Re: Detect a C128 from C64 mode

From: Gerrit Heitsch <>
Date: Thu, 23 May 2013 20:20:49 +0200
Message-ID: <>
On 05/23/2013 08:08 PM, Spiro Trikaliotis wrote:
> Hello Greg,
> * On Thu, May 23, 2013 at 10:17:23AM -0400 Greg King wrote:
>>> Unfortunately, I don't think any of these work; they all set
>>> the test-mode bit (bit 1) to 1 (on)?
>> Does that matter?  The original test bit (in $d016) crashed the
>> system because it disabled the dynamic RAM refresh (therefore, it
>> was removed).
> You mean: There are chip revisions where the test bit actually does
> something? On my 6569 (datecode from early 1983), it was a no-op.

Besides, in the C64 DRAM-refresh happens in EVERY RAS-cycle, no matter 
whether CPU or VIC are acessing ROM or RAM. This is due to RAS being 
connected directly to the RAM while CAS is gated by the PLA. So even if 
CPU or VIC access ROM, the lower 8 address bits are latched by RAS into 
the RAM and address a row. Which will be written back at the end of the 
cycle and therefore refreshed.

Unless you have RAM that really insists on 128cycles/2ms for the 4164 or 
256cyles/4ms for the 41464 (most do not but keep the data quite a bit 
longer), just the memory read cycles by VIC necessary to display the 
image should be enough to keep the RAM from becoming forgetful.


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Received on 2013-05-23 19:01:02

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