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{{Short description|Access method to read and write datasets sequentially}}
In IBM mainframe [[Operating system|operating system]]s, '''Basic sequential access method''' ('''BSAM''')<ref>{{cite manual|publisher=IBM|title=IBM System/360 Operating System Sequential Access Methods Program Logic Manual|id=Y28-6604-1|date=January 1967|url=http://www.bitsavers.org/pdf/ibm/360/os/plm_1966-67/Y28-6604-1_Sequential_Access_Methods_PLM_Jan67.pdf}}</ref> is an [[Access method|access method]] to read and write [[Data set (IBM mainframe)|dataset]]s sequentially. BSAM is available on OS/360, OS/VS2, [[Software:MVS|MVS]], z/OS, and related operating systems.

BSAM is used for devices that are naturally sequential, such as [[Engineering:Punched card|punched card]] readers, punches, line printers, and [[Physics:Magnetic tape data storage|magnetic tape]]. It is also used for data on devices that could also be addressed directly, such as [[Disk storage|magnetic disks]]. BSAM offers device independence: to the extent possible, the same [[Application programming interface|API]] calls are used for different devices.

BSAM allows programs to read and write physical [[Block (data storage)|blocks]] of data, as opposed to the more powerful but less flexible [[Software:Queued Sequential Access Method|Queued Sequential Access Method]] (QSAM) which allows programs to access logical records within physical blocks of data.
The BSAM user must be aware of the possibility of encountering short (truncated) blocks (blocks within a dataset which are shorter than the BLKSIZE of the dataset), particularly at the end of a dataset, but also in many cases within a dataset. QSAM has none of these limitations.

==Application program interface==
The programmer specifies <code>DSORG=PS</code> in his [[Data Control Block]] (DCB) to indicate use of BSAM.
As a ''basic'' access method BSAM reads and writes member data in [[Block (data storage)|blocks]] and the I/O operation proceeds asynchronously and must be tested for completion using the <code>CHECK</code> macro.<ref name=Macro>{{cite book|last1=IBM Corporation|title=OS Data Management Macro Instructions|date=June 1973|url=http://bitsavers.informatik.uni-stuttgart.de/pdf/ibm/360/os/R21.7_Apr73/GC26-3794-1_OS_Data_Management_Macro_Instructions_Rel_21.7_Jun73.pdf|accessdate=August 19, 2016}}</ref> BSAM uses the standard system macros <code>OPEN</code>, <code>CLOSE</code>, <code>READ</code>, <code>WRITE</code>,and <code>CHECK</code>. The <code>NOTE</code> macro instruction returns position of the last block read or written, and the <code>POINT</code> macro will reposition to the location identified by a previous <code>NOTE</code>.<ref name=Macro />

If the dataset is unblocked, that is, the logical record length (LRECL) is equal to the physical block size (BLKSIZE), BSAM may be utilized to simulate a directly accessed dataset using <code>NOTE</code> and <code>POINT</code> on any supported direct access device type (DEVD=DA), and some primitive applications were designed in this way.

==Similar facilities==
The BSAM 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 [[Software:Unix|Unix]] and [[Software:Microsoft Windows|Windows]]. <code>POINT</code> provides an analog of ''seek'' or ''lseek'',and ''ftell'' is the equivalent of <code>NOTE</code>.

==See also==
*[[Software:Queued Sequential Access Method|Queued Sequential Access Method]] (QSAM)
*Hierarchical Sequential Access Method (HSAM)
*Basic Indexed Sequential Access Method (BISAM)
*Queued Indexed Sequential Access Method (QISAM)
*Hierarchical Indexed Sequential Access Method (HISAM)

==References==
<references />

{{Mainframe I/O access methods}}

[[Category:IBM mainframe operating systems]]

{{Sourceattribution|Basic sequential access method}}