View source for PCLSRing

'''PCLSRing''' means PC-lusering (from [[Program Counter]] and [[luser]], the [[Incompatible Timesharing System|ITS]] jargon for [[user]]).

[[Image:PCLSR.png|thumb|right|400px|Early PCLSRing code from ITS 1.0.]]

It's the procedure of ensuring that the PC of a [[process]] is in user space.  This makes it easier for one process to observe and reason about another process, because the observed process does not exhibit any state hidden in [[kernel]] space.

PCLSRing is mostly associated with the ITS [[operating system]].  In some form it can also be found in the [[Berkeley Timesharing System]] and [[SITS]].

If a process is executing in kernel space and needs to be PCLSRed, typical strategies are:

* Update the user space state and return immediately.  This can only be done in special cases when the user state can express the intermediate state of a kernel operation.  An example is an I/O read/write operation where user space state includes the I/O length and target address.
* Back out the side effects of a kernel operation and return to a user space point before the operation was started.

A strategy not considered proper PCLSRing is to cancel the kernel operation and return an error to user space.  This was the topic of the essay known as "Worse Is Better".

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==External links==

* [http://fare.tunes.org/tmp/emergent/pclsr.htm PCLSRing: Keeping Process State Modular] - Alan Bawden's detailed memo on PCLSRing
* [https://github.com/larsbrinkhoff/mit-logo-and-sits-raw-files/blob/master/files/19741205_2of3_AI_KA_Arch_1113-1116_1117p/rjl/ch3.2 The PCLOSER Problem] - Ron Lebel's writeup for SITS.
* [https://www.dreamsongs.com/RiseOfWorseIsBetter.html The Rise of Worse is Better] - the philosophy of PCLSRing is discussed in this except of "Lisp: Good News, Bad News, How to Win Big"
** [https://www.dreamsongs.com/WorseIsBetter.html Worse Is Better] - background

[[Category: OS Basics]]