mirror of https://github.com/pevans/erc-c.git
95 lines
1.6 KiB
C
95 lines
1.6 KiB
C
/*
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* mos6502.branch.c
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*
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* This is all the logic we use for branch instructions, which are used
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* for conditional expressions.
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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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* This is just a minor convenience macro to wrap the logic we use in
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* branch situations, which is if `cond` is true, then we set the
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* program counter to the last effective address.
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*/
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#define JUMP_IF(cond) \
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if (cond) cpu->PC = cpu->eff_addr; else cpu->PC += 2
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/*
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* Branch if the carry flag is clear.
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*/
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DEFINE_INST(bcc)
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{
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JUMP_IF(~cpu->P & MOS_CARRY);
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}
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/*
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* Branch if carry is set.
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*/
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DEFINE_INST(bcs)
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{
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JUMP_IF(cpu->P & MOS_CARRY);
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}
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/*
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* Branch if the zero flag is set (that is, if our last instruction
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* resulted in something being _equal to zero_).
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*/
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DEFINE_INST(beq)
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{
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JUMP_IF(cpu->P & MOS_ZERO);
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}
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/*
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* Branch if the negative ("minus") flag is set.
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*/
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DEFINE_INST(bmi)
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{
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JUMP_IF(cpu->P & MOS_NEGATIVE);
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}
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/*
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* Branch if the zero flag is not set; which is to say, that the last
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* operation was _not equal_ to zero.
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*/
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DEFINE_INST(bne)
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{
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JUMP_IF(~cpu->P & MOS_ZERO);
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}
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/*
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* Branch if the negative flag is not set (meaning the last operation
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* was "plus", which includes zero).
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*/
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DEFINE_INST(bpl)
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{
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JUMP_IF(~cpu->P & MOS_NEGATIVE);
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}
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/*
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* This instruction will branch in all cases. It's not a true
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* conditional; it's analagous to a relative address mode JMP.
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*/
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DEFINE_INST(bra)
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{
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// Always jump!
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JUMP_IF(1);
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}
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/*
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* Branch if the overflow bit is clear.
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*/
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DEFINE_INST(bvc)
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{
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JUMP_IF(~cpu->P & MOS_OVERFLOW);
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}
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/*
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* Branch if the overflow bit is set.
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*/
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DEFINE_INST(bvs)
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{
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JUMP_IF(cpu->P & MOS_OVERFLOW);
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}
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