Re: R-DOS (REX DOS??) on an oceanic 1541 clone drive

From: Gerrit Heitsch <>
Date: Thu, 09 Oct 2014 17:36:07 +0200
Message-ID: <>
On 10/09/2014 07:45 AM, Baltissen, GJPAA (Ruud) wrote:
> Hallo Julian,
>> With some extra logic you could also use an SRAM in your C64
> This has been used to make a C64 run at 2 MHz: 64er, february 1994. If
> interested I can upload the magazine (100 MB). Uploading only the
> article will take more time because I first have to extract it out of
> the PDF.

Well, my circuit was simpler, it just replaced the DRAMs with a single 
SRAM. Besides the 128Kx8 SRAM (only 64KB used), you need a 74HCT573 and 
a 74HCT32.

Also, to get around the VSP-bug, it might help to run _RAS through a few 
logic gates (a 74LS08 comes to mind) to delay _RAS a bit.

> What I wonder about is that the article says to use a 3 MHz 6502. I
> would say you would need a 5 MHz one:

Well, if you want to stick to NMOS, you can't get more than the 6502B (3 
MHz). If you want to use CMOS,  back then 4 MHz was max and you got the 
problem that the illegal opcodes no longer work.


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Received on 2014-10-09 16:00:08

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