Patent US 4,374,409

Synchronization of processes in a multiprogramming/multiprocessing system is provided by P and V instructions that are executed during execution of processes and operate on data structures known as semaphores which represent processes and events. A process that performs P instruction requests data. The P instruction addresses a semaphore data structure stored in memory. If the semaphore indicates that the data is available, the process continues execution of its next instruction; otherwise, the process is stopped and placed into a wait state until the data become available. A process that performs a V instruction, on the other hand, delivers data to another process. The V instruction addresses a semaphore to determine whether another process is awaiting the data and data are transferred to an awaiting process. If no process is available, the V instruction enables one of the processes in the wait state tied to the same addressed semaphore to be transferred to the ready state. P and V instructions are provided for semaphore structures without messages as well as semaphore structures with messages. Each of the P and V instructions includes test instructions that enable a process to continue in the running state in the event that the test P or V instruction is unable to be executed.