Question

Why IFU has 6 bytes? when increasing the number of words in IFU would improve the performance?

Answer 1

IFU(Instruction Fetch Unit) has a 6 bytes prefetch instruction pipeline 8086 architecture which means that the bus interface unit reads up to six bytes of opcodes in advance from the memory. Here the 6 bytes is chosen because in 8086, the largest instruction size in 8086 is 6 bytes long.

The purpose of IFU is to separate the fetching of instruction from the execution of instruction so that both can be done parallely. Increasing number of words will make IFU to load more advance instructions, however this is not needed since loading one instruction in advance is sufficient to make parallel execution of fetching process and execution process and hence increasing number of words in IFU will not improve performance.