6502/cc65
1
0
Fork 0
mirror of https://github.com/cc65/cc65.git synced 2025-04-06 04:41:08 +00:00
Code Issues Projects Releases Wiki Activity

Fixed the behavior of the JMP (ind) instruction in sim65 when it runs with the "6502X" CPU type.

Browse Source 
The JMP (ind) bug is present in the 6502 which is emulated by both the "6502" and "6502X"
emulation targets of sim65; specifically, the OPC_6502_6C handler. In the old code, the
bug-exhibiting code was not executed when the target was set to 6502X, which is incorrect.
the patch removes the (CPU == CPU_6502) check, which made no sense.

The JMP (ind) bug was actually fixed in the 65c02. Indeed, the OPC_65C02_6C opcode handler
has code that implements the 'right' behavior.
This commit is contained in:
Sidney Cadot 2024-11-30 12:36:35 +01:00
parent 05a653d3f9
commit 2abd66ea0c
1 changed files with 9 additions and 17 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

26
src/sim65/6502.c
View File

@ -1964,25 +1964,17 @@ static void OPC_6502_6C (void)
PC = Regs.PC;
Lo = MemReadWord (PC+1);
if (CPU == CPU_6502)
{
/* Emulate the 6502 bug */
Cycles = 5;
Regs.PC = MemReadByte (Lo);
Hi = (Lo & 0xFF00) | ((Lo + 1) & 0xFF);
Regs.PC |= (MemReadByte (Hi) << 8);
/* Emulate the buggy 6502 behavior */
Cycles = 5;
Regs.PC = MemReadByte (Lo);
Hi = (Lo & 0xFF00) | ((Lo + 1) & 0xFF);
Regs.PC |= (MemReadByte (Hi) << 8);
/* Output a warning if the bug is triggered */
if (Hi != Lo + 1)
{
Warning ("6502 indirect jump bug triggered at $%04X, ind addr = $%04X",
PC, Lo);
}
}
else
/* Output a warning if the bug is triggered */
if (Hi != Lo + 1)
{
Cycles = 6;
Regs.PC = MemReadWord(Lo);
Warning ("6502 indirect jump bug triggered at $%04X, ind addr = $%04X",
PC, Lo);
}
ParaVirtHooks (&Regs);