Allocation

Enhanced Definition

In the z/OS environment, **allocation** refers to the process of assigning system resources—such as datasets, I/O devices (tape drives, printers), memory, and CPU time—to a specific job, job step, or program for its exclusive use during execution. This ensures that the required resources are available and properly managed for the task at hand.

Key Characteristics

- Resource Assignment: Involves the system identifying, reserving, and making available specific hardware (e.g., a tape drive) or software (e.g., a dataset) resources to a running job or program.
- Static vs. Dynamic: Resources can be statically allocated via JCL DD statements before a job step begins, or dynamically allocated by an executing program using system services (e.g., SVC 99) during its runtime.
- Scope and Duration: Allocation can be for the duration of a job step (most common for DD statements), for the entire job, or for a specific period within a program's execution, with resources being deallocated upon completion or explicit release.
- Resource Contention: The z/OS operating system manages resource contention, ensuring that a resource allocated to one job is not simultaneously allocated to another, often using enqueue/dequeue mechanisms.
- Error Handling: If a requested resource is unavailable (e.g., a dataset does not exist, or all tape drives are in use), the allocation process will fail, typically resulting in a JCL error (e.g., S813 abend) or a return code to the requesting program.

Use Cases

- Dataset Access: A COBOL program needs to read from an input sequential file and write to an output VSAM KSDS. JCL DD statements are used to allocate these datasets before the program starts.
```
jcl
//STEP01 EXEC PGM=MYCOBPROG
//INFILE   DD DSN=MY.INPUT.SEQ,DISP=SHR
//OUTFILE  DD DSN=MY.OUTPUT.KSDS,DISP=(NEW,CATLG),
//            SPACE=(CYL,(10,2)),DCB=(LRECL=80,BLKSIZE=0,RECFM=FB)
```
- Tape Drive Assignment: A backup utility requires a dedicated tape drive to write archive data. The system allocates an available tape device to the job step.
- Dynamic File Creation: A utility program needs to create a temporary dataset whose characteristics are determined at runtime. The program uses dynamic allocation services (e.g., DYNALLOC macro or SVC 99) to define and allocate the dataset.
- Memory Management: When a program is loaded and executed, z/OS allocates virtual storage (memory) for the program's code, data areas, and working storage within its address space.