Such questions are best answered by reviewing the topic "Block-Concurrent References" in Principles of Operation.
Jim Mulder z/OS Diagnosis, Design, Development, Test IBM Corp. Poughkeepsie NY > From: Phil Smith <[email protected]> > To: [email protected] > Date: 07/31/2017 02:21 AM > Subject: Re: Question about CPUs > Sent by: IBM Mainframe Assembler List <[email protected]> > > Charles Mills wrote, in part: > >The effect of multiple CPUs on a multi-task program is even worse than the > >load, add, store example above. With two CPUs, it is possible for even > >single machine instructions to interleave. So if one task executes MVC > >FOO,=C'Now is the time for all good men to come to the aid of the party' and > >another CPU executes MVC FOO,=C'The quick brown fox jumps over the lazy dog' > >it is at least in theory* possible for FOO to end up containing 'Now is the > >time fox jumps over the lazy dog the aid of the party'. > > Mmm...I'm pretty sure a single instruction is still atomic. I'm sure > Peter Relson or one of the other IBMers will chime in, but it there > has to be some sort of interlock at some level. And I've debugged > plenty of concurrency problems, never seen a mixture from a single > instruction! > > ...phsiii
