Re: Using large DRAM modules inside the C64

From: Daniel Kahlin (
Date: 2005-10-12 19:53:43

On Wed, 12 Oct 2005, Spiro Trikaliotis wrote:

> I always thought the VIC refreshed the DRAM simply by accessing it. If
> you "swap" A0-A7 and A8-15 inside of the VIC, the VIC just accesses
> every row while generating its screen ($0400-$07E7), as the last byte
> cycles through all possible values. Thus, it could refresh all 64 KB RAM
> just by displaying the screen.
> Is this wrong? Does the VIC really generate some refresh cycles?

This is correct.   This is why messing with $d011, like FLD, FLI and 
line-crunch may in some cases corrupt memory.  This is especially true 
with line-crunch.   How sensitive a machine is to this is dependent on 
which revision of the c64, and probably what type of DRAM chips there are
Also, the 256Kbit chips (41256) does not require you to cycle more than 
the lowest 8-bits.  You can put 41256 chips into a regular c64, and put a 
very simple circuit for manual bank select on A8 if you like.


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