danl@intelca.UUCP (Dan Lau) (04/15/86)
*** REPLACE THIS LINE WITH YOUR MESSAGE *** I just bought a 10M hard disk for my IBM/PC, but after I formatted it, format reported a size of 10584064 bytes total available instead of the usual 10592256 that is reported on a PC/XT. What is wrong? The particulars are: IBM/PC with ROM BIOS dated when original PC/XT introduced IBM/PC-DOS V3.1 OMTI Hard disk controller 5510-3 CMI 5412 drive (asme drive as used by IBM, 306 cylinders, 4 heads) FDISK was used to install one DOS partition using the whole disk. No bad sectors were reported when doing the hard and soft format. Does anyone know why I am missing 16-sectors (2 clusters of 8K each). Thanks in advance. D. Lau (408) 987-7414 intelca!danl
bmw@aesat.UUCP (Bruce Walker) (04/17/86)
In article <240@intelca.UUCP> danl@intelca.UUCP (Dan Lau) writes: >I just bought a 10M hard disk for my IBM/PC, but after I formatted >it, format reported a size of 10584064 bytes total available instead >of the usual 10592256 that is reported on a PC/XT. What is wrong? >The particulars are: > > IBM/PC with ROM BIOS dated when original PC/XT introduced > IBM/PC-DOS V3.1 > OMTI Hard disk controller 5510-3 > CMI 5412 drive (asme drive as used by IBM, 306 cylinders, 4 heads) > >FDISK was used to install one DOS partition using the whole disk. No bad >sectors were reported when doing the hard and soft format. > >Does anyone know why I am missing 16-sectors (2 clusters of 8K each). "Elementary, my dear Watson..." I'll bet you were using DOS2.1 on the XT and just now switched to DOS3.1 when you FDISK'ed and formatted the new drive. Every release of DOS fiddles some parameter or another and 3.1 is no exception. The culprit here is FDISK. FDISK's task is to create and maintain "partitions" on the fixed disk. It does this by storing a special boot program and partition table on the first sector of the disk (ie - cylinder 0, head 0, sector 1). In the table are structures which describe the starting cylinder and ending cylinder numbers for up to four separate areas, or OS'es, only one of which may be bootable at a time. Although the partitions may be specified as beginning and ending at an arbitrary sector, FDISK only allows the user to specify partitions to a cylinder boundary (remember: cylinders are the "detents" that the mechanism carrying the heads may step to, "tracks" is the product of "number-of-heads" times "number-of-cylinders"; eg: the XT standard drive has 306 cylinders and four heads, giving it a total of 1224 tracks). In DOS2.1, if you specified to FDISK "gimme the whole disk", it would set up the partition slot number 4 with the beginning of the disk as cylinder zero, head zero, sector 2 and the end of the disk as the last sector on the last track minus one (the last track is reserved for diagnostics to scribble on). Now the value for the start of the partition disobeys already the rule we said about starting on an even track boundary (eg: given cylinder, given head, sector 1) because the partition and boot block lives on the first sector. I suspect some purist somewhere (and perhaps some confused software) convinced the writers of DOS3.[01] to change that. So they did. Now, partitions begin at a minimum of cylinder zero, head ONE, sector one, which means you lose 16 sectors (the remainder of track zero after the boot block). ********** A complete description of the contents of the "Boot Record/Partition Table" is found in Appendix G of the PC/XT Technical Reference Manual. Bruce Walker {allegra,ihnp4,linus,decvax}!utzoo!aesat!bmw "I'd feel a lot worse if I wasn't so heavily sedated." -- Spinal Tap