Simultaneous access to memory
in [Hardware]
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From: Peter Olcott on 6 Apr 2010 16:47 What happens when two processes access memory at literally the same time, with one reading and another writing, does the read or the write get mangled? I know the whole semaphore lock stuff, I want to know what happens when one skips this step.
From: Sjouke Burry on 6 Apr 2010 17:46 Peter Olcott wrote: > What happens when two processes access memory at literally > the same time, with one reading and another writing, does > the read or the write get mangled? I know the whole > semaphore lock stuff, I want to know what happens when one > skips this step. > > Skip that step and you have a pile of junk instead of a computer.
From: Peter Olcott on 6 Apr 2010 18:54 "Sjouke Burry" <burrynulnulfour(a)ppllaanneett.nnll> wrote in message news:4bbbabc4$0$14122$703f8584(a)textnews.kpn.nl... > Peter Olcott wrote: >> What happens when two processes access memory at >> literally the same time, with one reading and another >> writing, does the read or the write get mangled? I know >> the whole semaphore lock stuff, I want to know what >> happens when one skips this step. > Skip that step and you have a pile of junk instead of a > computer. I am taking this informal message to mean: On and given hardware platform, a simultaneous read operation interferes with a write operation such that the write operation is garbled.
From: Peter Olcott on 6 Apr 2010 18:55 "Peter Olcott" <NoSpam(a)OCR4Screen.com> wrote in message news:zuWdncZMA8wIJibWnZ2dnUVZ_rCdnZ2d(a)giganews.com... > > "Sjouke Burry" <burrynulnulfour(a)ppllaanneett.nnll> wrote > in message > news:4bbbabc4$0$14122$703f8584(a)textnews.kpn.nl... >> Peter Olcott wrote: >>> What happens when two processes access memory at >>> literally the same time, with one reading and another >>> writing, does the read or the write get mangled? I know >>> the whole semaphore lock stuff, I want to know what >>> happens when one skips this step. >> Skip that step and you have a pile of junk instead of a >> computer. > > I am taking this informal message to mean: > On ANY given hardware platform, a simultaneous read > operation interferes with a write operation such that the > write operation is garbled. >
From: Paul on 6 Apr 2010 19:27
Peter Olcott wrote: > "Peter Olcott" <NoSpam(a)OCR4Screen.com> wrote in message > news:zuWdncZMA8wIJibWnZ2dnUVZ_rCdnZ2d(a)giganews.com... >> "Sjouke Burry" <burrynulnulfour(a)ppllaanneett.nnll> wrote >> in message >> news:4bbbabc4$0$14122$703f8584(a)textnews.kpn.nl... >>> Peter Olcott wrote: >>>> What happens when two processes access memory at >>>> literally the same time, with one reading and another >>>> writing, does the read or the write get mangled? I know >>>> the whole semaphore lock stuff, I want to know what >>>> happens when one skips this step. >>> Skip that step and you have a pile of junk instead of a >>> computer. >> I am taking this informal message to mean: >> On ANY given hardware platform, a simultaneous read >> operation interferes with a write operation such that the >> write operation is garbled. >> The read or the write will not be mangled. Operations are atomic enough, to complete, and all you have to worry about from a programming perspective, is whether the order they complete in matters or not. If a read-modify-write instruction is involved, that is atomic enough to complete, without being snipped in half. The benefit of using an actual RMW type instruction on the computer, is the processor takes care of it being atomic. The processor won't allow an RMW to be split in half. http://en.wikipedia.org/wiki/Read-modify-write Paul |