A multi-processor is a system with more than one CPU. There are a tremendous range of designs, from 'tightly-coupled' (where they share access to some main memory, which usually has to be multi-port memory), onward.
Another axis for dividing them is among:
- MIMD - 'multiple instruction streams, multiple data sets'
- SIMD - 'single instruction streams, multiple data sets'
In the latter, each CPU is fed different data, but they all execute the same instructions in the same order.
Among tightly-coupled systems, one can distinguish between 'symmetric' and 'asymmetric' systems. In the former, all the CPUs are 'equal', and no CPU has a special, distinguished role (so often any CPU can e.g. field interrupts from any peripheral). The latter are often so-called 'master/slave' systems, with one CPU in the first class, and one or more in the second.
With the advent of very-high-speed LANs, it is now common to create multi-processors by simply linking together a number of ordinary systems by use of such a network.
- TOPS-10 Symmetric Multiprocessing - includes a very good overview of multi-processing in general