RE: Using large DRAM modules inside the C64

From: Gideon Zweijtzer (
Date: 2005-10-10 20:24:37

Hello Ruud,

There are two different kinds of 72-pin SIMM modules; the FPM (Fast page
mode) and EDO (Extended Data Out) modules. I would say that the FPM
SIMMs are the most compatible with the C-64 hardware. I think that you
have to make sure that the 72-pin modules are 5V as well; some are 3.3V.

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 is
not necessay to use this feature. Secondly, they support bursting, by
asserting cas more often, but once again; you don't need to use this

The biggest difference is the geometry. The C-64 rams use an 8x8 address
bit matrix (resulting in 2^8 * 2^8 addresses per chip (65536 addresses);
1 bit per chip, 8 chips => 64kB). The 72-pin SIMMs have different
geometries, such as 9*9, 8*10, 9*10, etc.. I think it would not be hard
to multiplex those from the bank register that you need for bank
switching anyway, but the pitfall is the refresh. Since the VIC chip
generates the refresh cycles by actually counting through the rows, only
2^8 of all rows will be refreshed on the SIMM. In addition to the
multiplexing of the additional row/column bits, you need to make a
distinction also between an access and a refresh cycle. It can be done,
but it requires some logic. I think it can be done by using a counter
that counts the 8 MHz clock (synchronized by the PHI2), and switching
the mux according to the output of this three-bit counter.

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?


> Hallo allemaal,
> Has anybody experience with using a 72 pins DRAM module in 
> combination with the C64? I need RAM but much much more than 
> the 256 KB I get when replacing the original 4164s with 41256s.
> Thanks for any input!
> --
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