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Reimplement idx and idy to use 16-bit addresses

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While I have found some conflicting details on how this should be done,
it seems pretty clear to me that IDX and IDY work with 16-bit addresses,
not 8-bit. The inability to do so was preventing us from probing
peripheral ROM (e.g. at $C705, for the disk controller).
This commit is contained in:
Peter Evans 2018-01-27 19:54:55 -06:00
parent 9402eccc04
commit 2c3d6a4dfa
1 changed files with 23 additions and 5 deletions
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28
src/mos6502.addr.c
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@ -169,11 +169,19 @@ DEFINE_ADDR(ind)
DEFINE_ADDR(idx) DEFINE_ADDR(idx)
{ {
vm_8bit addr; vm_8bit addr;
vm_16bit caddr;
addr = mos6502_get(cpu, cpu->PC + 1); // The address we are given as an operand must be immediately
addr = mos6502_get(cpu, addr + cpu->X); // incremented by the content of the X register.
addr = mos6502_get(cpu, cpu->PC + 1) + cpu->X;
EFF_ADDR(addr); // And the combined address will then be the point of the LSB to a
// 16-bit pointer; so addr+1 holds the MSB, and we combined it in
// the usual, little-endian way.
caddr = (mos6502_get(cpu, addr + 1) << 8) | mos6502_get(cpu, addr);
// And that's really it--that's our effective address.
EFF_ADDR(caddr);
return mos6502_get(cpu, eff_addr); return mos6502_get(cpu, eff_addr);
} }
@ -187,11 +195,21 @@ DEFINE_ADDR(idx)
DEFINE_ADDR(idy) DEFINE_ADDR(idy)
{ {
vm_8bit addr; vm_8bit addr;
vm_16bit caddr; // combined address
// The immediate address we know is the operand following the
// opcode.
addr = mos6502_get(cpu, cpu->PC + 1); addr = mos6502_get(cpu, cpu->PC + 1);
addr = mos6502_get(cpu, addr);
EFF_ADDR(addr + cpu->Y); // But that's just the first part of the combined pointer address we
// care about; caddr therefore is the combined address pointed at by
// addr and addr + 1, with respect to little-endian order (ergo
// mem[addr+1] is the MSB, mem[addr] is the LSB).
caddr = (mos6502_get(cpu, addr + 1) << 8) | mos6502_get(cpu, addr);
// But that's not all! We also need to increment that combined
// address by the content of the Y register.
EFF_ADDR(caddr + cpu->Y);
return mos6502_get(cpu, eff_addr); return mos6502_get(cpu, eff_addr);
} }