[Coco] CoCo 3 display offset question.

[jdaggett at gate.net]

On 14 Sep 2006 at 18:38, Phill Harvey-Smith wrote:

> Hi,
> 
> Can anyone tell me if on the CoCo 3 the display generation in the GIME 
> goes through the mapped memory or if it is limited to a particular block 
> (or blocks). A quick check using mess seems to suggest that it honours 
> the tradition display offset regs at $FFC6-$FFD3, but I just verified 
> this with a quick 4 line basic program, which will have had the mapping 
> regs turned off (well CoCo 1/2 compatible mode anyway).
> 
*****************

Phil the GIME chip sees the complete 512K memory map. In the Coco compatible mode, 
bit#7 of the INIT0 register($FF90) is set then the GIME chip will respond to data plugged 
into the registers at $FFC6 to $FFD3. Then the GIME chip will only see the upper 64K of the 
512K memory map from what I understand. 

Clearing bit#7 allows the user to program the starting address of the video page in 8 bit 
boundaries by programming the upper 16 bits of the address in memory at $FF9D and 
$FF9E. Another way is to take the 19 bit address and shift right three times(divide by 8). 
Also remember that the GIME chip sees the memory as 256Kx16 and not 512Kx8. Thus 
A19 down to A4 is programmed into the Also the refresh counter in the GIME is only an 8 bit 
counter. Therefore the refresh is 256 cycles in 4 mS. the greates density is 256Kx1 drams 
unless you add an external refresh counter  or other means of refresh. 

james

 
> I'm asking as one of the things I am working on is to use a 1M 30 pin 
> SIMM, and build a banked system for the CoCo 1/2 & Dragon, using a CPLD, 
>   a small SRAM to do the banking. The current design however takes the 
> Address bus input to the SAM, and modifies this through the bank 
> registers before it goes into the SAM. This should work as far as the 
> CPU is concerned, but a side effect may well be that the CPU and the VDG 
> will see the display memory differently, whilst this probably won't be a 
> problem it's something I'll have to bear in mind :)
> 
> Cheers,
> 
> Phill.
> 
> -- 
> Phill Harvey-Smith, Programmer, Hardware hacker, and general eccentric !
> 
> "You can twist perceptions, but reality won't budge" -- Rush.
> 
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