75 lines
1.6 KiB
ArmAsm
75 lines
1.6 KiB
ArmAsm
;================================
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;================================
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; mockingboard interrupt handler
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;================================
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;================================
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; On Apple II/6502 the interrupt handler jumps to address in 0xfffe
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; This is in the ROM, which saves the registers
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; on older IIe it saved A to $45 (which could mess with DISK II)
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; newer IIe doesn't do that.
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; It then calculates if it is a BRK or not (which trashes A)
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; Then it sets up the stack like an interrupt and calls 0x3fe
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; Note: the IIc is much more complicated
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; its firmware tries to decode the proper source
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; based on various things, including screen hole values
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; we bypass that by switching out ROM and replacing the
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; $fffe vector with this, but that does mean we have
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; to be sure status flag and accumulator set properly
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interrupt_handler:
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php ; save status flags
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; we don't use decimal mode so no need to clear it?
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; A is saved in $45 by firmware
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; we are assuming a II/II+/IIe here
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txa
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pha ; save X
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tya
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pha ; save Y
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; inc $0404 ; debug (flashes char onscreen)
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ay3_irq_handler:
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;==========================================
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; clear 6522 interrupt by reading T1C-L ; 4
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bit MOCK_6522_T1CL
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.include "play_frame.s"
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;=================================
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; Finally done with this interrupt
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;=================================
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done_ay3_irq_handler:
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pla
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tay ; restore Y
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pla
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tax ; restore X
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; on II+/IIe the firmware saves A in $45
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; this won't work on a IIc/IIgs
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lda $45 ; restore A
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plp ; restore flags
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rti ; return from interrupt ; 6
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;============
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; typical
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; ???? cycles
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