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 Post subject: 6502 Perfect Timing
PostPosted: Fri Sep 30, 2005 12:59 pm 
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Just a quick word about the 6502' interrupts. If an interrupt happens during an opcode (lets say cycle 3 of 6 ASL ZPX) is the opcode finished before the interrupt is executed? Also apparently the BRK opcode's breakpoint is ignored if the occurs, doe anyone have any more information regarding this?

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PostPosted: Fri Sep 30, 2005 2:40 pm 
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Interrupts are not handled until after an instruction has finished executing. NMI/IRQ during BRK is a special case because the 6502 handles interrupts by literally executing a BRK instruction with special flags active to specify which vector to load (and to push the 'B' flag in the proper state).

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PostPosted: Fri Sep 30, 2005 2:54 pm 
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Thanks for letting me know that. Just to clarify about the BRK bug. If an interrupt occurs during the BRK opcode then the opcode is not finished at all?

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PostPosted: Fri Sep 30, 2005 6:54 pm 
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If an interrupt happens during a BRK, then the end of the BRK will be replaced with the end of the interrupt.

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