[Coco] Kicking the Z-80 butts...

[Dave Philipsen]

That is true.  Really, in my mind what determines whether a processor is 
8-bit or 16-bit is the width of the data bus internally.  I say 
'internally' because there are some weird processors floating around 
like the 68008 which is 16-bits internal but only 8-bits on the 
outside.  The 6809s data bus is 8 bits wide and when it fetches 
instructions it does so 8-bits at a time.

What might have been sort of cool would be for the 6809 to have a larger 
address bus like 20 or 24 bits.  You'd have to increase the size of some 
registers internally too, though, and that would sort of mess things up.

Dave Philipsen


On 4/28/2015 3:06 PM, Hugo Dufort wrote:
> I've always found this "8-bit vs 16-bit classification" debate quite 
> strange.
> Most processors of the early late 1970s to early 1980s era had a 
> mostly 8-bit structure. Their working registers and most operations 
> were mostly 8-bit.
> Sure you have a D register on the 6809, but most operations 
> (especially bitwise operations) are strictly 8-bit (A, B). The joint 
> A:B=D register is necessary to use the multiplication results, for 
> instance, or to manipulate memory addresses.
> Having a joint 2x 8-bit register (A:B=D) and 3x 16-bit address 
> registers don't qualify as "16-bit computing" in my opinion.
> You can't perform much accumulator-like operations on X,Y and U. 
> Except for LD, ST and LEA.
>
> And if you look at true 16-bit processors such as the M68k, they all 
> had a few 32-bit operations, as well as 32-bit "joint" registers, but 
> that don't make them "32-bit processors".
>
> It is the same for all generations of processors.
>
> Hugo
>
> Le 2015-04-28 15:49, Dennis Bathory-Kitsz a écrit :
>> On Tue, April 28, 2015 9:24 am, CoCo Demus wrote:
>>> Who needs a Z-80 anyway ???
>> Ha! Okay, can I bit?
>>
>> It's kind of an unfair comparison as the 6809 is really a 16-bit 
>> machine on an
>> 8-bit bus. Take out those 16-bit structures and you're pretty much 
>> left with a
>> 6502 -- the most elegant processor of all time, I think.
>>
>> My first published article was in "Byte" in 1979: "Relative 
>> Subroutines for
>> the Z-80", emulating the 6502's behavior.
>>
>> Now give a Z-80 a 16-bit internal structure...
>>
>> No, don't answer me! :) :)
>>
>> Dennis
>>
>>
>>
>
>
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