Path: utzoo!censor!geac!torsqnt!news-server.csri.toronto.edu!rutgers!usc!sdd.hp.com!uakari.primate.wisc.edu!crdgw1!sixhub!davidsen From: davidsen@sixhub.UUCP (Wm E. Davidsen Jr) Newsgroups: comp.sys.ibm.pc.hardware Subject: Re: ESDI harddisk >512MB: WHY is there a 16head-limit? Message-ID: <2351@sixhub.UUCP> Date: 23 Nov 90 02:21:10 GMT References: Reply-To: davidsen@sixhub.UUCP (bill davidsen) Distribution: comp Organization: *IX Public Access UNIX, Schenectady NY Lines: 27 In article boegehol@informatik.uni-stuttgart.de (Harald Boegeholz) writes: | Looking for ESDI controllers, I've come across the following problem: | All ESDI controllers remap the disk in some way to get below the 1024 | cylinder limit of BIOS, DOS etc. The number of sectors per track is | limited to 64 for the same reasons. | | But WHY -- and this is the question -- is the number of heads after | remapping restricted to 16? I.e., 3 ESDI controllers I know of support | no more than 16 heads: | WD1007V-SE2 | Adaptec 2322D | DTC 6180HD | And by multiplying 1024cyl*64sec*16hd*512Bytes one gets 512MB. In the header on each sector there is a 2 byte field which holds the cylinder number in 12 bits, and the head number in 4 bits. Therefore you can't write a header for a head number > 15 or a cylinder number > 4096. The second doesn't seem to be a problem today. I thought the DOS mapper packages took care of this. You pay a penalty in performance by using the mapping. -- bill davidsen - davidsen@sixhub.uucp (uunet!crdgw1!sixhub!davidsen) sysop *IX BBS and Public Access UNIX moderator of comp.binaries.ibm.pc and 80386 mailing list "Stupidity, like virtue, is its own reward" -me