The following message is a courtesy copy of an article that has been posted to bit.listserv.ibm-main,alt.folklore.computers as well.
[EMAIL PROTECTED] (David Logan) writes: > Test and set works just fine too. They are both atomic. re: http://www.garlic.com/~lynn/2008b.html#31 How does ATTACH pass address of ECB to child? http://www.garlic.com/~lynn/2008b.html#47 How does ATTACH pass address of ECB to child? the issue with atomic test&set was that it was a purely binary value ... locked or not locked. charlie was working on fine-grain cp67 multiprocessing locking at the science center (virtual machine operating system running on 360/67) http://www.garlic.com/~lynn/subtopic.html#545tech when he invented compare&swap (as previously noted, CAS was chosen because it is charlie's initials). http://www.garlic.com/~lynn/subtopic.html#smp compare&swap ... allowed for full word binary value ... as opposed to simple set/notset (or locked/notlocked) ... and compare-double&swap allowed for double word value atomic update. in the multithreaded/multiprogramming value ... compare&swap allowed for atomic updating of a wide variety of values. the test&set scenario allowed for "locking" a section of code, non-atomic update of value within the locked code section ... and then unlocking the code section. independent executing paths would arrive at the locked code section and "spin" until the other executable path had released the lock. the problem in the multiprogramming/multithreaded sequence ... is that the executable thread could be interrupted while doing the non-atomic update (but still holding the lock) ... with execution then passing to another thread which went into unending "spin-loop" (waiting for the suspended thread to release the lock). as noted in the examples added to the principles of operation was examples of atomic updates of more complex values (than simple set/not-set) ... which could be used even in interruptable code. prior to compare&swap ... such multiprogramming/multithreaded operation always required the overhead of supervisor call (for performing non-atomic updates in non-interruptable code ... and avoiding possible application spin loops). from long ago and far away A.6 Multiprogramming and Multiprocessing Examples http://publibz.boulder.ibm.com/cgi-bin/bookmgr_OS390/BOOKS/DZ9ZR003/A.6?SHELF=DZ9ZBK03&DT=20040504121320 for other topic drift ... it has now been 40yrs since three people from the science center came out and installed cp67 at the university. ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [EMAIL PROTECTED] with the message: GET IBM-MAIN INFO Search the archives at http://bama.ua.edu/archives/ibm-main.html
