[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk BIOS and Capacity Factors | BIOS and the Hard Disk ]

Since the system BIOS on virtually every PC provides native support for IDE/ATA hard disks, there are a number of parameters that can be set to tell the BIOS what hard disks are in the system, and how to control them. Each hard disk in the system will have its own settings, so there is one set for the primary master, one for the primary slave, and so on. This normally applies only to IDE/ATA hard disks; SCSI hard disks are configured through their host adapter and built-in SCSI BIOS.

Most modern BIOSes support hard disk autodetection, which allows the BIOS to interrogate each hard disk to determine its logical geometry, supported transfer modes and other information. This can be done either at setup time or dynamically each time the machine is booted, depending on the BIOS. This is described in detail here.

Dynamic autodetection is the standard way of setting up modern drives, especially since drives over 8 GB in size cannot be described using traditional IDE/ATA BIOS geometry parameters. In some situations, especially with older PCs, you may still set some drive parameters manually. The following are the settings normally found in the BIOS setup program for configuring IDE/ATA hard disks. Since these are described in full detail in the BIOS chapter, I include only a very brief description of each here. Note that on modern systems some of the oldest compatibility settings may not even be present any more:

• Disk Type: Originally used to allow you to pick your hard disk from a predefined list, this is now used to control automatic or manual parameter setup for the drive. (The old tables describing ancient drives are now often not even present on many modern BIOSes, since they no longer serve any purpose.)
• Size: This is the size of the drive in decimal megabytes. It is calculated from the other parameters.
• Cylinders: The number of logical cylinders on the disk. The value used depends on whether BIOS translation is enabled, in some BIOSes. For a drive set to "Auto", no number may be shown.
• Heads: The number of logical heads on the disk. The value used depends on whether BIOS translation is enabled, in some BIOSes. For a drive set to "Auto", no number may be shown.
• Sectors: The number of logical 512-byte sectors in each logical track on the disk. This is usually 63 for modern drives. Again, for a drive set to "Auto", you may not see a number here.
• Write Precompensation: A compatibility setting that specifies at which cylinder number write adjustments should be made, for very much older drives.
• Landing Zone: The cylinder where the heads are parked by the BIOS when the drive is shut off; not used on modern drives since they automatically park their heads.
• Translation Mode: The BIOS translation mode being used, for support of hard disks over 504 MB. Translation issues are discussed in detail here.
• Block Mode: Controls the BIOS's ability to perform disk transfers in blocks.
• PIO or DMA Mode: The programmed I/O mode or DMA mode used to perform transfers to and from the hard disk.
• 32-Bit Transfer Mode: Controls the use of higher-performance 32-bit data transfers.

Next: Hard Disk Size Barriers

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