Editing CICS (section)

===Nucleus===
The original CICS nucleus consisted of a number of functional modules written in 370 assembler until V3:
* Task Control Program (KCP)
* Storage Control Program (SCP)
* Program Control Program (PCP)
* Program Interrupt Control Program (PIP)
* Interval Control Program (ICP)
* Dump Control Program (DCP)
* Terminal Control Program (TCP)
* File Control Program (FCP)
* Transient Data Control Program (TDP)
* Temporary Storage Control Program (TSP)

Starting in V3, the CICS nucleus was rewritten into a kernel-and-domain structure using IBM's [[PL/AS]] language{{snd}} which is compiled into assembler.

The prior structure did not enforce separation of concerns and so had many inter-program dependencies which led to bugs unless exhaustive code analysis was done. The new structure was more modular and so resilient because it was easier to change without impact. The first domains were often built with the name of the prior program but without the trailing "P". For example, Program Control Domain (DFHPC) or Transient Data Domain (DFHTD). The kernel operated as a switcher for inter-domain requests{{snd}} initially this proved expensive for frequently called domains (such as Trace) but by utilizing PL/AS macros these calls were in-lined without compromising on the separate domain design.

In later versions, completely redesigned domains were added like the Logging Domain DFHLG and Transaction Domain DFHTM that replaced the Journal Control Program (JCP).