Hardware Reference
In-Depth Information
term
bit-shift
is used because the division and multiplication math is actually done in the
BIOS software by shifting bits in the CHS address.
With CHS bit-shift translation, the drive reported (physical) parameters are referred to as
P-CHS
, and the BIOS-altered logical parameters are referred to as
L-CHS
. After the set-
tingsaremadeintheBIOSSetup,L-CHSaddressesareautomaticallytranslatedtoP-CHS
at the BIOS level. This enables the operating system to send commands to the BIOS us-
ing L-CHS parameters, which the BIOS automatically converts to P-CHS when it talks to
the drive using ATA commands.
Table 7.21
shows the rules for calculating CHS bit-shift
translation.
CHS bit-shift translation is based on dividing the physical cylinder count by a power of 2
to bring it under the 1,024 cylinder BIOS INT13h limit and then multiplying the heads by
the same power of 2, leaving the sector count unchanged. The power of 2 used depends
on the cylinder count, as indicated in
Table 7.20
.
Table 7.20 CHS Bit-Shift Translation Rules
Here is an example of CHS bit-shift translation:
Bit-shift
P-CHS
L-CHS
Parameters
Parameters
———————————————————————
Cylinders 8,000 1,000
Heads 16 128
Sectors/Track 63 63
==============================================
Total Sectors
8,064,000
8,064,000
———————————————————————
Total Bytes 4,128,768,000 4,128,768,000
Megabytes (MB)
4,129
4,129
Mebibytes (MiB)
3,938
3,938
Gigabytes (GB)
4.13
4.13
Gibibytes (GiB)
3.85
3.85
