Each statement requires an integral number of time units to execute. The running

Question

Each statement requires an integral number of time units to execute. The running program is permitted to continue executing instructions for a period of time called its quantum. When a program’s time quantum expires, another ready program will be selected to run. Any instruction currently being executed when the time quantum expires will be allowed to complete.

Programs are queued first-in-first-out for execution in a ready queue. The initial order of the ready queue corresponds to the original order of the programs in the input file. This order can change, however, as a result of the execution of lock and unlock statements.

Explanation / Answer

The lock and unlock statements are used whenever a program wishes to claim mutually exclusive access to the variables it is manipulating. These statements always occur in pairs, bracketing one or more other statements. A lock will always precede an unlock, and these statements will never be nested. Once a program successfully executes a lock statement, no other program may successfully execute a lock statement until the locking program runs and executes the corresponding unlock statement. Should a running program attempt to execute a lock while one is already in effect, this program will be placed at the end of the blocked queue. Programs blocked in this fashion lose any of their current time quantum remaining. When an unlock is executed, any program at the head of the blocked queue is moved to the head of the ready queue. The first statement this program will execute when it runs will be the lock statement that previously failed. Note that it is up to the programs involved to enforce the mutual exclusion protocol through correct usage of lock and unlock statements. (A renegade program with no lock/unlock pair could alter any variables it wished, despite the proper use of lock/unlock by the other programs.)

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