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Clarify behavior of pop_stack

This commit is contained in:
Peter Evans 2018-04-13 16:22:58 -05:00
parent ebb5f068fd
commit 3887f52119
1 changed files with 4 additions and 7 deletions
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11
src/mos6502/mos6502.c
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@ -211,18 +211,15 @@ mos6502_push_stack(mos6502 *cpu, vm_8bit addr)
}
/*
* Pop an address from the stack and return that.
* Increment the stack register and return the value that is pointed at
* by the new offset. Because the S register is 8-bit, the following
* (correct) behavior is exhibited: when S overflows beyond 0xFF, it
* rolls over to 0x00.
*/
vm_8bit
mos6502_pop_stack(mos6502 *cpu)
{
// The first thing we want to do here is to decrement S by 2, since
// the value we want to return is two positions back.
cpu->S++;
// We need to use a bitwise-or operation to combine the hi and lo
// bytes we retrieve from the stack into the actual position we
// would use for the PC register.
return mos6502_get(cpu, 0x0100 + cpu->S);
}