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Path: utzoo!linus!philabs!cmcl2!seismo!hao!noao!terak!doug
From: doug@terak.UUCP
Newsgroups: net.micro.cbm
Subject: Re: 1541 drive speedups?
Message-ID: <177@terak.UUCP>
Date: Mon, 15-Oct-84 16:30:46 EDT
Article-I.D.: terak.177
Posted: Mon Oct 15 16:30:46 1984
Date-Received: Thu, 18-Oct-84 06:44:42 EDT
References: <3377@ecsvax.UUCP>
Organization: Terak Corporation, Scottsdale, AZ, USA
Lines: 42

[]

> Has anyone seen soft- or hardware that speeds up the data transfer rate
> of the 1541 disk drive to something more reasonable?  I have a copy of
> the 4 minute disk copy program from Compuserve, and I've timed its track
> reads at about 2 seconds, for a data rate of at least 16k bits/sec, if my
> arithmetic is right.  In normal operation, the transfer rate is about 2k
> bits/sec, slower by a factor of 8!...

Some background information:

The primary limitation on the speed of the disk drive is that
the serial cable is operated at a very low rate (8K bits per sec).
This is because the C-64 does not use a USART chip to take 8 bits
from the cable and make a byte out of them, but rather requires
that the CPU chip receive each individual bit.  This worked OK in
the VIC-20, but the C-64's CPU chip can be rendered "unconscious"
by the VIC-II video chip for up to 50 microseconds at a time.
This occurs every 8 scan lines, when the VIC-II needs to look at
what characters will be displayed for the next 8 scan lines.

So Commodore had to slow down the communications speed on the
serial cable so that even when the CPU chip was asleep for 50
microseconds that it wouldn't miss a data bit.  This is the
difference between a VIC-1540 disk drive and the VIC-1541
drive.  The 1541 is slowed down.

It is possible to make the C-64/1541 combination run as fast as
a VIC-20/1540.  First you have to set the bit in the VIC-II chip
that causes it to blank the screen (like in cassette saving and
loading).  Then send the "UI-" command to the disk drive.  This
sets it in the faster mode.  You can now operate at the faster
VIC-20 speed.  Except your screen will be totally blank.

Another way to increase the speed is to transfer large blocks
of data in one disk operation.  Normally, for each byte you want
to read, about half a dozen others are passed back and forth
with the disk to select the proper disk drive and file.

Hope this provides you with some clues.

        Doug Pardee -- Terak Corp. -- ...!noao!terak!doug