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70 lines
1.5 KiB
C
70 lines
1.5 KiB
C
/*
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* mos6502.exec.c
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*
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* These instructions concern program execution; things like JMP, JSR,
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* BRK, and so forth.
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*/
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#include "mos6502.h"
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#include "mos6502.enums.h"
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/*
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* The BRK instruction will set the interrupt bit; will push the current
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* PC address to the stack; and will advance the counter by 2 positions.
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*/
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DEFINE_INST(brk)
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{
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mos6502_push_stack(cpu, cpu->PC);
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mos6502_push_stack(cpu, cpu->P);
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cpu->P |= MOS_INTERRUPT;
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cpu->PC += 2;
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}
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/*
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* A jump is straight forward; whatever the effective address is, that
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* is now the new value of the PC register.
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*/
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DEFINE_INST(jmp)
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{
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cpu->PC = cpu->last_addr;
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}
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/*
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* Meanwhile, a JSR (or jump to subroutine) is a little more nuanced. We
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* record our current position, plus two, to the stack, and jump the
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* effective address.
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*/
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DEFINE_INST(jsr)
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{
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mos6502_push_stack(cpu, cpu->PC);
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cpu->PC = cpu->last_addr;
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}
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/*
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* The NOP instruction is short for no-operation. It does nothing except
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* waste cycles (which happens elsewhere).
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*/
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DEFINE_INST(nop)
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{
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// do nothing
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}
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/*
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* Here we return from an interrupt, which effectively resets the PC
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* register to the last value on the stack.
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*/
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DEFINE_INST(rti)
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{
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cpu->P = mos6502_pop_stack(cpu);
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cpu->PC = mos6502_pop_stack(cpu);
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}
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/*
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* The RTS instruction (return from subroutine) works the same as the
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* RTI instruction, which may or may not be a misconception on my part.
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*/
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DEFINE_INST(rts)
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{
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cpu->PC = mos6502_pop_stack(cpu);
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}
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