[Coco] 6309 Weirdness ??

[Robert Gault]

Darren A. wrote:
 ><SNIP>
> Probably best to avoid using the PC register with these instructions 
> unless interrupts are masked.
> 

It is probably best to avoid using code that modifies itself in the 
middle of an instruction regardless of interrupts. The glitches you have 
found with PC probably should constitute illegal instructions but the 
6309 is not doing that.

If your examples were desirable code, there are other ways to get 
correct results using opcodes that are known to work. For addr x,pc use 
something like leax x,pcr tfr x,pc

I believe you don't have any intentions of using instructions like addr 
x,pc but rather won't to be sure that assemblers/disassemblers function 
correctly. That being the case, you are doing a great job of finding 
6309 bugs.

Here is an equivalent "bug" in the 6809. Experiment with leax 0,pc(r) 
with different assemblers and see what the 6809 actually does. The 
expected result would be that regX gets the address of the next instruction.