Most IBM PC style multiprocessor motherboards combine Intel 486 or Pentium processors and glue chipsets with a hardware/software specification. The specification places much of the onus for hard work on the chipset and hardware rather than the operating system.
The Intel pentium processors have a wide variety of inbuilt facilities for supporting multiprocessing, including hardware cache coherency, built in interprocessor interrupt handling and a set of atomic test and set, exchange and similar operations. The cache coherency in particular makes the operating systems job far easier.
The specification defines a detailed configuration structure in ROM that the boot up processor can read to find the full configuration of the processors and busses. It also defines a procedure for starting up the other processors.