Perspectives from DPT....
IDE versus SCSI: Which disk interface is best

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The first PC/AT computers introduced by IBM in the early eighties used
--low-capacity, ST506 disk drives. The disk controller used in these early
machines, a Western Digital WD1003, was quite unsophisticated by today's
standards. The WD1003 controlled up to two ST506 drives, and used the PC's
processor to move the data-to and from a buffer in the controller.

Demand for higher disk storage capacities soon reached the limits of ST506
drives. A new disk interface which offered a solution to this limitation
was ESDI (Enhanced Small Disk Interface). Because of the large degree of
similarity between the older ST506 and new ESDI interfaces, it was easy
for controller manufacturers to build ESDI controllers which could emulate
the original WD1003 ST506 controller register set and protocol. This
avoided the necessity of writing a new BIOS as well as new disk drivers
for every PC operating system. As a result, ESDI was initially the
interface of choice for high-capacity drives used in PCs.

Unfortunately, ESDI also had limitations. Like the ST506, it transferred
data serially, one bit at a time. This placed practical limitations on the
data transfer rate, and because of the fixed rotational speed, also the
maximum number of bits that could be squeezed onto each track on the disk
media. In the late eighties, drive manufacturers introduced the first
fully intelligent SCSI disk drives by including an embedded disk
controller in the drive electronics.

In addition to offering faster, parallel data transfer, SCSI drives offered
significant architectural advantages over the previous ST506 and ESDI
drives. Since each SCSI drive had it own embedded disk controller, it was
now possible for the computer to issue simultaneous commands to every
drive in the system. Each drive could then fully overlap its seek,
rotational positioning, and read or write operations with the other SCSI
drives. The data could be fully buffered on the drive and transferred at
high speed over the shared SCSI bus when a time slot was available.

The primary problem with SCSI drives was initially the lack of software
support. Unlike ESDI, SCSI looked nothing like the old ST506 interface,
and thus SCSI controllers which emulated the WD1003 ST506 disk-controller
were much more difficult to build. In the late eighties, DPT built such a
controller - the PM2001. The PM2001 required no special software drivers
since it emulated the old WD1003 ST506 controller register set. The drive
companies also introduced an equivalent product by embedding a WD1003
register set compatible controller on the disk drive. These drives were
called Integrated Drive Electronics, or IDE, drives.

In addition to requiring no special software, IDE drives offered the
advantage of eliminating the need for any external controller pr host bus
adapter. Only a simple paddle card with  a couple of ICs to implement the
drive selection logic was required. Since the cost to build an IDE drive
was identical to the cost of a SCSI drive, many low-end desktop PCs
offered special connectors for one or two IDE drives, thereby freeing up a
bus slot and reducing cost. However, because of a limitation of the
original WD1003 controller register set, no more than two IDE drives are
supported by the standard BIOS.

Each advantage of IDE, however, came with a cost. As PCs advanced beyond
single-user applications, the ability to overlap the operation of multiple
drives became instrumental in obtaining higher l/O rates from the disk
subsystem. IDE drives, which were tied  to the old WE1003 ST506 controller
protocol, could only single-task l/O operations. Once a command had been
issued to an IDE drive, the operating system had to wait until the command
had been completed before issuing another command to any drive.

In addition, WD1003 protocol relied on the processor to move the data one
word at a time between a buffer on the controller and the computer's RAM.
This limited the speed of the data transfer, as well as precluding
hardware scatter-gather support which can be Implemented by bus-mastering
SCSI controllers.

Although IDE drives do not require a host adapter card, more and more PCs
require tape backup, CD-ROM or optical drive support, and thus must still
be configured with a SCSI host bus adapter.

In short, in the late eighties, IDE drives initially offered a high degree
of software compatibility at a low cost. However, the incremental cost of
supporting SCSI drives has dropped to zero in many cases since the SCSI
host bus adapter can be shared with other devices like tape and optical
drives. In addition, all major operating systems today include software
support for a large range of SCSI devices. The disadvantages of IDE,
although not as apparent under DOS, severely limit the performance of
multiuser operating systems: like Novell Netware, Unix, or Windows NT,
which realize large benefits from multitasked, overlapped I/O, bus
mastering, scatter/gather memory access, and I/O command queuing, all
offered by SCSI drives with intelligent host bus adapters.

Why are the limitations of IDE not-apparent under DOS?

1. IDE does not support overlapped, multitasked I/O. Neither does DOS.

2. IDE does not support I/O command queuing. Neither does DOS.

3. IDE does not support bus mastering, thus data transfer must be done via
slower PIO (processor I/O). However, unless the record size is quite
large, reducing data transfer time through bus mastering produces little
additional performance under single-user operating systems like DOS.

4. Since IDE supports only PIO data transfer, scatter/gather memory access
is not possible. However, simple operating systems like DOS do not use
virtual memory addressing which causes memory fragmentation, and thus DOS
does not benefit from scatter-gather memory access.

5. No more than two IDE drives can be supported in a PC system without a
special, non-standard BIOS. However, most DOS users do not require more
than two drives.

6. Because of limitations in the original WD1003 ST506 controller register
set addressing, IDE imposes drive capacity limitations of 528MB per drive
without a special, non-standard BIOS. However, few DOS systems require
drive capacity greater than 528MB.

7. Unlike SCSI, IDE does not support tape or optical drives. However, most
DOS systems today are still not configured with tape or optical drives.

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