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Bus Interface Unit (BIU)

The BIU contains a prefetch queue, a bus controller, segment registers, and the instructions pointer (IP). The main purpose of the BIU are to keep the prefetch queue filled with instructions, to generate and accept the system control signals, to provide the system with a memory address or input/ output (I/O) port number, and to act as a window between the EU and memory for data.

The BIU ensures that the queue is filled with instructions by fetching the next single byte of an instruction if the 8088 queue has space, and two bytes if the 8086 queue has space. Prefetching allows the execution unit to obtain the next instruction directly from the BIU instead of the memory as required by older, 8-bit microprocessors. Because the next instruction is already inside of the 8086/8088 when execution is imminent, the microprocessor executes software at a much higher speed than if every instruction were fetched directly form the memory.

(The Intel Microprocessors, Barry B. Brey, Maxwell MacMillan International Editions, 1991, pg. 4.)