In
IBM mainframe
IBM mainframes are large computer systems produced by IBM since 1952. During the 1960s and 1970s, IBM dominated the large computer market. Current mainframe computers in IBM's line of business computers are developments of the basic design of th ...
operating system
An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs.
Time-sharing operating systems schedule tasks for efficient use of the system and may also i ...
s, queued sequential access method (QSAM)
is an
access method to read and write
dataset A data set (or dataset) is a collection of data. In the case of tabular data, a data set corresponds to one or more database tables, where every column of a table represents a particular variable, and each row corresponds to a given record of the ...
s sequentially. QSAM is available on
OS/360
OS/360, officially known as IBM System/360 Operating System, is a discontinued batch processing operating system developed by IBM for their then-new System/360 mainframe computer, announced in 1964; it was influenced by the earlier IBSYS/IBJOB ...
,
OS/VS2 Operating System/Virtual Storage 2 (OS/VS2) is the successor operating system to OS/360 MVT in the OS/360
OS/360, officially known as IBM System/360 Operating System, is a discontinued batch processing operating system developed by IBM for thei ...
,
MVS
Multiple Virtual Storage, more commonly called MVS, was the most commonly used operating system on the System/370 and System/390 IBM mainframe computers. IBM developed MVS, along with OS/VS1 and SVS, as a successor to OS/360. It is unrelated ...
,
z/OS
z/OS is a 64-bit operating system for IBM z/Architecture mainframes, introduced by IBM in October 2000. It derives from and is the successor to OS/390, which in turn was preceded by a string of MVS versions.Starting with the earliest:
* ...
, and related operating systems.
QSAM is used both for devices that are naturally sequential, such as
punched card
A punched card (also punch card or punched-card) is a piece of stiff paper that holds digital data represented by the presence or absence of holes in predefined positions. Punched cards were once common in data processing applications or to di ...
readers and punches and line printers, and for data on devices that could also be addressed directly, such as
magnetic disk
Magnetic storage or magnetic recording is the storage of data on a magnetized medium. Magnetic storage uses different patterns of magnetisation in a magnetizable material to store data and is a form of non-volatile memory. The information is ac ...
s. QSAM offers device independence: to the extent possible, the same
API
An application programming interface (API) is a way for two or more computer programs to communicate with each other. It is a type of software interface, offering a service to other pieces of software. A document or standard that describes how ...
calls are used for different devices.
QSAM is—as its name says—queued, in this specific context meaning buffered with deblocking of reads and blocking of writes. It allows programs to read and write logical records within physical blocks of data, as opposed to the less advanced
basic sequential access method
In IBM mainframe operating systems, Basic sequential access method (BSAM) is an access method to read and write datasets sequentially. BSAM is available on OS/360, OS/VS2, MVS, z/OS, and related operating systems.
BSAM is used for devices that ...
(BSAM) which allows programs to access physical blocks of data, but provides no support for accessing logical records within blocks.
Indeed, QSAM manages truncated final blocks and truncated embedded blocks completely transparently to the user.
The QSAM application program interface can be compared with the interface offered by ''open'', ''read'', ''write'' and ''close'' calls (using file handles) in other operating systems such as
Unix
Unix (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, an ...
and
Windows
Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for ser ...
.
See also
*
Sequential access memory
In computing, sequential access memory (SAM) is a class of data storage devices that read stored data in a sequence. This is in contrast to random access memory (RAM) where data can be accessed in any order. Sequential access devices are usually a ...
(SAM)
*
Basic sequential access method
In IBM mainframe operating systems, Basic sequential access method (BSAM) is an access method to read and write datasets sequentially. BSAM is available on OS/360, OS/VS2, MVS, z/OS, and related operating systems.
BSAM is used for devices that ...
(BSAM)
*
Hierarchical sequential access method
The IBM Information Management System (IMS) is a joint hierarchical database and information management system that supports transaction processing.
History
IBM designed the IMS with Rockwell and Caterpillar starting in 1966 for the Apollo pr ...
(HSAM)
*
Basic indexed sequential access method
ISAM (an acronym for indexed sequential access method) is a method for creating, maintaining, and manipulating computer files of data so that records can be retrieved sequentially or randomly by one or more keys. Indexes of key fields are mainta ...
(BISAM)
*
Queued indexed sequential access method
ISAM (an acronym for indexed sequential access method) is a method for creating, maintaining, and manipulating computer files of data so that records can be retrieved sequentially or randomly by one or more keys. Indexes of key fields are mainta ...
(QISAM)
*
Hierarchical indexed sequential access method (HISAM)
References
{{Mainframe I/O access methods
IBM mainframe operating systems