[Coco] MC6809 Inards

[KnudsenMJ at aol.com]

In a message dated 10/4/06 2:32:04 PM Eastern Daylight Time,  
jdaggett at gate.net writes:

>I  did  not follow every reference patent, but if one did then one maybe led 
 to 
>those patents. I did not specific search on the PDP-11.  

OK, understood.  I suspect that if one did search on patents specific  to the 
PDP-11 (provided that DEC listed them that way, maybe not; specific  products 
rarely appear in patent briefs), you would find a treasure trove of  great 
ideas, many used in every CPU since then.

>The  MC6800 and offspring use an incrementer linked with the PCR and a 16 
bit  
>temp register. There is a patent that does describe how to do  precharge 
from a 
>carry. Looks as if this does the actual  increment/decrement by one or by 
two. 


Every CPU has to auto-increment its PCR. I built a computer out of TTL DIPs  
at Bell Labs that had a set of adder chips whose only function was to 
increment  the PCR.
 
But the PDP-11 could auto-inc/dec the stack pointer reg, and I think any of  
the general-purpose regs, which could serve as index registers or  
accumulators.
The 6809 can auto-inc/dec X,Y, SP, and of course PC.
 
PS: I've been of he Coco List for a while, but this time I remembered to  
manually edit in the angle brackets to quote your text.  My emailer forgot  how 
to do that. :-(
--Mike K.