Synchronization Hardware

Synchronization Hardware

AU: May-19

Test and Set Operations

• Test and set is a single individual machine instruction. It is simply known as TS. It was introduced by IBM for its multiprocessing system.

• In a one machine cycle, it tests to see if the key is available and, if it is, sets it to unavailable. TS are a hardware solution for critical section problem.

• TS instruction eliminates the possibility of preemption occurring during the of interval. The instruction :

testAndset (p, q)

• The instruction reads the value of q, which may be either true or false. Then the value is copied into "p" and the instruction sets the value of "q" to true.

• A process P1 would test the condition code using TS instruction before entering a critical section. If no other process was in this critical section, then process P1 would be allowed to proceed and the condition code would be changed from zero to one.

• When process P1 exist the critical section, the condition code is reset to zero so another process can enter into critical section.

• If process P1 finds a busy condition code then it is placed in a waiting loop where it continues to test the condition code and waits until it is free.

• The "TestandSet" instruction is a tool that programmers use to simplify software solution to mutual exclusion but the instruction itself does not enforce mutual exclusion.

Advantages:

1. Simple to implement.

2. It works well for a small number of processes.

3. It can be used to support multiple critical sections..

Drawbacks:

1. It suffers from starvation.

2. There is possibility of busy waiting.

3. There may be deadlock.

University Question

1. Write an algorithm using test-and-set() instruction that satisfy all the critical section requirements.  AU May 19, Marks 05