;--------------------------------------------------------------
; snd_irq -- Timer-1 handler: ONE 4-bit sample for every live voice, then the
;   timer is switched off if that emptied the mixer. The OS enters it with CLD +
;   JMP (VIMIRQ) and does NOT save anything, hence the pha/pla.
;   2026-09-29 (drac030): X AND Y are pushed at full width -- the body's
;   sep #$30 clears both high bytes of the code it interrupted.
;--------------------------------------------------------------
.proc snd_irq
        rep #$10                     ; X/Y 16-bit first, pushed at that width
        phx
        phy
        sep #$30                     ; 8-bit A/X/Y for the body
        pha
    .ifdef ANTONIA2
        phb                          ; DBR = $01 inside the card's divide (math.asm
        phk                          ;   udiv24): bank 0 for the absolute operands,
        plb                          ;   or POKEY is never acked (K = 0 here)
    .endif
        lda IRQST_R
        and #$01
        beq ?mine                    ; bit0 = 0 -> Timer-1, ours
        lda PORTB                    ; foreign IRQ. snd_old_irq points INTO THE
        lsr                          ;   OS ROM, and the frame loop runs with it
        bcs ?chain                   ;   banked OUT (rom_out + urom_init), so the
        lda POKMSK_R                 ;   old chain jumped into whatever RAM lies
        sta IRQEN_R                  ;   under $E000-$FFFF and, never acking, came
        jmp ?out                     ;   straight back -- the freeze the 2026-08-07
                                     ;   trace caught parked here.
?chain
    .ifdef ANTONIA2
        plb
    .endif
        pla                          ; A (one byte), then Y and X at the width
        rep #$10                     ;   they were pushed; the OS chain gets
        ply                          ;   8-bit X/Y
        plx
        sep #$10
        jmp (snd_old_irq)
?mine
        stz IRQEN_R                  ; ack Timer-1: drop bit 0, then restore it.
        lda POKMSK_R                 ;   POKMSK is $01 here -- this IS its timer,
        sta IRQEN_R                  ;   and snd_init/snd_disarm are the only
                                     ;   writers (it holds $00 or $01, never
                                     ;   more) -- so `and #$FE` of it is 0

        ldx #SND_VTOP
?voice  lda sv_act,x                 ; 0 = idle, 1 = playing
        beq ?next
        ; --- WHICH NIBBLE: bit 7 of the voice's fraction. sv_frc counts the ...
        lda sv_cur,x
        bit sv_frc,x                 ; N = bit 7 (A untouched): the LOW nibble
        bmi ?nib                     ;   is current
        lsr                          ; (NOT `?out` -- that is the handler's own
        lsr                          ;  exit label further down, and reusing it
        lsr                          ;  silently pointed the empty-mixer sweep's
        lsr                          ;  `beq ?out` back INTO this loop)
?nib    and #$0F
        ora #$10
        bit sv_side,x                ; snd_out INLINE on the sample path
        bmi ?so_r                    ;   (2026-09-15: the jsr/rts went, ~3,960
                                      ;   times a second while anything plays).
        sta.l AUDC1_R,x              ; 2026-09-22 (rapidus-bus-timing): long,x has no
        bvs ?so_d                    ;   dummy read of POKEY before the store -- abs,x
?so_r   sta.l AUDC1_R+$10,x          ;   paid a second chip cycle for it
?so_d                                ; output ASAP -- less jitter. STEREO
                                     ;   (2026-08-31): the voice's side routes ...
        lda sv_frc,x
        clc
        adc sv_stp,x
        sta sv_frc,x
        bcc ?next                    ; still inside this byte
        inc sv_rl,x                  ; count UP toward $0000 = sample finished
        bne ?adv
        inc sv_rh,x
        bne ?adv
        inc sv_fin                   ; a voice just went quiet -- the ONLY way
                                     ;   the mixer can empty, so the sweep below
                                     ;   is owed exactly here (see ?any)
        lda #0                       ; done: this voice only. The timer belongs
        sta sv_act,x                 ;   to ALL of them, so only the sweep after
        jsr snd_out                  ;   the loop may switch it off. snd_out's
        beq ?next                    ; (always: BIT with A=0 leaves Z=1, and
                                     ;   both stores keep it -- the contract
                                     ;   this beq was already leaning on)
?adv    inc sv_al,x                  ; advance this voice's read address
        bne ?fetch
        inc sv_ah,x                  ;   (a sound never crosses a bank end, so
                                     ;    sv_ab is never touched -- wadsound.py)
        ; --- fetch the next byte.
?fetch  jsr snd_fetch
?next   dex
        dex
        bpl ?voice

        ; --- did that empty the mixer?
        lda sv_fin
        beq ?out
        stz sv_fin
        ldx #SND_VTOP
        lda #0
?any    ora sv_act,x
        dex
        dex
        bpl ?any
        cmp #0
        bne ?out
                                      ; 2026-09-22 idiom: snd_disarm inlined (-12; its
        stz POKMSK_R                 ;   live body is these two stz -- see the proc)
        stz IRQEN_R
?out
    .ifdef ANTONIA2
        plb
    .endif
        pla                          ; A ...
        rep #$10                     ; ... then Y and X at the FULL width the
        ply                          ;     entry pushed them
        plx
        rti                          ; (RTI's pulled P puts the real widths back)
.endp