Read a button on SG-1000

Build the complete caller with make -C sega-sg-1000/patterns/assembly/input/read-action. Press button 1 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. Load the cartridge produced by the Makefile in the named configuration; retain its reset/header setup.

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

    in a,($dc)
    cpl
    and $10
    ld b,a

Controller port $DC returns active-low button-1 bit 4. B receives 0/$10; 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=$C001; turn=$C000, field clock=$C002/$C003; stack below $C3F0. 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

Keep pad sampling separate from display modes; shared pad/PSG ideas do not make the SG-1000 VDP a Master System VDP.

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 $be
VDP_CTRL equ $bf
TURNS equ $c000
SOUND_TICKS equ $c005
    org 0
    jp start
    defs $66-$,0
    retn
    defs $100-$,0
start:
    di
    ld sp,$c3f0
    xor a
    ld ($c000),a
    ld ($c001),a
    ld ($c002),a
    ld ($c003),a
    call init_picture
main:
    call wait_frame
    ld hl,($c002)
    inc hl
    ld ($c002),hl
    include "../sample.inc"
    ld a,($c001)
    ld a,b
    nop
    jr z,no_action
    ld a,($c000)
    inc a
    ld ($c000),a
    call trigger_sound
no_action:
    ld a,b
    ld ($c001),a
    call draw_picture
    call service_sound
    jp main
wait_frame:
    in a,($bf)
wait_new:
    in a,($bf)
    and $80
    jr z,wait_new
    ret
    include "../picture.inc"
    include "../sound.inc"
    defs $7ff0-$,0
    db "TMR SEGA"
    db 0,0,0,0,0,0,0,$4c

The complete caller

Working setup and integration

; Original capability caller: owns VDP, one audio channel and declared RAM.
VDP_DATA equ $be
VDP_CTRL equ $bf
TURNS equ $c000
SOUND_TICKS equ $c005
    org 0
    jp start
    defs $66-$,0
    retn
    defs $100-$,0
start:
    di
    ld sp,$c3f0
    xor a
    ld ($c000),a
    ld ($c001),a
    ld ($c002),a
    ld ($c003),a
    call init_picture
main:
    call wait_frame
    ld hl,($c002)
    inc hl
    ld ($c002),hl
    include "sample.inc"
    ld a,($c001)
    cpl
    and b
    jr z,no_action
    ld a,($c000)
    inc a
    ld ($c000),a
    call trigger_sound
no_action:
    ld a,b
    ld ($c001),a
    call draw_picture
    call service_sound
    jp main
wait_frame:
    in a,($bf)
wait_new:
    in a,($bf)
    and $80
    jr z,wait_new
    ret
    include "picture.inc"
    include "sound.inc"
    defs $7ff0-$,0
    db "TMR SEGA"
    db 0,0,0,0,0,0,0,$4c

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

Verification and sources

Target: NTSC stock SG-1000 cartridge configuration. 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.

Sega, SG-1000 technical documentation: controller/PSG I/O; TI SN76489A and TMS9918A data manuals.