Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!apple!marc
From: marc@Apple.COM (Mark Dawson)
Newsgroups: comp.sys.mac
Subject: Re: SE question
Message-ID: <35681@apple.Apple.COM>
Date: 16 Oct 89 14:25:18 GMT
References: <50037@<1989Oct2> <8400174@m.cs.uiuc.edu> <1989Oct12.231203.15676@NCoast.ORG> <670@wet.UUCP>
Organization: Apple Computer Inc, Cupertino, CA
Lines: 29

In article <670@wet.UUCP> tempest@wet.UUCP (Ken Lui) writes:
>In article <1989Oct12.231203.15676@NCoast.ORG> allbery@ncoast.ORG (Brandon S. Allbery) writes:
>...
>>(Why do Macs use variable speed drives?  Because 400K floppies are that much
>>nicer than 360K floppies (SS/DD), and 800K better than 720K (DS/DD).  (Do
>...
>
>It's interesting that you brought this subject up because a
>friend of mine has an IBM PC, and is using a program that
>reformats DS/DD disks to 800K and (either 1.2Mb diskettes --> 1.44Mb
>or 1.44Mb diskettes --> 1.6Mb).  So, it's possible to get the
>extra capacity with just fixed-speed drives.  Given this
>situation, is there a program or is it possible to patch the
>MacOS to give us increased storage capacity?
>
>BTW, that program is only needed to format the diskette, and once
>formatted, any drive can read it.  I always thought this variable
>drive speed was nice--400K on SS 3.5s and 800K on DS 3.5s--until
>I learned that the Amiga gets 880K on DS 3.5s.
>

I believe its a function of the disk controller chip.  Kennect's Rapport
device can get 1.2mb on DSDD 3.5" disks using a section of Apple's SWIM chip.
Apple's old IWM disk controller chip just couldn't handle it.  If you don't 
have a variable speed drive, to get some of the high densities (like the 1.2mb
format), you need to be able to vary the data rate on the disk controller chip.
I believe the IWM doesn't support variable data rates, while the SWIM does.

    Mark