Multiprocessors in Computer Architecture

Multiprocessors in Computer Architecture – A multiprocessor is a computer system with two or more central processing units (CPUs) that share full access to a common RAM. The main objective of using a multiprocessor is to boost the system’s execution speed, with other objectives being fault tolerance and application matching.

Types of Multiprocessors

There are two types of multiprocessors:

1. Shared memory multiprocessor
2. Distributed memory multiprocessor

Shared Memory Multiprocessor

1. Also called a tightly coupled multiprocessor.
2. Processor communication is done by means of a shared address space.
3. Processor communication is done by means of shared memory read/write.
4. Convenient on small-scale devices.
5. Lower latency.
6. Non-uniform memory access (NUMA) or symmetric multiprocessing (SMP).

Distributed Memory Multiprocessor

1. Every CPU has its own private memory.
2. Also known as a loosely-coupled system.
3. Each processor has its local memory with the address space available only for this processor.
4. Communication through message passing.

Applications of Multiprocessors

1. As a uniprocessor, such as single instruction, single data stream (SISD).
2. As a multiprocessor, such as single instruction, multiple data stream (SIMD), which is usually used for vector processing.
3. Multiple series of instructions in a single perspective, such as multiple instruction, single data stream (MISD), which is used for describing hyper-threading or pipelined processors.
4. Inside a single system for executing multiple, individual series of instructions in multiple perspectives, such as multiple instruction, multiple data stream (MIMD).

Characteristics of Multiprocessors

1. A multiprocessor is an interconnection of two or more CPUs with memory and input-output equipment.
2. The term processor in a multiprocessor system can mean either a central processing unit (CPU) or an input-output processor (IOP).
3. Multiprocessors are classified as multiple instruction stream multiple data stream (MIMD) systems.
4. A multiprocessor system is controlled by one operating system that provides interaction between processors, and all the components of the system cooperate in the solution of a problem.
5. Increased throughput because of the execution of multiple jobs in parallel.
6. Multiprocessing improves the reliability of the system so that a failure in one part has a limited effect on the rest of the system.
7. If a fault causes one processor to fail, another processor can be assigned to perform the functions of the disabled processor.
8. The benefit derived from a multiprocessor organization is improved system performance.
9. Multiprocessing can improve performance by decomposing a program into parallel executable tasks.

The performance of a multiprocessor can be achieved in the following ways-

• The user can explicitly declare that certain tasks are to be executed in parallel.
• The other efficient way is to provide a compiler that can automatically detect parallelism in a user’s program.
• Multiple independent jobs can be made to operate in parallel.
• A single job can be partitioned into multiple parallel tasks.