Hardware Reference
In-Depth Information
than9.44ZB(zettabytes,orquadrillionbytes).Thatisthesameassaying9.44trillionGB,
which is 9.44×10
21
bytes or, to be more precise, 9,444,732,965,739,290,427,392 bytes! I
say theoretical capacity because even though by 1998 the BIOS could handle up to 2
64
sectors, ATA drives were still using only 28-bit addressing (2
28
sectors) at the ATA in-
terface level. This limited an ATA drive to 268,435,456 sectors, which was a capacity of
137,438,953,472 bytes, or 137.44GB. Thus, the 8.4GB barrier had been broken, but an-
other barrier remained at 137GB because of the 28-bit LBA addressing used in the ATA
interface. The numbers work out as follows:
Max. Values
————————————————-
Total Sectors
268,435,456
————————————————-
Total Bytes 137,438,953,472
Megabytes (MB) 137,439
Mebibytes (MiB) 131,072
Gigabytes (GB) 137.44
Gibibytes (GiB) 128.00
By using the new extended INT13h 64-bit LBA mode commands at the BIOS level, as
well as the existing 28-bit LBA mode commands at the ATA level, no translation would
be required and the LBA numbers would be passed unchanged. The combination of LBA
at the BIOS and the ATA interface levels meant that the clumsy CHS addressing could
finally die. This also means that when you install an ATA drive larger than 8.4GB in a PC
thathasanEDD-capableBIOS(1998ornewer),boththeBIOSandthedriveareautomat-
ically set to use LBA mode.
An interesting quirk is that to allow backward compatibility when you boot an older op-
erating system that doesn't support LBA mode addressing (DOS or the original release
of Windows 95, for example), most drives larger than 8.4GB report 16,383 cylinders, 16
heads, and 63 sectors per track, which is 8.4GB. For example, this enables a 120GB drive
to be seen as an 8.4GB drive by older BIOSs or operating systems. That sounds strange,
but I guess having a 120GB drive being recognized as an 8.4GB is better than not having
it work at all. If you did want to install a drive larger than 8.4GB into a system dated be-
fore 1998, the recommended solution is either a motherboard BIOS upgrade or an add-on
BIOS card with EDD support.
The 137GB Barrier and Beyond
By 2001, the 137GB barrier had become a problem because 3 1/2-inch hard drives were
poised to breach that capacity level. The solution came in the form of ATA-6, which was
being developed during that year. To enable the addressing of drives of greater capacity,
