Editing Direct-access storage device (section)

{{Short description|Secondary storage device}}
{{About|a class of computer storage devices|other uses of DASD|DASD (disambiguation)}}
{{More citations needed|date=December 2009}}

A '''direct-access storage device''' ('''DASD''') (pronounced {{IPAc-en|ˈ|d|æ|z|d|iː}}) is a [[secondary storage]] device in which "each physical record has a discrete location and a unique address". The term was coined by [[IBM]] to describe devices that allowed [[random access]] to data, the main examples being [[drum memory]] and [[hard disk drive]]s.<ref name=Intro>{{cite book|author=IBM Corporation|title=Introduction to IBM Direct-Access Storage Devices and Organization Methods|date=1975|page=1-1|url=http://bitsavers.trailing-edge.com/pdf/ibm/dasd/GC20-1649-9_DASDintro_Dec75.pdf|access-date=2013-12-23|archive-date=2013-12-24|archive-url=https://web.archive.org/web/20131224113411/http://bitsavers.trailing-edge.com/pdf/ibm/dasd/GC20-1649-9_DASDintro_Dec75.pdf|url-status=dead}}</ref>  Later, [[optical disc drive]]s and  [[flash memory]] units are also classified as DASD.<ref>{{cite web |author=IBM Corporation |title=Serial Direct Access Storage Device Subsystem |url=https://www-01.ibm.com/support/knowledgecenter/ssw_aix_61/com.ibm.aix.kernextc/serial_direct_storage.htm |work=IBM AIX V6.1 Documentation |date=2015 |access-date=December 28, 2015 |archive-url=https://web.archive.org/web/20160304070746/https://www-01.ibm.com/support/knowledgecenter/ssw_aix_61/com.ibm.aix.kernextc/serial_direct_storage.htm |archive-date=2016-03-04}}</ref><ref>{{cite web |last1=DellEMC |title=Powermax 8000 Joins the Mainframe Solutions Portfolio |url=https://www.dellemc.com/en-us/storage/mainframe.htm |website=Dell EMC Products for Mainframe Environments |access-date=August 7, 2018 |archive-url=https://web.archive.org/web/20180808104243/https://www.dellemc.com/en-us/storage/mainframe.htm |archive-date=2018-08-08}}</ref>

The term DASD contrasts with [[sequential access]] storage device such as a [[tape drive|magnetic tape drive]], and [[unit record equipment]] such as a [[punched card]] device. A record on a DASD can be accessed without having to read through intervening records from the current location, whereas reading anything other than the "next" record on tape or deck of cards requires skipping over intervening records, and requires a proportionally long time to access a distant point in a medium. [[Access method]]s for DASD include sequential, [[Data set (IBM mainframe)#Partitioned data set|partitioned]], [[ISAM|indexed]], and [[Basic direct access method|direct]]. 

The DASD storage class includes both fixed and removable media.