Write - Storing data to output
In the context of mainframe computing, a **write operation** refers to the process of transferring data from a program's internal memory (e.g., a COBOL working-storage section or an Assembler buffer) to an external output device or storage medium. This fundamental I/O operation makes processed data persistent, available for subsequent use, or consumable by other systems or users.
Key Characteristics
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- Data Transfer: Moves data from a program's address space to I/O buffers managed by the operating system or specific access methods, which then handle the physical transfer to the device.
- Device/Medium Agnostic: Can target various output devices and media, including Direct Access Storage Devices (DASD), magnetic tape, printers (via
SYSOUT), and display terminals. - Access Method Dependent: The mechanics and efficiency of a write operation are heavily influenced by the chosen access method (e.g.,
QSAMfor sequential,VSAMfor indexed/relative,BSAMfor basic,EXCPfor direct control). - Record-Oriented or Block-Oriented: Data is typically written as logical records, which the access method often groups into physical blocks for optimized physical I/O operations.
- Buffering: Utilizes I/O buffers in memory to stage data before it is physically written to the device, significantly reducing the number of costly physical I/O operations.
- Error Handling: Requires robust error handling mechanisms (e.g.,
FILE STATUSin COBOL,SYNADexits in Assembler) to manage issues like full datasets, device errors, or data integrity problems.
Use Cases
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- Batch Report Generation: A COBOL program processes transaction data and writes formatted report lines to a
SYSOUTdataset, which is then spooled for printing or viewing. - Creating Master Files: Updating a master file by reading an old master and transaction records, then writing new or updated records to a new master file on DASD.
- Logging Transactions: An online CICS transaction writes audit trail records to a
VSAM KSDSor a sequential log file for later batch processing, recovery, or compliance. - Backup and Archiving: Writing critical datasets from DASD to magnetic tape for long-term storage, disaster recovery, or regulatory compliance.
- Inter-program Communication: A program writes intermediate results to a temporary dataset (
DD DSN=&&TEMP) which is then read as input by a subsequent job step within the same job.
- Batch Report Generation: A COBOL program processes transaction data and writes formatted report lines to a
Related Concepts
A write operation is central to Data Management on z/OS, relying heavily on Access Methods (like QSAM, BSAM, VSAM) to interact with physical storage. It is orchestrated by JCL (Job Control Language) through DD (Data Definition) statements, which define the characteristics of the output dataset or device. Within application programs, languages like COBOL use WRITE statements, while Assembler uses macros like PUT or WRITE (for EXCP or