Direct-access storage device

A direct-access storage device (DASD) (pronounced /ˈdæzdiː/) is a <a href="/facts/Secondary_storage/wPHKFpdx">secondary storage</a> device in which "each physical record has a discrete location and a unique address". The term was coined by <a href="/facts/IBM/QszA7nwd">IBM</a> to describe devices that allowed <a href="/facts/Random_access/1cxrCk3r">random access</a> to data, the main examples being <a href="/facts/Drum_memory/yul0r8nG">drum memory</a> and <a href="/facts/Hard_disk_drive/kekLSGSE">hard disk drives</a>. Later, <a href="/facts/Optical_disc_drive/IM9ugGbU">optical disc drives</a> and <a href="/facts/Flash_memory/Cn8JsStx">flash memory</a> units are also classified as DASD.
The term DASD contrasts with <a href="/facts/Sequential_access/8NCL847H">sequential access</a> storage device such as a <a href="/facts/Tape_drive/9MfLuCa0">magnetic tape drive</a>, and <a href="/facts/Unit_record_equipment/xkRvng9j">unit record equipment</a> such as a <a href="/facts/Punched_card/44WsPulQ">punched card</a> 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. <a href="/facts/Access_method/ITDHX6ag">Access methods</a> for DASD include sequential, <a href="/facts/Data_set_(IBM_mainframe)/ker4c5te">partitioned</a>, <a href="/facts/ISAM/J1Q7sty3">indexed</a>, and <a href="/facts/Basic_direct_access_method/5GPgnnxl">direct</a>. 
The DASD storage class includes both fixed and removable media.

Direct-access storage device open-in-new

Direct-access storage device