Code Like It's 198x

Read a button on Atari 5200

Reuse the actual machine-specific reader, with its caller, state, ownership and a diagnosed input mistake.

Build the complete caller with make -C atari-5200/patterns/assembly/input/read-action. Press fire 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.

Atari 5200 native caller after accepted input.
Captured from the running caller in the configuration below.

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

sample.inc
    lda $c010
    and #1                    ; GTIA TRIG0 reports bit 0, unlike the TIA
    eor #1
    sta current

GTIA TRIG0 at $C010 uses active-low bit 0. The inline sampler stores 0/1 in current=$1004, leaving the edge decision to its caller; A/flags 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=$1001, current=$1004; turn=$1000, field clock=$1002/$1003. 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

This reads the fire trigger only. The 5200’s analogue axes require their own sampling/calibration policy.

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

no-edge.asm
; Capability caller: original input/frame service, explicit drawing and sound.
; atari-5200: untimed turns from a new fire press, with independent field work.
; Owns display/input and $1000-$1004; stack is page 1. IRQ disabled.
turns = $1000
previous = $1001
clock_ticks = $1002
current = $1004
* = $4000
start:
    sei
    cld
    ldx #$ff
    txs
    lda #0
    sta $d40e                 ; disable ANTIC NMI ownership from BIOS
    sta $d400                 ; display-list DMA off: background-only output
    lda #0
    sta turns
    sta previous
    sta clock_ticks
    sta clock_ticks+1
    jsr init_picture
main:
    jsr wait_frame
    inc clock_ticks
    bne sample
    inc clock_ticks+1
sample:
    !source "../sample.inc"
    lda current
    nop
    nop
    nop
    nop
    beq no_action
    inc turns
    jsr trigger_sound
no_action:
    lda current
    sta previous
    jsr draw_picture
    jsr service_sound
    jmp main

wait_frame:
    lda $d40b                 ; VCOUNT bit 7 covers the tail of the NTSC field
    bmi wait_frame
wait_new:
    lda $d40b
    bpl wait_new
    rts
!source "../picture.inc"
!source "../sound.inc"
    !fill $bffe-*, $ff
    !word start               ; genuine BIOS cartridge handover address
Atari 5200 deliberately broken input caller.
Observed fault; use the working caller to repair it.

The complete caller

Working setup and integration

demo.asm
; Capability caller: original input/frame service, explicit drawing and sound.
; atari-5200: untimed turns from a new fire press, with independent field work.
; Owns display/input and $1000-$1004; stack is page 1. IRQ disabled.
turns = $1000
previous = $1001
clock_ticks = $1002
current = $1004
* = $4000
start:
    sei
    cld
    ldx #$ff
    txs
    lda #0
    sta $d40e                 ; disable ANTIC NMI ownership from BIOS
    sta $d400                 ; display-list DMA off: background-only output
    lda #0
    sta turns
    sta previous
    sta clock_ticks
    sta clock_ticks+1
    jsr init_picture
main:
    jsr wait_frame
    inc clock_ticks
    bne sample
    inc clock_ticks+1
sample:
    !source "sample.inc"
    lda previous
    eor #$ff
    and current
    beq no_action
    inc turns
    jsr trigger_sound
no_action:
    lda current
    sta previous
    jsr draw_picture
    jsr service_sound
    jmp main

wait_frame:
    lda $d40b                 ; VCOUNT bit 7 covers the tail of the NTSC field
    bmi wait_frame
wait_new:
    lda $d40b
    bpl wait_new
    rts
!source "picture.inc"
!source "sound.inc"
    !fill $bffe-*, $ff
    !word start               ; genuine BIOS cartridge handover address

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

Verification and sources

Target: NTSC 32 KiB cartridge, recorded BIOS handover. 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.

Atari, 5200/GTIA hardware documentation: trigger inputs; POKEY data sheet: AUDF/AUDC/AUDCTL/SKCTL; ANTIC VCOUNT.