[Coco] Replacing Dynamic RAM with Static RAM

[sales at gimechip.com]

I just noticed the mention of using a single 2-Meg SRAM. That's what I've 
been working on, but the SRAM is 1024x16 for a total of 2-Megs by eight. The 
GIME has a 16-bit databus (sort of). Even bytes are in one bank and odd 
bytes in the other, thus a 16-bit SRAM or two 8-bit SRAMs are ideal. It can 
be done with an 8-bit wide SRAM as well, I suppose, but I haven't tried that 
yet.

I am using a 16x4-bit RAM array with associated multiplexing circuits to 
extend the MMU registers by 8-bits.

Since I'm using Static RAM, no modified refresh is required, plus if the 
SRAM is battery backed, the various RAMdisk software can be non-volatile 
(although some of the RAMdisks initialize the disk automatically - that 
behavior would have to be patched out)

The video can only exist within a 512K block - this is a GIME limitation. 
Using Tony's method, a register holds the Video Bank data. During a video 
cycle, the video bank data is presented to the RAM to select 1 of 4 of the 
512K banks for video.

The CPU, however, can access the entire 2-Megs via the normal paging method. 
During a CPU cycle the DAT register addresses are presented to the Static 
RAM.

There's a little bit more to it than that (for example, the dual WE* signals 
must be accounted for - I've done this by multiplexing them and using A0 to 
select which one passes to the SRAM for R/W* control - A0=0=even bank, 
A0=1=odd bank). This should be completed soon. I'll upload everything to 
coco3.com and maltemedia for any who want to make use of it.
J&R

----- Original Message ----- 
From: <sales at gimechip.com>
To: "CoCoList for Color Computer Enthusiasts" <coco at maltedmedia.com>
Sent: Friday, June 11, 2010 1:50 PM
Subject: Re: [Coco] Replacing Dynamic RAM with Static RAM


> Yes, that's what these were for, but I also uploaded a file that uses a
> 256Kx16 SRAM for the CoCo3 for 512K. It needs a bit of work. It's here:
> http://www.coco3.com/community/2010/06/512k-simm-and-sram-files
>
>
> ----- Original Message ----- 
> From: "Stephen Adolph" <twospruces at gmail.com>
> To: "CoCoList for Color Computer Enthusiasts" <coco at maltedmedia.com>
> Sent: Friday, June 11, 2010 1:21 PM
> Subject: Re: [Coco] Replacing Dynamic RAM with Static RAM
>
>
> Interesting.
> I've got a background activity thinking about how to use a single 2MB SRAM
> chip.
> Seems that this is a way to replace each individual DRAM with an SRAM.
>
> Similar, yet different. hmm more thinking.
> Steve A
>
>>> On 6/11/2010 8:57 AM, sales at gimechip.com wrote:
>>>>
>>>> Over the past year, I've had some success at replacing DRAM with SRAM. 
>>>> I
>>>> have posted some of the schematics and designs to CoCo3.com in these
>>>> threads:
>>>>
>>>> http://www.coco3.com/community/2010/05/512k-upgrade-question
>>>>
>>>>
>>>> http://www.coco3.com/community/2010/06/4164-dram-sram-replacement-1-by-jr-of-gimechip-com
>>>>
>>>>
>>>> http://www.coco3.com/community/2010/06/4164-dram-%E2%80%93-sram-replacement-2-by-jr-of-gimechip-com-c19-march-2010
>>>>
>>>> It is relatively simple to demultiplex the multiplexed address lines 
>>>> and
>>>> to replace DRAM with SRAM. The above links should provide a great deal
>>>> of
>>>> information on how I've gotten it to work.
>>>>
>>>> As soon as I get the rest of my documents together on the 512K, 1M& 2M
>>>> CoCo 3 Static RAM upgrades, I'll get those posted as well. -J&R
>>>>
>>>> --
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>>>>
>>>>
>>>
>>>
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