System for distributed multiprocessor communication

A tightly coupled communication scheme based on a common shared resource circuit and adapted particularly to a multiprocessing system including 2 CPUs. A mechanism has been added that allows data in a shared register to be read and incremented as a single instruction, eliminating the need for semaphore manipulations during the operation. A second mechanism has been added to permit the use of indirect addressing in the addressing of semaphore bits and shared registers. Operating systems can relocate semaphore bits and message areas to permit simultaneous execution of the same function within a single task. In addition, an instruction has been added which tests of the semaphore bit and acts upon the state of that bit. If the semaphore bit is not set then the processor takes control of the semaphore bit by setting it. If the semaphore bit is set, the processor will execute a branch and execute other instructions. Thus, jobs assigned to a processor in a multiprocessing, multitasking application do not block or wait for the semaphore bit to clear.