[coco] wondering about 68k machines

jdaggett at gate.net jdaggett at gate.net
Sun Jul 10 17:44:41 EDT 2005

The 6809 was initially designed in 1977 time frame and was somewhat an upgrade 
from the 6800/6802/6804 processor. The 68K was on  the heels of the  6809 and 
came out in  the early 80's. The 6809 state machine and instruction set was all 
logic. There is no microcode in the 6809. The 68K had a redesigned state machine 
and new microcode for its instruction set. This savings alone allowed for larger 
busses to be ran along with newer 1.5 micron technology. 

Motorola wanted to have a line of 8 and 16 bit specialized processors that could be 
used for dedicated tasks. What we now call embeeded processors. It appears that 
the 6809 was not going to be that processor core. Motorola took what they learned 
from the 6809/68K  and plowed that into the 6811. About 1985 the HC11 series 
started. If I remember correctly, Motorola decide not to enter into the 8 bit PC 
market and that kind of really set the death watch for the 6809. That and the 
acceptance of the HC11 series. 

In some ways the HC12 which came out in 1987/8 time frame was the  successor 
for the 6809. It is a 16 bit machine that is u pward compatible for the HC11. 
Interestingly the indexed addressig modes of the 6809 were added as well as 
indirect indexed addressing. The HC12 is a 16 bit machine. What they failed to 
implement was the U pointer register. Outside that, a large portion of the 6809 code 
is source upward compatible. I am surprised that no one wrote a translation tool to 
translate 6809 code to HC12 code. 

I once told one of the managers of the HC11 line that the one thing that would have 
made the HC11/12 a better processor was to have had the U pointer register like 
the 6809. He agreed with me. In some ways, I think the H C12 would be better if it 
did have the U pointer register. It would be then the 16 bit version of the 6809.

I once looked at what it would take to port Extended Color Basic to the HC12. It is 
not difficult but a bit time consuming. Definitely need more stack space and more 
swapping on and off the stack. The code size will grow a bit in bytes but I think there 
is enough room in the memory map to handle it. 


On 10 Jul 2005 at 16:30, RJRTTY at aol.com wrote:

From:           	RJRTTY at aol.com
Date sent:      	Sun, 10 Jul 2005 16:30:47 EDT
Subject:        	Re: [coco]  wondering about 68k machines
To:             	coco at maltedmedia.com
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> In a message dated 7/10/05 3:21:02 PM Eastern Daylight Time, 
> jdaggett at gate.net writes:
> << 
>  A bit ackward to try and compare the 6809 and 68K execution times
>  with regard to peripheral access. The 6809 and 68K use different
>  state machines. >>
>        What was the purpose of that tho?   Is there some advantage to
> this configuration?     Why didn't they just extend the buses and
> clock speed of the 6809 and maybe even give it a 6809 compatiblity
> mode sorta like Intel did for the 8086 in later 80x86  cpu's?   
> Wouldn't that have been sweet?  :))   
> Roy
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