Hardware Reference
In-Depth Information
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(2
10
)maximum,numbered0-1,023.Becausetheheadisidentifiedbyan8-bitnum-
ber, 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 ad-
dressing. Using the maximum numbers possible for CHS at the BIOS level, you can ad-
dress a drive with 1,024 cylinders, 256 heads, and 63 sectors per track. Because each sec-
tor 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
