Small Computer System Interface

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The Small Computer System Interface, better known by the acronym SCSI is a set of standards that dictate the physical connection between a computer and peripheral devices and the hardware and software implementation needed to make all that work together.

Even though nowhere as popular as it once was it is still pretty much a requirement for anyone doing low level programming to have at the least a basic understanding of the SCSI subsystem and the standard commands etc. Most if not all alternative data transport systems for peripheral devices copy the SCSI command structure to some degree and storage centered standards like SATA copy the command structure down to a tee, even though they are electrically very different systems. You are therefore going to find SCSI in the oddest of places when you are doing any programming that involves interfacing with storage systems of some sort.

In hardware terms SCSI still has a number of advantages over alternatives such as SATA, although its main advantage is of processing independence from the host was partially lost in the mid-90's on PC systems (PC architecture issue rather than an OS or driver issue).

Advanced SCSI Programming Interface

Better known as just ASPI, it is a programming interface introduced by Adaptec in the 1990's that simplifies the programming for end user applications that need to access a SCSI device or devices with compatible command interface such as ATAPI, most but not all SCSI drivers provide an ASPI interface. Most operating systems provide an ASPI interface for their ATAPI (IDE/SATA) implementation or have them as optional installs although with the decline in use of CD-ROM technology the use of ASPI has gone down as well.

SCSI over Ethernet

A networking protocol that allows computers to access storage devices and other peripherals to be accessed over Ethernet as if they where local SCSI devices, via encapsulation.

Fibre channel

High speed networking standard that is in practice mostly used to connect high speed storage devices to computer servers using a SCSI based protocol very similar to SCSI over Ethernet.


A very high speed interconnect standard primarily used in high speed networking hardware, it allows for a form of iSCSI for peripheral or storage devices best described as "SCSI over InfiniBand".

USB Attached SCSI

Introduced with the USB version 3 specification but has also been made to work with USB 2. Basically replaces the USB Mass Storage Bulk-Only Transport (BOT) with the SCSI protocol. In general UAS is faster than the old BOT especially when dealing with SSD or other fast flash memory technology. At this point in time there are no OS/2 drivers for UAS.

Universal Flash Storage

Or UFS is a new(ish) standard for flast storage on consumer devices, the hardware interface part is either M-PHY or UniPro but the software interface copies SCSI.


Introductory material
  • Friedhelm Schmidt: SCSI-Bus und IDE-Schnittstelle (4. Auflage) - Addison-Wesley 2001, ISBN 3827318289
A much improved fourth edition of this book, IDE in particular improved and all in all one of the better introductory texts but only available in German.
English translation of an older version
  • Gary Field; Peter Ridge: Book of SCSI: I/O for the New Millennium (2nd edition) - No Starch Press 2000, ISBN 1886411107
This is targeted towards the beginner but as such is a rather good introduction.
Targeted towards the end user rather than the programmer.
  • Marc D. Brooks; Duane J. Stein; Barry B. Shiloff: Making SCSI Work Paralan Corporation 1999, ISBN 096574650X
  • NCR Corp: SCSI: Understanding the Small Computer System Interface - Prentice Hall 1990, ISBN 0-13-796855-8
Well written and clear introduction, but outdated. Short, almost a booklet rather than a book.
A short article that explains what SCSI standards are out there, what they define and how they came to be.
Quite good introduction to SCSI and more modern than most of the above.
Reference works
Aimed squarely at the technician rather than the software engineer, but as such very good although outdated.
Example chapters and index PDF
Actually a technical sales document if a bit long at 377 pages, and outdated but does contain loads of relevant SCSI information to a point where some implementations point to this publication rather than the published standard. There were later editions released by the company but they are not to be found on the net.
Very good in its day and still has some value, but outdated.
Hardware reference texts
Fibre Channel
  • Ralph O. Weber and Jeffrey D. Stai: FCBR-2: Fibre Channel Bench Reference, Second Edition - ENDL Publications 2005, ISBN 1879936186
USB Attached SCSI