dos33fsprogs/demos/second/interrupt_handler.s
2023-10-04 01:23:02 -04:00

75 lines
1.7 KiB
ArmAsm

;================================
;================================
; mockingboard interrupt handler
;================================
;================================
; On Apple II/6502 the interrupt handler jumps to address in 0xfffe
; This is in the ROM, which saves the registers
; on older IIe it saved A to $45 (which could mess with DISK II)
; newer IIe doesn't do that.
; It then calculates if it is a BRK or not (which trashes A)
; Then it sets up the stack like an interrupt and calls 0x3fe
; Note: the IIc is much more complicated
; its firmware tries to decode the proper source
; based on various things, including screen hole values
; we bypass that by switching out ROM and replacing the
; $fffe vector with this, but that does mean we have
; to be sure status flag and accumulator set properly
interrupt_handler:
php ; save status flags
cld ; clear decimal mode
pha ; save A ; 3
; A is saved in $45 by firmware
txa
pha ; save X
tya
pha ; save Y
; inc $0404 ; debug (flashes char onscreen)
lda IRQ_COUNTDOWN
beq skip_irq_dec
dec IRQ_COUNTDOWN
skip_irq_dec:
.include "pt3_lib_irq_handler.s"
jmp exit_interrupt
;=================================
; Finally done with this interrupt
;=================================
quiet_exit:
stx DONE_PLAYING
jsr clear_ay_both
ldx #$ff ; also mute the channel
stx AY_REGISTERS+7 ; just in case
exit_interrupt:
pla
tay ; restore Y
pla
tax ; restore X
pla ; restore a ; 4
; on II+/IIe (but not IIc) we need to do this?
interrupt_smc:
lda $45 ; restore A
plp
rti ; return from interrupt ; 6
;============
; typical
; ???? cycles