[Coco] Any reason to put a 6309 in a Coco2?

[jdaggett at gate.net]

On 30 Dec 2003 at 10:37, Alex wrote:

> 
> On Dec 30, 2003, at 6:39 AM, John Guin wrote:
> 
> > So can anyone think of a reason to install it in a Coco2?
> 
> In a Co
> Co2 you can still put it in the high speed mode and see some benefit
> there, though you will undoubtedly run into compatibility problems. 
> You can always write your own stuff to take advantage of its extra
> features.
> 
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
In a Coco 1,2,3 if you have a MC6809E or a HD6309E you will see little 
difference in performance between the two devices if you are operating the Hitachi 
part as a Motorola equivalent. 

Why? Internally they are different chips. The Hitachi part does have extra registers 
and extra instructions. It also executes some of the identical instructions faster, it 
also executes a few slower. So there will be a net difference but that will be 
dependant on what the software does. On an average you may see a 5% 
improvement at best for Hitiachi part over the Motorola part with both operating at 
the same clock speed. Personally a 5% improvement is negligable irregardless of 
clock speed.

Where the performnce difference really improves is if the software is written to take 
advantages of the registers and instructions the Hitachi part offers. Anytime you can 
take advantage of register movements internal to the processor then you get a big 
speed improvement. The 16 bit multiply and internal divide instructions speed 
things up if you are doing a lot of math and number crunching. 

The real advantages of the Hitachi part is when it comes to number crunching as 
that the new math instructions reduce code space. It is a considrable savings in 
code space to load two registers with data and issue a relative instruction to divide 
and wait about 30 machine cycles and there is you answer. On the 6809 you have 
to move data in and out of the processor several times to do a divide. Slightly 
longer in machine cycles and way more in code space. 

just some of my thoughts on the subject

james


> > And a related question:  was the Coco3 100% Coco2 compatible?  Was
> > there anything (at all) a Coco 1 or 2 could do which a Coco3 could
> > not?  It seems to me if the answer to this question is no, there is
> > nothing a Coco2 can do which a Coco3 can not, then there is no
> > reason to install the chip in a Coco2.
> 
> A CoCo3 is not 100% CoCo2 compatible.  The main incompatibility is
> that most of the semigraphics modes supported by the CoCo1/2 are not
> supported by the CoCo3.
> 
> 
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