CIA timer interrupt handler called twice during mod playback

[absence]

I've been wondering why some old menu program plays a mod file with slower arpeggios than regular trackers. I started disassembling and found out that the player is the stock standard unchanged PT-Play.s from Protracker 1.1b. The difference is that the menu program calls it from a CIA timer interrupt handler instead of at vertical blank. The strange thing is that the CIA timer runs at 25 Hz instead of the regular 50 Hz. That explains the slower arpeggio, but the tune still plays at the right speed. Eventually I found out that somehow the interrupt handler is called twice each time the timer underflows (the second time immediately after the RTE from the first run). I'm curious about why this happens, as I'm trying to learn how the old Amiga hardware works. Here's the relevant code I've found so far:

init:
    ; ...
    MOVE.L #int_handler,$00000078
    MOVE.B #$7f,$00bfdd00
    MOVE.B #$6f,$00bfd500
    MOVE.B #$03,$00bfd400
    MOVE.B #$11,$00bfde00
    MOVE.B #$81,$00bfdd00
    ; ...

int_handler:
    MOVEM.L D0-D7/A0-A6,-(A7)
    MOVE.W #$2000,$00dff09c
    TST.B $00bfdd00
    BSR.W mt_music
    MOVEM.L (A7)+,D0-D7/A0-A6
    RTE.L

Is that enough to explain what goes on, or is something important missing?

[Toni Wilen]

Originally Posted by absence

MOVE.W #$2000,$00dff09c
TST.B $00bfdd00
BSR.W mt_music

Paula interrupt is cleared but CIA interrupt line is still active -> Paula interrupt flag gets retriggered instantly.

(ADDED: I remember fixing this few years ago in UAE..)

[absence]

Originally Posted by Toni Wilen

Paula interrupt is cleared but CIA interrupt line is still active -> Paula interrupt flag gets retriggered instantly.

Ahh, I see! But why does it only retrigger once instead of continously?

Also, what code would have to be added to the interrupt handler to avoid the retrigging?

[Toni Wilen]

Originally Posted by absence

Ahh, I see! But why does it only retrigger once instead of continously?

Because tst.b $bfdd00 clears the CIA interrupt

Also, what code would have to be added to the interrupt handler to avoid the retrigging?

Clear CIA interrupts before writing to INTREQ.

[absence]

Originally Posted by Toni Wilen

Clear CIA interrupts before writing to INTREQ.

Doh! I thought I had already tried swapping the MOVE and TST, but I must have goofed up the debugger commands, because it didn't work the last time. When I tried again now, it nicely slowed down to 25 Hz. Thanks for solving the mystery!

I can't help wondering whether the effect was intended, or if the coder couldn't figure out why the music played twice as fast and simply solved it by reducing the CIA frequency.

[Toni Wilen]

Originally Posted by absence

I can't help wondering whether the effect was intended, or if the coder couldn't figure out why the music played twice as fast and simply solved it by reducing the CIA frequency.

I vote "couldn't figure out" guess