Re: OS / hardware support for Semphores

Divick wrote:
Hi, I have question related to semapores / lock. Does the OS / hardware
need to provide some support for Semaphores / locks? Why is it necessary?
Why can't it be implemented without the support of OS/ hardware?

Think about what a semaphore (or lock) has to do.

It must be at a higher level of abstraction than a process because it must
be shared by multiple processes. Thus the implementation cannot be as low as
a process. So it must be implemented at the OS or hardware level.

Now part of the implementation might be done at the level of individual
processes, but not much -- because operations on semaphores (or locks) must
be indivisible and process switching can occur anytime between the
individual instructions of a processor (assuming the processor is not
hyperthreaded or multi-core) and that the machine has only a single
processor. If the machine is multi-processor, multi-threaded, or
multi-cored, then there is no evading the need for hardware support in some
form or other.

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