dos33fsprogs/demos/demosplash2022/interrupt_handler.s
2022-11-12 18:40:59 -05:00

63 lines
1.6 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)
ay3_irq_handler:
bit MOCK_6522_T1CL ; clear 6522 interrupt by reading T1C-L ; 4
.include "play_frame.s"
.include "ay3_write_regs.s"
;=================================
; Finally done with this interrupt
;=================================
done_ay3_irq_handler:
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 ; restore flags
rti ; return from interrupt ; 6
;============
; typical
; ???? cycles