[Coco] "Reading" non-readable bytes with PEEK vs ZBUG

[Darren A.]

> 
> I can't remember who researched this before (Alan Dekok?), but the 6809  
> does do a bit of prefetching for the next instruction, which is why some  
> commands can complete in 2 cycles.
> The 6309 does full pre-fetching for native mode, of course.
> 
> -- 
> L. Curtis Boyle
> 
> --


I pulled out the 6809 datasheet and read the following on page 20:

"Each instruction begins with an opcode fetch. While the opcode is being internally decoded, the next program byte is always fetched. (Most instructions use the next byte, so this technique considerably speeds throughput)."

Looking at the cycle-by-cycle flow chart for indexed instructions on page 25, I think it shows that during cycle 3, [instruction address + 2] is presented on the address bus. The loading of the data from the effective address occurs in the last cycle (cycle 4 for LDB ,X).

Darren


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