Hardware Reference
In-Depth Information
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 interface 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 another 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 that has an EDD-capable BIOS (1998 or newer), both the
BIOS and the drive are automatically set to use LBA mode.
An interesting quirk is that to allow backward compatibility when you boot an older operating 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 before 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, ATA-6 upgraded the LBA
functions from using 28-bit numbers to using larger 48-bit numbers.
The ATA-6 specification extends the LBA interface such that it can use 48-bit sector addressing. This
means that the maximum capacity is increased to 248 (281,474,976,710,656) total sectors. Because
each sector stores 512 bytes, this results in the maximum drive capacity shown here:
Click here to view code image
Max. Values
--------------------------------------
Total Sectors 281,474,976,710,656
--------------------------------------
Total Bytes 144,115,188,075,855,872
Megabytes (MB) 144,115,188,076
Mebibytes (MiB) 137,438,953,472
Gigabytes (GB) 144,115,188
Gibibytes (GiB) 134,217,728
 
 
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