SCSI Pass-Through Programmer's Guide

What is “SCSI Pass Through”?
“SCSI Pass Through” (SPT) refers to the ability of an application to send a SCSI Device
Command through the MPE/iX OS and HP e3000 hardware to a SCSI Device: This is
illustrated below in Figure #1.
Generic SCSI
Device
I/O
Card
User A
pp
lication
SCSI Command
Bytes
(12 00 00 00 80 00)
I/O Bus
Hp e3000
Abstract SCSI “Pass Through”
MPE/iX O.S.
Figure #1
Why and when one might use SPT?
Most succinctly SPT allows a programmer to closely control the behavior of a SCSI device.
On MPE and most Operating Systems SCSI devices are controlled by a device-specific
driver. Device-specific drivers, such as those for SCSI direct access (disk) and sequential
access (tape) devices, coordinate device and driver states to accomplish correct logical device
behavior. The SCSI pass-through interface enables one to interact directly with a SCSI
device rather than depending upon MPE’s File System or “High-Level I/O” interfaces. With
SPT MPE will allow both the usage of a “generic” SCSI device driver to allow one to
interface to devices not supported by MPE/iX and the ability to talk to supported devices in a
manner not currently allowed by MPE/iX Intrinsics.
The following are two examples of how this might be used.
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