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It turns out we don't use carry for addr modes.

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This fix actually corrects a rather irritating bug where the Apple ][
logo was displaying one-off--e.g., as "pple ][D".
This commit is contained in:
Peter Evans 2018-01-26 21:18:42 -06:00
parent 4509f61be6
commit ca5276cb6b
1 changed files with 9 additions and 14 deletions
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23
src/mos6502.addr.c
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@ -106,14 +106,13 @@ DEFINE_ADDR(abs)
/*
* The absolute x-indexed address mode is a slight modification of the
* absolute mode. Here, we consume two bytes, but add the X register
* value to what we read -- plus one if we have the carry bit set. This
* is a mode you would use if you were scanning a table, for instance.
* value to what we read. This is a mode you would use if you were
* scanning a table, for instance.
*/
DEFINE_ADDR(abx)
{
ADDR_HILO(cpu);
MOS_CARRY_BIT();
EFF_ADDR(addr + cpu->X + carry);
EFF_ADDR(addr + cpu->X);
return mos6502_get(cpu, eff_addr);
}
@ -125,8 +124,7 @@ DEFINE_ADDR(abx)
DEFINE_ADDR(aby)
{
ADDR_HILO(cpu);
MOS_CARRY_BIT();
EFF_ADDR(addr + cpu->Y + carry);
EFF_ADDR(addr + cpu->Y);
return mos6502_get(cpu, eff_addr);
}
@ -166,8 +164,7 @@ DEFINE_ADDR(ind)
* The indirect x-indexed address mode, as well as the y-indexed mode,
* are a bit complicated. The single, next byte we read is a zero-page
* address to the base of _another_ zero-page address in memory; we add
* X to it, which is the address of what we next dereference. Carry does
* not factor into the arithmetic.
* X to it, which is the address of what we next dereference.
*/
DEFINE_ADDR(idx)
{
@ -183,13 +180,12 @@ DEFINE_ADDR(idx)
* In significant contrast, the y-indexed indirect mode will read a
* zero-page address from the next byte, and dereference it immediately.
* The ensuing address will then have Y added to it, and then
* dereferenced for the final time. Carry _is_ factored in here.
* dereferenced for the final time.
*/
DEFINE_ADDR(idy)
{
ADDR_LO(cpu);
MOS_CARRY_BIT();
EFF_ADDR(mos6502_get(cpu, addr) + cpu->Y + carry);
EFF_ADDR(mos6502_get(cpu, addr) + cpu->Y);
return mos6502_get(cpu, eff_addr);
}
@ -235,7 +231,7 @@ DEFINE_ADDR(zpg)
/*
* In zero-page x-indexed mode, we read the next byte; add X to that;
* and dereference the result. Carry is not a factor here.
* and dereference the result.
*/
DEFINE_ADDR(zpx)
{
@ -247,8 +243,7 @@ DEFINE_ADDR(zpx)
/*
* This is, as with absolute y-indexed mode, the mirror opposite of the
* zero-page x-indexed mode. We simply use the Y register and not the X,
* and here as well, we do not factor in the carry bit.
* zero-page x-indexed mode. We simply use the Y register and not the X.
*/
DEFINE_ADDR(zpy)
{