RE: Using large DRAM modules inside the C64

From: Baltissen, GJPAA (Ruud) (
Date: 2005-10-11 12:05:29

Hallo Gideon,

Thank you for your extensive answer!

> some are 3.3V.

Oops, I indeed completely forgot about that.

> ... but the pitfall is the refresh. 

I realised that as well. The CBM series use some counters to generate the
refresh address and are very easy to expand. Regarding the C64, here the
VIC-II takes care of the refresh but the problem is that it is impossible to
expand. IMHO you're better off building a generator à la CBM. 
But here comes another problem in view: the CBM refreshes the DRAM with 1
MHz. Good enough for 4164's and 41256's. But AFAIK most DRAM's need a
referesh every 1 ms. and that means only 10*10 configurations, read 1M*?
DRAM's, can be serviced. Tripling the refresh rate can service 16M*? DRAM's.
Am I right?

> you need to make a distinction also between an access and a refresh
> cycle. It can be done,

In the C64 the VIC chip takes care of the refresh. And IMHO it is impossible
to detect whether a refresh or an access cycle is going on, please anyone,
proof me wrong. And even if possible, that one will be stuck to 1 MHz with
the same limitations as described above. Which means I need to build my own
generator anyway. But then I have to keep the original DRAM's and the new
RAM can only be used outside the original 64 KB.

> And then... will you only use 8 bits from the 32-bits that a SIMM
> offers? Or do you want to multiplex the data as well?

I have a datasheet of a 72 pins 1M*32 module and the pinouts show RAS0..3
and CAS0..3. IMHO this means the data can be accessed 8 bits wise. And the
PC is able to perform 8 bits operations, which is difficult with 32 bits
only memory. 

> FPM is slightly different from the 'normal' DRAMs as used in the
> C-64. For example, they support auto-refresh modes (cas-before-ras),
> but it isnot necessay to use this feature. Secondly, they support
> bursting, by asserting cas more often, but once again; you don't
> need to use this feature.

Why don't I need it? To me it seems it is much simpler to use cas-before-ras
because I need less hardware (no generator and extra multiplexers needed).
Or do I overlook something?

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