Path: utzoo!news-server.csri.toronto.edu!cs.utexas.edu!asuvax!ncar!zaphod.mps.ohio-state.edu!sdd.hp.com!elroy.jpl.nasa.gov!decwrl!uunet!mcsun!ukc!mucs!uk.ac.man.cs From: rogersh%t5e@uk.ac.man.cs (Huw J. Rogers) Newsgroups: comp.sys.acorn Subject: Re: BBC disc drives.. Keywords: 8-bit Message-ID: <2301@m1.cs.man.ac.uk> Date: 15 Mar 91 19:14:38 GMT References: <1991Mar12.024637.12084@monu6.cc.monash.edu.au> <%`J&`M-@warwick.ac.uk> Sender: news@cs.man.ac.uk Organization: Murder Inc. Lines: 56 In article gilbertd@p4.cs.man.ac.uk (Dave Gilbert) writes: >I thought 1.2Mb drives had a different type of magnetic head - something to do >with needing a different amount of magnatisation for the different media > >(at least thats the story I heard why the 1.2Mb discs aren't SUPPOSED to work >on other drives) There are three separate issues in disk formatting: 1) The track/sector layout (no. sectors/track, sector numbering, etc.) 2) The magnetic density (single, double, or quad) 3) The number of heads (sides) used (1 or 2) BBC DFS is 40/80 track, single density (FM encoding), 1 sided. BBC ADFS is 80 track, double density (MFM encoding), 2 sided. PC 1.44Mb (1.2Mb formatted) is 80 track, quad density (MFM2 encoding), 2 sided. Drives can step to either 40 or 80 tracks in the 5.25'' world, and can be single or double sided. All 3.5'' drives step to 80 tracks, but apart from this 3.5'' and 5.25'' drives are identical and can have as many configurations as each other. The density affects the amount of data you can store per track: FM - 2.5k; MFM - 5k; MFM2 - 10k. This data is split up into sectors (of either 128, 256, 512 or 1024 byte length) on the track. Drives are limited at the h/ware level as to what they can read by the following possibilities: 1) drive is limited to 40 track step resolution. 2) drive has only one head. 3) drive's FDC (Floppy Disk Controller) is FM only. 4) drive's FDC is FM/MFM only. The FDC is mounted internally in the computer and communicates the FM/MFM/MFM2 signal direct to the drive via the interface cable. BBC drives were so ubiquitous and supplied by so many different manufacturers that they fell into some or all of the above limitations. However none of them could read MFM2. The 8271 FDC was FM only, the later WD1770 was FM/MFM. The Archimedes 3.5'' drive also has this latter limitation - it cannot read MFM2 because it's FDC is a VL1772 (which is a WD1772 clone which is essentially a faster stepping variant of the BBC's WD1770.) The actual magnetic head is not, as far as I am aware, an issue here. STs and PCs also use the WD177x FDC standard as do IBM minis/mainframes, DEC vaxes (inc. MicroVaxes), Suns, and almost every other computer/embedded system manufacturer: Hence the ability to read each others disks given the right software/FDC combination. Sorry about this lecture, but the subject needed some airing judging from recent discussion... ;-) -Huw [ H.J.Rogers (INTERNET: rogersh%p4%cs.man.ac.uk@cunyvm.cuny.edu) ] [ ,_, (BITNET/EARN: rogersh%p4%cs.man.ac.uk@UKACRL.BITNET) ] [ :-(_)-o (UUCP: ...!uunet!cunyvm.cuny.edu!cs.man.ac.uk!p4!rogersh) ] [ _} {_ (JANET: rogersh%p4@uk.ac.man.cs) ]