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NOTE Hard drive makers talk about hard drive capacities in millions and billions of bytes, not megabytes and gigabytes.

Unfortunately, the BIOS routines for the original AT command set allowed a hard drive size of only up to 528 million bytes (or 504 MB—remember that a mega = 1,048,576, not 1,000,000). A drive could have no more than 1024 cylinders, 16 heads, and 63 sectors/track:

1024 cylinders × 16 heads 63 sectors/track × 512 bytes/sector 504 MB

For years, this was not a problem. But when hard drives began to approach the 504 MB barrier, it became clear that there needed to be a way of getting past 504 MB. The ATA-2 standard defined a way to get past this limit with logical block addressing (LBA). With LBA, the hard drive lies to the computer about its geometry through an advanced type of sector translation. Let’s take a moment to understand sector translation, and then come back to LBA.

Sector Translation Long before hard drives approached the 504 MB limit, the limits of 1024 cylinders, 16 heads, and 63 sectors/track gave hard drive makers fits. The big problem was the heads. Remember that every two heads means another platter, another physical disk that you have to squeeze into a hard drive. If you wanted a hard drive with the maximum number of 16 heads, you would need a hard drive with eight physical platters inside the drive. Nobody wanted that many platters: it made the drives too tall, it took more power to spin up the drive, and that many parts cost too much money (see Figure 11-12).

Manufacturers could readily produce a hard drive that had fewer heads and more cylinders, but the stupid 1024/16/63 limit got in the way. Plus, the traditional sector arrangement wasted a lot of useful space. Sectors toward the inside of the drive, for example, are much shorter than the sectors on the outside. The sectors on the outside don’t need to be that long, but with the traditional geometry setup, hard drive makers had no choice. They could make a hard drive store a lot more information, however, if they could make hard drives with more sectors/track on the outside tracks (see Figure 11-13).

Figure 11-12 Too many heads

Figure 11-13 Multiple sectors/track

The ATA specification was designed to have two geometries. The physical geometry defined the real layout of the CHS inside the drive. The logical geometry described what the drive told the CMOS. In other words, the IDE drive “lied” to the CMOS, thus sidestepping the artificial limits of the BIOS. When data was being transferred to and from the drive, the onboard circuitry of the drive translated the logical geometry into the physical geometry. This function was, and still is, called sector translation.

Let’s look at a couple of hypothetical examples in action. First, pretend that Seagate came out with a new, cheap, fast hard drive called the ST108. To get the ST108 drive fast and cheap, however, Seagate had to use a rather strange geometry, shown in Table 11-1.

Table 11-1 Seagate’s ST108 Drive Geometry

Notice that the cylinder number is greater than 1024. To overcome this problem, the IDE drive performs a sector translation that reports a geometry to the BIOS that is totally different from the true geometry of the drive. Table 11-2 shows the actual geometry and the “logical” geometry of our mythical ST108 drive. Notice that the logical geometry is now within the acceptable parameters of the BIOS limitations. Sector translation never changes the capacity of the drive; it changes only the geometry to stay within the BIOS limits.

Table 11-2 Physical and Logical Geometry of the ST108 Drive

Back to LBA Now let’s watch how the advanced sector translation of LBA provides support for hard drives greater than 504 MB. Let’s use an old drive, the Western Digital WD2160, a 2.1-GB hard drive, as an example. This drive is no longer in production, but its smaller CHS values make understanding LBA easier. Table 11-3 lists its physical and logical