE),a
    endif
    call checked_step
    call emit_state
; CHARACTER = 52
    ld a,52
    ld (CHARACTER),a
    if BROKEN
; VALUE = 0
    ld a,0
    ld (VALUE),a
    endif
    call checked_step
    call emit_state
    ret
    else
; CHARACTER = 50
    ld a,50
    ld (CHARACTER),a
    if BROKEN
; VALUE = 0
    ld a,0
    ld (VALUE),a
    endif
    call checked_step
    call emit_state
; CHARACTER = 53
    ld a,53
    ld (CHARACTER),a
    if BROKEN
; VALUE = 0
    ld a,0
    ld (VALUE),a
    endif
    call checked_step
    call emit_state
; CHARACTER = 54
    ld a,54
    ld (CHARACTER),a
    if BROKEN
; VALUE = 0
    ld a,0
    ld (VALUE),a
    endif
    call checked_step
    call emit_state
; CHARACTER = 53
    ld a,53
    ld (CHARACTER),a
    if BROKEN
; VALUE = 0
    ld a,0
    ld (VALUE),a
    endif
    call checked_step
    call emit_state
    endif
    ret

Customise it

Parse 104 rather than the normal 2,5,6,5 sequence. 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
CHARACTER: defb 0
STATUS: 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

The custom diagnostic row is 68 34 01 00 00 00 00 00. The full client trace in the source explains how it gets there.

Mistake and repair

The wrong caller resets VALUE before every character. The final digit replaces the whole number. Initialise once at the start of input, then retain the accepted prefix through later characters and errors.

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
CHARACTER: defb 0
STATUS: 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 174 code bytes; public state is 3 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.

Constant work per character. Shifts and additions replace multiplication by ten; they are justified only after the overflow precheck.

No sign, whitespace, backspace, decimal point or end-of-line handling is implicit. A prompt owns those actions. Wider integer parsing uses the same precheck idea with a wider quotient and remainder, rather than accepting wrapped results.

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

Text-entry buffers: A text buffer keeps characters for editing; the parser turns accepted digit events into a bounded numeric value.

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