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Task Control Block

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The Task Control Block (TCB) contains the state of a task[a] in, e.g., OS/360 and successors on IBM System/360 architecture and successors.

The TCB in OS/360 and successors

In OS/360, OS/VS1, SVS, MVS/370, MVS/XA, MVS/ESA and z/OS, the TCB[1][2][3]: 311–321  contains, among other datas, the general and floating point registers for a task that is not currently assigned to a CPU.

A TCB provides an anchor for a linked list of other, related request blocks[3]: 269–301  (RBs); the top-linked RB for a TCB contains the Program status word (PSW) when the task is not assigned to a CPU.

When the control program's dispatcher selects a TCB to be dispatched, the dispatcher loads registers from the TCB and loads the PSW from the top RB of the TCB, thereby dispatching the unit of work.

Request Blocks

OS/360 has the following types of request block

Interruption Request Block[3]: 281–284 
An IRB is used to handle an asynchronous exit.[b]
Program Request Block[3]: 285–287 
A PRB represents a module invoked with an ATTACH macro, a LINK macro or a synchronous exit.[c]
System Interruption Request Block[3]: 289–291 
An SIRB is used to run I/O error recovery code.
Supervisor Request Blocks[3]: 293–301 
An SVRB represents the execution of a Type 2, Type 3 or Type 4 SVC routine

History

With the introduction of MVS/370 and successor systems, a whole new environment was introduced: the Service Request Block (SRB), which generally has a higher priority than any Task Control Block, and, indeed, which itself has two distinct priorities: a Global SRB (priority over all local address space SRBs and TCBs) and a Local SRB (priority over only the local address space TCBs); and MVS's dispatcher must manage all of these with absolute consistency across as many as two processors (MVS/370) and as many as sixteen processors (successor systems).

See also

Notes

  1. ^ Roughly analogous to a thread in UNIX-like operating systems.
  2. ^ An asynchronous routine to handle an exception or other signal.
  3. ^ Unprivileged callback routine.

References

  1. ^ IBM System/360 Operating System - Fixed-Task Supervisor - Program Number 360S-CI-505 (PDF) (Third ed.). February 1967. Y28-6612-2. {{cite book}}: |work= ignored (help)
  2. ^ IBM System/360 Operating System - MVT Supervisor (PDF) (Eighth ed.). May 1973. GY28-6659-7. Retrieved March 14, 2022. {{cite book}}: |work= ignored (help)
  3. ^ a b c d e f IBM System/360 Operating System: System Control Blocks - OS Release 21.7 (PDF) (Tenth ed.). IBM. April 1973. GC28-6628-9. Retrieved June 12, 2022. {{cite book}}: |work= ignored (help)
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