The PPC Guide to Hard Disk Sizes

If you're looking to drop a new drive into an old machine, you may need to check out the best way of configuring it. David Dorn plots the pitfalls.

Installing a new hard disk to an older computer is not just as straightforward as connecting the cables and tightening some screws. Early machines had limitations on how big a disk they could handle without some help. In order to understand what these limitations are, there are some terms I need to define before we move on.

CHS: Stands for Cylinders Heads and Sectors. PCs made prior to July 1994 used this scheme to address hard disks. CHS has a limitation of 1,024 cylinders, 16 heads and 63 sectors per track - which gives a maximum of 504MB (that's 528MB in manufacturer speak.)

Large mode: A now conventional naming for a more advanced system, where data was stored by numbering blocks of data from 0, but calculating where they sit in the hard disk by translating the logical block to a CHS system that ignores the 1024 cylinder limit. This system has a limit of 2GB and was available from July 1994 onwards.

LBA: Logical Block Addressing is a mathematical scheme for addressing sectors, beginning at cylinder 0, head 0 and sector 1, which is equal to Logical Block Address 1. This scheme maps the drive linearly until the final physical sector is reached. LBA is efficient because it reduces some system overhead by not having to convert the operating system's LBA to the BIOS CHS and then back to drive LBA. LBA can see drives up to 8.4GB

Int 13: The maximum parameters at the 8.4 GB mark are 16,383 cylinders, 16 heads and 63 sectors for a capacity of 8.455 GB. To go beyond this capacity, a new extended INT 13 function is needed from the BIOS as a support feature for the drives - and now we're talking capacities up to (currently) 80GB in one disk.

So, what do you do if your machine can handle only CHS mode drives and you've just bought yourself a spanking new 8.4GB drive - or your PC can handle Large mode, but not LBA, and you've got a 20GB drive? Have you dropped a king-sized clanger?

Well, yes and no. Most hard disk manufacturers supply translation software that loads before the operating system, and gets between it and the BIOS/Hard disk chain. What this software does is to intercept any calls to the hard disk and (effectively) handle them itself , bypassing the limitations of the PC's BIOS. In this way, you can install a decent sized drive in a PC that can only really handle a 504MB drive. It may not, though, be the best bet - there are two further options.

New Interface

The first of these is to purchase a new IDE interface card that has a replacement BIOS on it and install that. The problem here is that the combined cost of the card and new drive may well exceed the value of your old machine - it could be more cost effective to upgrade to a new system box!

New BIOS

It may also be possible to obtain a new system BIOS for your old machine, that will support the larger drive sizes. In general, you should approach your motherboard manufacturer for a new BIOS, or, if it's a flash BIOS, obtain the necessary software for doing the upgrade yourself. The latter approach seems fine - and, indeed, most of the time it's fairly straightforward, but there are times when the wrong BIOS update files get shipped or downloaded, and, if you send them to your BIOS, the machine can be rendered dead!

This last fix is especially applicable to AWARD BIOS users with machines produced prior to June 1999. AWARD BIOSES prior to that date cannot address IDE hard disks larger than 32GB (which probably seemed reasonable at the time) unless they are updated. Again, the fix will come from your motherboard manufacturer, not Award, so try the appropriate website for your motherboard.

If all of this seems just too much to handle, and you have a limited machine, print this page and take it, with your PC, to your local supplier when you want to add a larger hard disk. Your supplier ought to be able to sort out what you need in order to upgrade. Be aware, though, as I said earlier, that it's often better to change the system box completely. A modern hard disk in an old machine will almost certainly not perform to its best, and, indeed, may well slow the rest of the machine down.

 

David Dorn