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From: asm@utcsri.UUCP (Anees Munshi)
Newsgroups: comp.sys.atari.st
Subject: External ramdisk
Message-ID: <4270@utcsri.UUCP>
Date: Mon, 2-Mar-87 00:30:22 EST
Article-I.D.: utcsri.4270
Posted: Mon Mar  2 00:30:22 1987
Date-Received: Tue, 3-Mar-87 03:43:43 EST
Distribution: net
Organization: CSRI, University of Toronto
Lines: 51


[]

Hi all.

	For my thesis I needed to use the ST's
cartridge port for output as well as input. So I designed an interface
which uses 2 latches and one PAL (16L8) which fits on a tiny cart
plugged into the cartridge port and converts the cartridge port from a
128Kbyte read only port to a 64Kbyte read/write port.  The writing
process is a two step process (it involves two reads), and allows you
to write 16bit data through the cartridge port.

	I have written software to read and write bytes and words
through this port (in assembler) and have been using this adapter for
nearly 6 months without any problems. (If you are interested in the
design mail me and I'll try to send you the relevent parts of the
documentation of my thesis which I am in the process of writing.)

	Since I didn't need too much speed on this port, I never really
benchmarked it until I read those postings about the hardware
ram-disk.  Just to see if it would be feasible to use this port to
connect to some memory and make a ramdisk, I decided to benchmark it.
The speed I obtained is about 75978 bytes/second for writes and more
than twice as much for reads. Although this is slower than the DMA
port, given that I don't know of any piece of documentation that
describes the DMA port properly, it might be worth putting a RAM-disk
on the cart. port. In fact, it should be pretty easy to put 2K of
battery-backed CMOS ram, some N meg of dynamic ram, and perhaps a
clock-calendar IC. By using a segmentation register, you could access
the DRAM in 48K chunks or so (leaving some addressable space for things you
might think of later).

	So, does anyone have a well commented ramdisk program that I
can modify easily to serve as a driver for the external ram disk?
I would prefer a program in C which I can modify quickly and tune
for efficiency later.

	-anees

-- 
	Anees Munshi @ University of Toronto Engineering.

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