Hardware Reference
In-Depth Information
the drive. IBM originally wrote the INT13h interface for the BIOS on the PC XT hard disk controller
in 1983, and in 1984 the company incorporated it into the AT motherboard BIOS. This interface used
numbers to define the particular cylinder, head, and sector being addressed.
Table 7.17
, which shows
the standard INT13h BIOS CHS parameter limits, includes the maximum values for these numbers.
Table 7.17. INT13h BIOS CHS Parameter Limits
The concept of a maximum value given a number of digits is simple: If you had, for example, a hotel
with two-digit decimal room numbers, you could have only 100 (10
2
) rooms, numbered 0-99. The
CHS numbers used by the standard BIOS INT13h interface are binary, and with a 10-bit number
being used to count cylinders, you can have only 1,024 (210) maximum, numbered 0-1,023. Because
the head is identified by an 8-bit number, the maximum number of heads is 256 (2
8
), numbered 0-255.
Finally, with sectors per track there is a minor difference. Sectors on a track are identified by a 6-bit
number, which would normally allow a maximum of 64 (2
6
) sectors; however, because sectors are
numbered starting with 1 (instead of 0), the range is limited to 1-63, which means a total of 63
sectors per track is the maximum the BIOS can handle.
These BIOS limitations are true for all BIOS versions or programs that rely on CHS addressing.
Using the maximum numbers possible for CHS at the BIOS level, you can address a drive with 1,024
cylinders, 256 heads, and 63 sectors per track. Because each sector is 512 bytes, the math works out
as follows:
Max. Values
---------------------------
Cylinders 1,024
Heads 256
Sectors/Track 63
===========================
Total Sectors 16,515,072
---------------------------
Total Bytes 8,455,716,864
Megabytes (MB) 8,456
Mebibytes (MiB) 8,064
Gigabytes (GB) 8.4
Gibibytes (GiB) 7.8
From these calculations, you can see that the maximum capacity drive addressable via the standard
BIOS INT13h interface is about 8.4GB (where GB equals roughly 1 billion bytes), or 7.8GiB (where
GiB means
gigabinarybytes
).
Unfortunately, the BIOS INT13h limits are not the only limitations that apply. Limits also exist in the
ATA interface. The ATA CHS limits are shown in
Table 7.18
.
Table 7.18. Standard ATA CHS Parameter Limitations
