Re: cbm 8032 motherboard + 4164

From: Gerrit Heitsch <>
Date: Thu, 13 Oct 2016 16:28:30 +0200
Message-ID: <>
On 10/12/2016 11:49 PM, smf wrote:
> I believe c128 came as standard with either 4164 or 41464s
> 41256 used on c128 ram upgrade
> (
> is 8 bit refresh (256 cycles)
> 414256 is 9 bit refresh (512 cycles)
> The apple 2 apparently doesn't have dram refresh, it uses the video
> fetches to refresh the memory. I'm not sure why the c64 is different.

More than one reason... First, on the C64 you can disable the display 
which disables the memory access. Second, with DRAMs that needs 128 
cycles, you have 2ms to complete those 128 cycles (with 256 it's 4ms). 
On an NTSC system a full frame takes 16ms. How much of it is spent in 
the upper and lower border and the vertical retrace where no memory 
access happens (not counting an open border with sprites)? More than 2 ms?

So in order to make sure that the DRAM is refreshed properly, they just 
implemented a refresh counter which does 5 cycles per scan line.


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Received on 2016-10-13 15:00:01

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