Display article   Subject:  SCSI Support and Advantages
   04/30/97   18:56:19


XENIX AND SCSI

As you may be aware, Radio Shack has complete SCSI support in the
version 2.2.4 SCO XENIX 386 product.  What you may not be aware of is
how SCSI delivers performance advantages with XENIX over other drive
architectures.

XENIX is a very disk intensive or disk bound operating system.  Up
until our SCSI implementation, the best measurement of a hard drive's
performance was the average seek time.  While this is still important,
it is also important to understand that we now have another method of
determining performance for the XENIX system that has been virtually
ignored by much of the industry:  OVERHEAD.

To understand overhead, you must first understand how XENIX does disk
I/O at the driver level.  With ST-506 and ESDI hard drives, the operating
system issues a command and waits for that command to be done.  Then the
drive issues an interrupt to the CPU acknowledging that a command has
been completed.  It is up to the CPU to then move the data from the
controller buffer to system memory.

During the time between when a command is sent and when the data is moved
to memory, the driver "sleeps."  Sleeping is a term used to refer to a
driver waiting for an interrupt to occur.  It is important to realize
that during a "sleep" all other activity ceases.  No other user processes
can be executed and currently executing processes are suspended.  The time
for all this to occur is referred to as overhead.

Overhead is substantially reduced with our SCSI implementation.  Our
driver never waits for a command to be completed.  In other words, our
driver never "sleeps."  This allows the CPU to allocate more time to user
processes (i.e. applications).  Of course, the end result is higher overall
system throughput.

Another way our SCSI implementation reduces overhead is by reducing the
number of interrupts the CPU must service.  For each and every command for
an ST-506 and ESDI drive, there are two interrupts.  With our SCSI
technology, there is only one interrupt per multiple commands.  In other
words, our adapter can queue up to 16 commands at a time, and when those
commands are completed, the CPU will receive one interrupt.

Our SCSI technology also reduces overhead by moving the data itself to
memory at the highest rate memory will allow.  This is much faster than
the CPU having to move the data at the speed of the AT bus which is
considerably slower than the clock rate of the CPU.

Another way we reduce overhead is through the use of multi-threaded I/O.
It is possible for the SCSI adapter to issue commands to more than one
device at a time.  This cannot be accomplished with ST-506 or ESDI.  For
instance, if you have 2 SCSI hard drives, the adapter can issue commands
to both of those hard drives at the same time.  By being able to issue
these two commands to both devices, overhead is cut in half for disk
accesses.

SCSI technology provides an extremely fast and efficient method for
accessing data storage devices.
