What are control blocks in Mainframes?

Control blocks are the memory units which represents/contains the status of what is going on or what has just happened, in short the TASK information when task is executed in the operating system. It provides the information about the job, CICS task and much other information.

Types of Control Blocks:

System Related Control Blocks

Resource Related Control Blocks

Job Related Control Blocks

Task Related Control Blocks

These provide debugging functionality in the event an operating system component or user application fails while the system is running.

Most commonly used system related control blocks are as follows:

1. CVT (Communication Vector Table)

2. SSCVT (Subsystem CVT)

3. ASCB (Address Space Control Block)

4. TCB (Task Control Block)

5. ASXB (Extended Address Space Control Block)

Where we use Control Blocks:

Dump Reading

CICS Abend Analysis

z/OS Data Areas

Diagnosing Dump

Abends  and many more..

TCBs represent tasks executing within an address space, such as user programs with multi tasking.

SRBs  executing from one address space and  perform a task in other some address space.

Each address space then has its own chain of ready SRBs and/or TCBs, pointed to from its ASCB. Whenever an event completes which changes the status of an address space, the relevant z/OS function updates the dispatching queues to reflect it.

ASCB – Address Space control Block

ASCB contains information and pointer addresses required for the management of this address space. Each address space is represented by an ASCB within the z/OS operating system.

ASVT – Address Space Vector Table

ASVT used to keep track of all address spaces in the system image, therefore it contains a list of all possible address space IDs, if assigned with associated ASCB. Remember an address space is created for each job, started task or mount request. CVTASVT which is the location of ASVT from system CVT.

ASXB – Address Space Extended Block

control block also contains information about an address space which has crucial information about number of TCB etc.     

PSA – Prefixed Save Area

Contains PSW ( Program Status Word) , interruption codes, registers  & register save area for lock manager & interrupt handler. It holds the basic information z/OS needs when scheduling work on a Central Processor (CP) , it always start at address ‘0’. One PSA per processor  & it contains processor related information.

FLCCVT - A pointer to the CVT

PSAAOLD - A pointer to the Address Space Control Block (ASCB) of the address space currently scheduled on this CP. The ASCB holds basic information about an address space, including Job name and Address Space ID. More information is held in the Address Space Extension Block (ASXB), which is pointed to by the ASCB.

PSATOLD - A pointer to the Task Control Block (TCB) of the task currently scheduled on this CP. The TCB holds information on a specific task.