[Coco] Today, I have seen the CoCo 4...

[jdaggett at gate.net]

On 27 May 2010 at 14:11, Frank Swygert wrote:

> I was wondering what the SRAM was used for! Other than power and retaining
> memory powered down, is there any real reason to use SRAM instead of
> standard DRAM SIMMs on a specially designed board? The CoCo will support
> at least 2MB, but isn't there a simple hack to allow a max of 4MB (adding
> an address line or two, IIRC)? Incorporating that in a new design would be
> ideal, then use a readily available SIMM. Two memory slots should be
> sufficient if the slots can be made to support from 256K-2MB SIMMs. That
> would at least make initial cost a bit lower, though memory isn't really
> so pricey that a board with 4MB would be all that much higher than one
> with 512K. Not when the overall price of the board is considered. But
> someone just wanting to run DECB wouldn't really need more than 512K, OS-9
> on the other hand benefits from maxing out. If there has to be a choice
> the board should be optimized for OS-9 (Nitros-9), as the majority of
> users will probably be running that. As far as I kn
>  ow there is nothing that OS-9/Nitros-9 does that would hamper DECB
> operation.


Frank

If you are referring to the 30 pin SIMM modules using EDO or Fast page DRAM, they would 
be to slow for a soft 6809 in an FPGA. At 25MHz clock speed you need 40nS access time or 
faster. That leaves the newer SDRAM chips or fast SRAM to give a soft COCO fast enough 
memory. 

The soft processor speed really is not fast enough to worry about DDR/DDR2/DDR3 ram 
either. The added complexity for the driver hardware is not worth it. OS9 stock can handle 
2Meg of memory. Using another register as a page register can extend the memory to 
essentially 256 pages of 2Meg. Theoretically 512 Megabuytes. 

The SRAM chip I have if ran at 2.5VDC has an acess time of 8 nS. At 3.3 VDC is 10nS. That 
is fast enough for 50 to 100 MHz range soft processor. 

james