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

L. Curtis Boyle curtisboyle at sasktel.net
Tue Jan 22 14:12:55 EST 2008

On Tue, 22 Jan 2008 12:31:13 -0600, Darren A. <darccml at hotmail.com> wrote:

> I can reproduce this reliably on my 128K CoCo3. So far, nobody else has  
> reported their results.
> In all my tests, when a LDB ,X instruction is executed with $FFA0 in X,  
> bits 6-7 of the value loaded into B match bits 6-7 of the next  
> instruction opcode.
> I am no hardware expert, but it seems to me that this would infer that  
> the next instruction opcode is appearing on the data bus prior to the  
> read of $FFA0. Is it possible that the 6809 does a "look ahead" when  
> executing an indexed instruction with a zero-displacement postbye?  
> Something like:
> Cycle 1:  Fetch LDB opcode
> Cycle 2:  Fetch Indexing postbyte
> Cycle 3:  Peek at the next instruction byte
> Cycle 4:  Read data from [X]
> Darren

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

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