[Coco] Coco 3 Memory Map Questions

[Neal Crook]

>>  It is in the realm of possibility to speed up the 6809 even more and
still retain full object code compatibility. Right now the 6809 core with
8-bit data bus really smokes at 25 MHz.

Let's say that the best-case improvement from switching the data bus from
8-bit to 16-bit is 2x. The 6809 spends its time doing (i) nothing (internal
cycle, VMA=0), (i) op-code fetches and (iii) data read/write.

16-bit bus obviously does nothing for (i) (worse than nothing: you are now
wasting 2x the bandwidth)
If you have 2 opcodes to fetch (with no internal cycles) and the first is
at an odd address, you get a potential speedup
If you have 8-bit data to fetch, no speedup
If you have 16-bit data to fetch, potential speed up.. but ONLY if your
data is at a 16-bit aligned address.
Given the processing power and tools available, you could run traces and
assess the impact of this change, but I suspect it would not be anything
like 2x.

Neal.

On 8 February 2016 at 06:49, Dave Philipsen <dave at davebiz.com> wrote:

> Yes, even though the 6809 has internal 16-bit registers it still has to
> make two accesses through an 8-bit bus to get it the data for them.  The
> 16-bit access would only help if the 6809 had a 16-bit data bus.  That also
> brings up an interesting question:  For the 6809 implemented in FPGA by
> John Kent, I wonder what kind of average speed increase could be had if
> your FPGA had 16-bit memory (which the DE1 does) and you designed some
> pipelining and a 16-bit data bus for the 6809.  It is in the realm of
> possibility to speed up the 6809 even more and still retain full object
> code compatibility. Right now the 6809 core with 8-bit data bus really
> smokes at 25 MHz.  Imagine what might be possible if a 16-bit data bus
> could be designed.
>
> Dave
>
>
> On 2/7/2016 4:25 PM, L. Curtis Boyle wrote:
>
>> Isn't the 16 bit access to memory more for the GIME, for accessing
>> graphics memory to send to the screen?
>>
>> Sent from my iPhone
>>
>> On Feb 7, 2016, at 3:59 PM, RETRO Innovations <go4retro at go4retro.com>
>>> wrote:
>>>
>>> On 2/7/2016 12:57 PM, Robert Gault wrote:
>>>> RETRO Innovations wrote:
>>>>
>>>> I'll try to answer these questions although I'm not sure I know just
>>>> what you meant by some of them. If you don't already have a copy, get the
>>>> Tandy Coco3 service manual, #26-3334. There are .pdf copies.
>>>>
>>> Looked over my copy before posting, and yes, it is a fine resource.
>>>
>>>> Since the cpu has 16-bit registers, it make sense for memory to be
>>>> accessed at that width.
>>>>
>>> That doesn't make sense to me.  The 6809 data path is 8 bits wide,
>>> regardless of the width of internal registers.  Just like the 68008 used an
>>> 8 bit data path even though the registers were 16 and 32 bits wide.
>>>
>>>
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>>>
>
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