[Coco] Coco 3 Memory Map Questions

Dave Philipsen dave at davebiz.com
Mon Feb 8 01:35:42 EST 2016

I know that the video memory must be constantly read by the GIME in 
order to generate the display in real time.  And, the CPU can access the 
memory without any wait states or contention with the video access.  I 
think I've heard others explain that the GIME reads video memory 16 bits 
at a time which effectively halves the number of accesses compared to 8 
bit reads.  In addition to all that I assume that the GIME must also 
make sure that certain address lines are stepped through sequentially in 
order to keep the contents of the DRAM stable.  So it makes sense that 
if you've got some tight timing for memory access that you get as much 
data as possible with every cycle.  I've never sat down and figured out 
all of the math behind it but it was probably a necessity to do 16-bit 
access for the video memory.


On 2/7/2016 9:05 PM, RETRO Innovations wrote:
> 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?
> I initially discounted that, because GIME has an 8 bit path.  But, I 
> think you are correct.
> U10 and U11 bridge the CPU data lines to the RAM lines when WE0 and 
> WE1 are low.  If I assume WE0 goes low on even memory address and WE1 
> goes low on odd, that's what that circuit does.
> U12 and U13 do indeed bring in 16 bits.  When CAS goes low, U12 
> bridges the even? data bank to GIME.  When CAS goes high, U13 latches 
> the odd? data bank and also bridges the latched data into GIME.
> Looking at the schematic, other items that makes me wonder:
>  * It looks like to save 1 pin on GIME, Tandy brought out S0,S1,S2
>    select lines to a '138 to select 4 different IO spaces (SCS, CTS,
>    ROM and the two PIAs).  I wonder if the other 4 select lines are
>    decoded to some spot in memory.
>  * Has anyone figured out what happens if the "test" line is brought
>    low?  I see some discussion about it online, but no results of it
>    being exercized.
> Jim

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