Hardware Reference
In-Depth Information
disks. Standards committees, like politicians, have a tendency to push problems
forward in time so the next committee has to solve them.
EIDE drives and controllers also had other improvements as well. For ex-
ample, EIDE controllers could have two channels, each with a primary and a sec-
ondary drive. This arrangement allowed a maximum of four drives per controller.
CD-ROM and DVD drives were also supported, and the transfer rate was increased
from 4 MB/sec to 16.67 MB/sec.
As disk technology continued to improve, the EIDE standard continued to
evolve, but for some reason the successor to EIDE was called
ATA-3
(
AT Attach-
ment
), a reference to the IBM PC/AT (where AT referred to the then-Advanced
Technology of a 16-bit CPU running at 8 MHz). In the next edition, the standard
was called
ATAPI-4
(
ATA Packet Interface
) and the speed was increased to 33
MB/sec. In ATAPI-5 it went to 66 MB/sec.
By this time, the 128-GB limit imposed by the 28-bit LBA addresses was
looming larger and larger, so ATAPI-6 changed the LBA size to 48 bits. The new
standard will run into trouble when disks reach 2
48
2
9
bytes (128 PB). With a
50% annual increase in capacity, the 48-bit limit will probably last until about
2035. To find out how the problem was solved, please consult the 11th edition of
this topic. The smart money is betting on increasing the LBA size to 64 bits. The
ATAPI-6 standard also increased the transfer rate to 100 MB/sec and addressed the
issue of disk noise for the first time.
The ATAPI-7 standard is a radical break with the past. Instead of increasing
the size of the drive connector (to increase the data rate), this standard uses what is
called
serial ATA
to transfer 1 bit at a time over a 7-pin connector at speeds start-
ing at 150 MB/sec and expected to rise over time to 1.5 GB/sec. Replacing the old
80-wire flat cable with a round cable only a few mm thick improves airflow within
the computer. Also, serial ATA uses 0.5 volts for signaling (compared to 5 volts on
ATAPI-6 drives), which reduces power consumption. It is likely that within a few
years, all computers will use serial ATA. The issue of power consumption by disks
is an increasingly important one, both at the high end, where data centers have vast
disk farms, and at the low end, where notebooks are power limited (Gurumurthi et
al., 2003).
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SCSI disks are not different from IDE disks in terms of how their cylinders,
tracks, and sectors are organized, but they have a different interface and much
higher transfer rates. SCSI traces its history back to Howard Shugart, the inventor
of the floppy disk, which was used on the first personal computers in the 1980s.
His company introduced the SASI (Shugart Associates System Interface) disk in
1979. After some modification and quite a bit of discussion, ANSI standardized it
in 1986 and changed the name to
SCSI
(
Small Computer System Interface
).
SCSI is pronounced ''scuzzy.'' Since then, increasingly higher bandwidth versions
