What is the X in X-mas?
(SMP) Two or more similar processors connected via a high-bandwidth link and managed by one operating system, where each processor has equal access to I/O devices. This is in contrast to the "compute server" kind of parallel processor where a front-end processor handles all I/O to disks, terminals and local area network etc.
The processors are treated more or less equally, with application programs able to run on any or perhaps all processors in the system, interchangeably, at the operating system's discretion. Simple MP usually involves assigning each processor to a fixed task (such as managing the file system), reserving the single main CPU for general tasks.
OS/2 currently supports so-called HMP (Hybrid Multiprocessing), which provides some elements of symmetric multiprocessing, using add-on IBM software called MP/2. OS/2 SMP was planned for release in late 1993.