PC Hardware in a Nutshell, 3rd Edition

IDE Capacity Limits

Various IDE capacity limits exist, which depend on the BIOS, interface hardware, operating system, and other factors. There’s a lot of information and misinformation about these limits, so it’s worth getting the facts straight before you accept unnecessary limitations. Most of these limits are a result of interactions between the methods that the BIOS and the ATA interface use to address sectors on a hard disk. Even the oldest BIOS or ATA interface can address large hard disks. In combination, however, interactions between the BIOS and the interface may limit the number of addressable sectors to a fraction of the number either could address alone. Table 13-5 summarizes these limits.

Table 13-5. ATA addressing and BIOS addressing limitations

Maximum addressable	ATA limit	Int13 limit	Shared limit
Cylinders	65,536 (216)	1,024 (210)	1,024
Heads	16 (24)	256 (28)	16
Sectors per track	255 (28 - 1)	63 (26 - 1)	63
Disk size (bytes)	136,902,082,560	8,455,716,864	528,482,304

ATA addressing uses four registers that total 28 bits. ATA numbers cylinders starting at 0. The cylinder address is a 16-bit value, divided as a least-significant 8-bit Cylinder Low register and a most-significant 8-bit Cylinder High register, allowing up to 65,536 cylinders to be addressed. Each cylinder has a number of heads, also numbered starting at 0, which are addressed using a 4-bit value stored in the lowest four bits of the Device/Head register, allowing up to 16 heads ...