Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!wuarchive!gem.mps.ohio-state.edu!uakari.primate.wisc.edu!dogie.macc.wisc.edu!uwvax!rang From: rang@cs.wisc.edu (Anton Rang) Newsgroups: comp.sys.mac Subject: Re: SE question Message-ID: Date: 14 Oct 89 20:58:41 GMT References: <50037@<1989Oct2> <8400174@m.cs.uiuc.edu> <1989Oct12.231203.15676@NCoast.ORG> <670@wet.UUCP> Sender: news@spool.cs.wisc.edu Organization: UW-Madison CS department Lines: 40 In-reply-to: tempest@wet.UUCP's message of 14 Oct 89 18:20:07 GMT [ Why do Macs have variable-speed drives? ] The basic reason is because using a variable-speed drive was the most reliable way of packing a lot of information onto a disk when the drive was chosen. With a fixed-speed drive, there are three approaches to putting lots of data on a disk (well, besides single-sided vs. double-sided): (1) Using a more compact encoding scheme. This is how Apple ][s went from 13-sector to 16-sector drives. The hardware for this can get expensive, though, and current systems come pretty close to "perfect" as far as this goes (at least, without $$$ decoding hardware). (2) Putting more sectors on a track--driving the read/write head faster. This physically packs data closer together, at the cost of needing a faster R/W head setup. Also, some magnetic media which can support a high density is needed. (3) Putting more tracks on the disk--putting them closer together. This requires more accuracy in positioning the head, a "smaller" head, and high-density media. Options (2) and (3) require high-density media with very high quality- control standards, because the bits are actually closer together. Using a variable-speed drive mechanism means that you can put more sectors on outer tracks, without packing data closer together on the inner tracks. In general, as long as the speed control works, this will be more reliable than (2) or (3) because a larger area of the disk is magnetized for each disk. (Does anyone have wear figures for 800K disks vs. HD disks? I looked through some back ASLE issues but couldn't find any...) +----------------------------------+------------------+ | Anton Rang (grad student) | rang@cs.wisc.edu | | University of Wisconsin--Madison | | +----------------------------------+------------------+