Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!bloom-beacon!apple!voder!pyramid!cbmvax!fred
From: fred@cbmvax.UUCP (Fred Bowen)
Newsgroups: comp.sys.cbm
Subject: Re: turbodisks
Keywords: Faster Serial Communications for C-64 and 1541
Message-ID: <5994@cbmvax.UUCP>
Date: 17 Feb 89 13:28:47 GMT
References: <8902130828.AA09474@MATH.Tau.Ac.IL> <2719@nunki.usc.edu>
Reply-To: fred@cbmvax.UUCP (Fred Bowen)
Organization: Commodore Technology, West Chester, PA
Lines: 24

In article <2719@nunki.usc.edu> aliu@nunki.usc.edu writes:
>One simple and unsophisticated (therefore, not that dramatic) approach
>to the speed-up problem is by using the 1541 fast mode.  I remember
>that the VIC-20 could make serial transfers 25% faster than the C-64,
>that's why C-64's had such a hard time reading the old 1540 drives.
>People who had VIC-20's could take advantage of this extra speed by
>sending the user command 'uj' to the disk (I am not 100% percent sure
>about this, I don't have my Disk Manual handy, but I think is fairly

The command is UI- to remove a 40us delay per bit, imposed to cope with VIC-II
DMA's.  As you indicated, blanking the VIC screen (POKE53265,PEEK(53265)AND239)
or placing a C128 in FAST mode stops VIC DMA's and so this delay can be
removed.  The command UI+ re-imposes the delay.

So, like the C64's MAX mode, the 1541 has a 1540 mode.  Guess that makes the
C128D something like, um, 7 machines in one?  The extremes to which we go in
the name of compatibility boggles the mind, doesn't it?
--
-- 
Fred Bowen			uucp:	{uunet|rutgers|pyramid}!cbmvax!fred
				arpa:	cbmvax!fred@uunet.uu.net
				tele:	215 431-9100

Commodore Electronics, Ltd.,  1200 Wilson Drive,  West Chester,  PA,  19380