Synchronization in Software Embed PDF 417 in Software Synchronization

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7.3 Synchronization use software pdf417 2d barcode implement todraw barcode pdf417 with software GS1 Standards Knowledge Centre paradigm fo Software PDF-417 2d barcode r the use of a lock, including its release, is: while (!acquire(lock)) {waiting algorithm} computation on shared data release (lock) Because several processes might want to acquire a lock simultaneously, the acquisition process must be atomic. Before proceeding on how to implement the acquisition, let us note that there are two main possibilities for the waiting algorithm that the processes that did not acquire the lock must execute. One is busy waiting, whereby the process repeatedly tries to acquire the lock, which in this case is called a spin lock (to be discussed).

The other is blocking, whereby the process suspends itself and releases the processor on which it was running. The blocked process will be reawakened when the lock is released. Whether busy waiting or blocking or a combination of both (several attempts using busy waiting and then, if unsuccessful, the use of blocking) is best is often application-dependent.

Since this distinction is more in the realm of operating systems, we do not consider it further. Let us now return to the lock acquisition problem. In its simplest form, a lock will be a variable stored in memory that can have only the values 0 (lock is free) or 1 (lock is already acquired).

The steps for acquire are therefore to test whether the value is 0 or 1 and, if it is 0, to set the value to 1. However, the test and the subsequent set must be indivisible; otherwise, two processes could test simultaneously that the lock has value 0 and both acquire the lock. What is needed is an instruction that can read, modify, and write a memory location without interference.

Such an instruction, called test-and-set, was already part of the ISA of the IBM System/360. It was implemented by preventing the use of the memory bus by other instructions while it was being executed. All microprocessors today have some form of test-andset in their ISA.

Releasing a lock is of course much simpler. We just need to write the value 0. Test-and-set is the simplest instance of an atomic exchange operation.

It can be generalized to exchange-and-swap, which allows values other than 0 and 1 by exchanging the values of a register and a memory location. Compare-and-swap is a further generalization that allows the value in memory to be changed only if it is equal to the test value supplied. Both these instructions are present in the ISAs of the Intel IA-32 and Sun SPARC.

In addition, in the Intel architecture, most integer arithmetic and logical instructions can be used with a lock pre x that makes them atomic. For the example of Figure 7.11, none of the above instructions present an advantage over test-and-set, for a simple Boolean lock is all that is needed for synchronization.

However, there are applications where more sophisticated atomic exchanges are useful, for example by using bits or bytes within a memory location (word) as individual locks for subsets of a data structure. A second type of atomic operations is the fetch-and- operation, which is a generic name for fetch-and-increment, fetch-and-add, fetch-and-store, and so on. The basic idea, for example for fetch-and-increment, is that the two operations of.

Multiprocessors loop:. test-and-set bnz ld addi st st R2,lock R2, loop R1, A R1,R1,1 R1, A R0, lock /* test and set the value in location lock*/ /* if the result is not zero, spin*/ /*the lock has been acquired*/ /*increment A*/ /*store A*/ /*release the lock; R0 contains 0*/. (a) Use of Software PDF-417 2d barcode test-and-set. This instruction returns the value of lock in R1 and sets the value in lock to 1. fetch-and-increment A.

(b) Use of fetch-and- , in this case fetch-and-increment loop: ll addi sc bz R1, A R1,R1,1 R2, A R2,loop /*A loaded in R1 and its address loaded in link register*/ /*increment A*/ /*test of the link register with address of A; if equal, sc returns 1*/ /* if failure try again; memory was not modified*/.
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