boegehol@informatik.uni-stuttgart.de (Harald Boegeholz) (11/22/90)
Hi everybody! I'm about to buy a new computer, and I'm just trying to decide which hard disk/controller to buy. I want to use a 660MB hard disk, and the only available choices seem to be SCSI and ESDI. Since SCSI requires special drivers for OS/2 and I don't want to rely on these in view of future OS/2 versions, I decided to use ESDI. 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. I currently have an IBM PS/2 Model 60 with two ESDI disks. The IBM controller maps these to 64 heads and 32 sectors/track, yielding exactly 1MB/cylinder. So there can't be any 16 head-limit in the software anywhere, or are things so very different between PS/2 and the rest of the world? Does anybody know of any ESDI controller (ISA or EISA) that can handle a 660MB disk in one piece? I know that the Adaptec 2322D can split the disk into two disks, but that's a rather ugly solution (and you probably won't be able to add a second physical disk then). Or does anybody know WHY there is no such controller? Does SCSI have the same problems? Why not? ADVmanythanksANCE for any information I receive Harald Boegeholz
davidsen@sixhub.UUCP (Wm E. Davidsen Jr) (11/23/90)
In article <BOEGEHOL.90Nov22135145@az3.informatik.uni-stuttgart.de> 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
boegehol@informatik.uni-stuttgart.de (Harald Boegeholz) (11/23/90)
In article <2351@sixhub.UUCP> davidsen@sixhub.UUCP (Wm E. Davidsen Jr) writes:
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.
What kind of header do you mean? The one physically recorded on the
hard disk? I'm not converned with that (at least I don't want to be
:-) ). The hard disk needs to be remapped (by the controller) anyway,
and I'd like to know why there can't be more than 16 heads _after_ remapping.
At least I haven't found an ESDI controller that can do it.
I thought the DOS mapper packages took care of this. You pay a penalty
in performance by using the mapping.
Yes, but I want to use OS/2!
Harald Boegeholz