[Coco] Coco 3 Memory Map Questions

Dave Philipsen dave at davebiz.com
Mon Feb 8 01:49:26 EST 2016

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.


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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