imported>Chatul: /* See also */ Thread, not process; OS/360

2025-04-04T19:04:59Z

See also: Thread, not process; OS/360

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{{Short description|Instance of a thread control block within IBM OS/360 and successor systems}}
{{for|the equivalent in Unix-derived systems|Thread control block}}
The '''Task Control Block''' ('''TCB''') contains the state of a task{{efn|Roughly analogous to a [[Thread (computing)|thread]] in [[UNIX]]-like operating systems.}} 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]], [[OS/VS2 (SVS)|SVS]], [[MVS/370]], [[MVS/XA]], [[MVS/ESA]], [[OS/390]] and [[z/OS]], the TCB<ref name=FixSup/><ref name=MVTSup/><ref name=SCB/>{{rp|pp=311–321}} contains, among other data, non-dispatchability flags and 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<ref name=SCB/>{{rp|pp=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 blocks
; Interruption Request Block<ref name=SCB/>{{rp|pp=281–284}}
: An IRB is used to handle an asynchronous exit.{{efn|An asynchronous routine to handle an exception, timer event or other signal.}}
; Program Request Block<ref name=SCB/>{{rp|pp=285–287}}
: A PRB represents a module invoked with an ATTACH macro, a LINK macro or a synchronous exit.{{efn|Unprivileged [[Callback (computer programming)|callback]] routine.}}
; System Interruption Request Block<ref name=SCB/>{{rp|pp=289–291}}
: An SIRB is used to run I/O error recovery code.
; Supervisor Request Blocks<ref name=SCB/>{{rp|pp=293–301}}
: An SVRB represents the execution of a Type 2, Type 3 or Type 4 SVC routine

An RB contains several fields, among them an old PSW, old general registers, a PSW and a wait count.

===Dispatching===
The ''Dispatcher'' is a routine in the nucleus that selects the work to be dispatched. It selects the highest priority task that:
#Is not running on another CPU
#Does not have any non-dispatchability flags set
#Has a top RB with a zero wait count.

The system maintains a pair{{efn|In [[65MP]] there is a pair for each CPU; in MVS there are pointers for each address space.}} of TCB pointers known as TCB old and TCB new. A TCB new pointer of zero causes the dispatcher to search for an eligible task.

When the dispatcher finds an eligible task, it sets the old and new TCB pointers. loads the registers from the TCB and loads the PSW from the top RB.

If the dispatcher fails to find eligible work, it enters an enabled wait.

==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==
*[[OS/360 and successors]]
*[[Thread control block]]

==Notes==
{{notelist}}

==References==
{{reflist|refs=
<ref name=FixSup>{{cite book
| title = IBM System/360 Operating System - Fixed-Task Supervisor - Program Number 360S-CI-505
| id = Y28-6612-2
| date = February 1967
| edition = Third
| url = http://www.bitsavers.org/pdf/ibm/360/os/plm_1966-67/Y28-6612-2_Fixed-Task_Supervisor_PLM_Sep67.pdf
| series = Program Logic
| access-date =
}}
</ref><ref name=MVTSup>{{cite book
| title = IBM System/360 Operating System - MVT Supervisor
| id = GY28-6659-7
| date = May 1973
| edition = Eighth
| url = http://bitsavers.org/pdf/ibm/360/os/R21.7_Apr73/plm/GY28-6659-7_MVT_Supervisor_PLM_Rel_21.7_May73.pdf
| series = Program Logic
| access-date = March 14, 2022
}}
</ref><ref name=SCB>{{cite book
| publisher = IBM
| title = IBM System/360 Operating System: System Control Blocks - OS Release 21.7
| id = GC28-6628-9
| edition = Tenth
| date = April 1973
| url = http://bitsavers.org/pdf/ibm/360/os/R21.7_Apr73/GC28-6628-9_OS_System_Ctl_Blks_R21.7_Apr73.pdf
| series = Systems Reference Library
| access-date = June 12, 2022
}}
</ref>
}}

[[Category:Process (computing)]]

Task Control Block - Revision history

imported>Chatul: /* See also */ Thread, not process; OS/360