[Coco] (no subject)

[Mike Pepe]

Dave R in Illinois wrote:
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> So If I understand correctly, faster memory would help a tad with simple 
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> calculations but when it comes to anything being drawn on screen, the 
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> GIME is the limiting factor, not the ram. Interresting. Reason I ask is, 
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> I was interested in building a coco from scratch, similar to CoCoZilla, but
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> Replacing the 6309/6809 with a Freescale 68HC11 to lower part count.
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> Thanks a bunch :D
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> Dave
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Dave,

You could put 2ns RAM in a CoCo and it wouldn't go any faster. (if it 
existed)

The cycle time of the machine is fixed at .89MHz, which is about 1100ns- 
though you have to divide that in half, since the VDG and CPU alternate 
accessing the RAM. And then you have to divide that in half again (not 
accurate, but for our purposes, just an assumption) to allow for the RAS 
portion of the memory cycle, which gives you about 279ns to work with. 
So, 200ns or faster DRAMs will work fine. (assuming my math is right- 
though the RAS to CAS time probably isn't half the cycle, so 250ns will 
probably work)

The way a DRAM works is that the first half of the address is presented 
(RAS) and after a certain time period, the second half is presented 
(CAS) - the time period between the presentation of the CAS signal and 
the output is guaranteed to be valid is what the access time actually means.

Since the cycle is fixed, there is no difference if you're running 200ns 
or 80ns memory in a CoCo 2. In an 80ns memory, the data is ready sooner 
than 200ns memory, but the CoCo doesn't care, it'll still end the cycle 
at the same time.

The faster memory works fine, but there is zero benefit in terms of 
performance.

I pretty much use whatever I can find these days for upgrades. I think 
I've got a bunch of 60ns DRAMs in one of my CoCo 3's. And, for 
completeness sake, since the CoCo3 can run at 1.79Mhz, you'd be best off 
with at least 120ns memory. My first 512k CoCo 3 had recycled 150ns 
memory from a Kaypro and worked fine, probably because they were good 
quality chips that actually worked a little quicker than advertised.