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

[Gene Heskett]

On Wednesday 31 December 2003 16:21, tim lindner wrote:
>Gene Heskett <gene.heskett at verizon.net> wrote:
>> AIUI, running in the default powerup 6809 mode, it perfectly
>> emulates the 6809 with the exception of doing a prefetch of the
>> next instruction if it has something else to do internally, and
>> this does increase the speed a few percentage points.  But unless
>> whatever you're doing is timed by the cpu instead of the clock,
>> you'll probably not be effected in any way you can detect.
>
>In startup mode then 6309 has the same instruction cycle count as
> the 6809.
>
I've always been told that for those intructions where it was doing 
something internally, and the 6809 didn't use its bus cycle, that the 
6309 took advantage of the spare bus cycle and fetched the first byte 
at least, of the next instruction.  Is this incorrect, and it still 
has to not only fetch, but then decode the next instruction at all 
times?

This is, ISTR from data supplied at the time by Chris Burke and Chet 
Simpson, each of whom published their versions of the new opcode maps 
complete with timing info for each command.

>You don't get faster instructions until you switch to native mode.
>
>If your programs stick to the instructions listed in Motorola's
> manual, you timing loops will be fine.

-- 
Cheers, Gene
AMD K6-III at 500mhz 320M
Athlon1600XP at 1400mhz  512M
99.22% setiathome rank, not too shabby for a WV hillbilly
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Copyright 2003 by Maurice Eugene Heskett, all rights reserved.