Read a button on Memotech MTX

Build the complete caller with make -C memotech-mtx/patterns/assembly/input/read-action. Press Space once, keep it held, then release and press again. The piece moves on each new press and returns to its starting position after two; holding does not repeat the action. Enter the generated DATA/POKE loader through stock BASIC, then RUN; USR takes over until reset.

Take the reader

sample.inc is an inline hardware sampler: include it where the caller samples input. It has no RET/RTS and must not be called as a subroutine. It supplies the held state described below; the caller supplies previous-state storage and decides what a new press does.

The reusable reader

    ld a,$7f
    out ($05),a               ; select column 7, active-low column drive
    in a,($06)                ; Space is bit 8 of the 10-bit return: high bit 0
    cpl
    and 1
    ld b,a

OUT ($05),$7F selects keyboard column 7; IN ($06) supplies the high part of its return. Space is active-low bit 0 of that read. B receives 0/1; AF/B change.

Calculate a new-press mask as current AND NOT previous, then store current as previous on every update, including release. Test held state for continuous movement and the new-press mask for an action that should happen once. A key already held at the caller’s first sample counts as a press because previous starts at zero.

Caller storage: previous=$DF01; turn=$DF00, field clock=$DF02/$DF03; stack below $F7F0. The sample’s display initialiser and any interrupts have their own ownership requirements; keep them with this complete example.

Change it and recognise a mistake

Preserve the read/write distinction and column drive. This direct reader takes over the keyboard configuration rather than consuming a BASIC character.

The broken caller feeds held state into the action decision. Keeping the button down advances repeatedly; the native fault run reaches 32 instead of remaining at one. Restore the previous-state comparison and update previous on release as well.

Deliberately broken input/update path

; Original capability caller: owns VDP, one audio channel and declared RAM.
VDP_DATA equ $01
VDP_CTRL equ $02
TURNS equ $df00
SOUND_TICKS equ $df05
; Entered from stock MTX BASIC using POKE and USR. Takes over until reset.
    org $e000
start:
    di
    ld sp,$f7f0
    xor a
    ld ($df00),a
    ld ($df01),a
    ld ($df02),a
    ld ($df03),a
    in a,($02)
    xor a
    out ($02),a
    ld a,$80
    out ($02),a          ; R0 legacy graphics
    xor a
    out ($02),a
    ld a,$81
    out ($02),a          ; R1 display and VDP IRQ disabled
    call init_picture
main:
    call wait_frame
    ld hl,($df02)
    inc hl
    ld ($df02),hl
    include "../sample.inc"
    ld a,($df01)
    ld a,b
    nop
    jr z,no_action
    ld a,($df00)
    inc a
    ld ($df00),a
    call trigger_sound
no_action:
    ld a,b
    ld ($df01),a
    call draw_picture
    call service_sound
    jp main

; AF clobbered; caller exclusively owns status/control-port latch.
wait_frame:
    in a,($02)
wait_new:
    in a,($02)
    and $80
    jr z,wait_new
    ret
    include "../picture.inc"
    include "../sound.inc"

The complete caller

Working setup and integration

; Original capability caller: owns VDP, one audio channel and declared RAM.
VDP_DATA equ $01
VDP_CTRL equ $02
TURNS equ $df00
SOUND_TICKS equ $df05
; Entered from stock MTX BASIC using POKE and USR. Takes over until reset.
    org $e000
start:
    di
    ld sp,$f7f0
    xor a
    ld ($df00),a
    ld ($df01),a
    ld ($df02),a
    ld ($df03),a
    in a,($02)
    xor a
    out ($02),a
    ld a,$80
    out ($02),a          ; R0 legacy graphics
    xor a
    out ($02),a
    ld a,$81
    out ($02),a          ; R1 display and VDP IRQ disabled
    call init_picture
main:
    call wait_frame
    ld hl,($df02)
    inc hl
    ld ($df02),hl
    include "sample.inc"
    ld a,($df01)
    cpl
    and b
    jr z,no_action
    ld a,($df00)
    inc a
    ld ($df00),a
    call trigger_sound
no_action:
    ld a,b
    ld ($df01),a
    call draw_picture
    call service_sound
    jp main

; AF clobbered; caller exclusively owns status/control-port latch.
wait_frame:
    in a,($02)
wait_new:
    in a,($02)
    and $80
    jr z,wait_new
    ret
    include "picture.inc"
    include "sound.inc"

For more responsive controls, compare polling, buffering and press edges in input choices.

Verification and sources

Target: MTX500 PAL, stock BASIC DATA/POKE/USR entry. These callers were executed with released Emu198x 0.44.1; assembly uses Asm198x 0.0.71. The sample folder contains reproduction instructions and retained observations. The independent checker binds source, native captures and actual output; its rejection tests exercise wrong state, missing files and false audio claims.

This qualifies the named emulator/firmware path. Pitch accuracy, original hardware and other peripherals require their own observations.

Memotech, MTX Series Operator’s Manual/technical description: keyboard matrix, sound I/O and USR; TI SN76489A data manual.