The Intel 8088 consists of a bus interface unit (BIU) and an execution unit (EU)

Question

The Intel 8088 consists of a bus interface unit (BIU) and an execution unit (EU), which form a 2-stage pipeline. The BIU fetches instructions into a 4-byte instruction queue. The BIU also participates in address calculations, fetches operands, and writes results in memory as requested by the EU. If no such requests are outstanding and the bus is free, the BIU fills any vacancies in the instruction queue. When the EU completes execution of an instruction, it passes any results to the BIU (destined for memory or I/O) and proceeds to the next instruction. a. Suppose the tasks performed by the BIU and EU take about equal time. By what factor does pipelining improve the performance of the 8088? Ignore the effect of branch instructions. b. Repeat the calculation assuming that the EU takes twice as long as the BIU.

Explanation / Answer

a) If both BIU and EU take equal time, there is no delay for instruction fetching of BIU, so no stalls in the pipelining.

b) If EU is taking more cycles, by that time BIU fetches more instructions into the queue and improves performance by a factor of twice.

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