Question

Explain the concept of pipelining in 8086. discuss its advantages and disadvantages

Answer 1

The process of fetching the next instruction when the present instruction is being executed is called as pipelining.Pipelining has become possible due to the use of queue.BIU (Bus Interfacing Unit) fills in the queue until the entire queue is full.BIU restarts filling in the queue when at least two locations of queue are vacant.Advantages of pipelining:The execution unit always reads the next instruction byte from the queue in BIU. This is faster than sending out an address to the memory and waiting for the next instruction byte to come.In short pipelining eliminates the waiting time of EU and speeds up the processing. -The 8086 BIU will not initiate a fetch unless and until there are two empty bytes in its queue. 8086 BIU normally obtains two instruction bytes per fetch.

The Instruction Queue:

The execution unit (EU) is supposed to decode or execute an instruction.Decoding does not require the use of buses.When EU is busy in decoding and executing an instruction, the BIU fetches up to six instruction bytes for the next instructions.These bytes are called as the pre-fetched bytes and they are stored in a first in first out (FIFO) register set, which is called as a queue.

Answer 2

pipeline in 8086 is a technique which is used in advanced microprocessors, were the microprocessor execute a second instruction before the completion of first. That is many instruction are simultaneously pipelined at different processing stage.

The advantages of pipelining is performance improvement, we are able to pump more instructions and get improved in processor speed as we are able to execute parts of instructions in parallel to parts of other instruction.

Disadvantage of pipeline is that it makes things complex, for example if we need to take care of branch penalty and forwarding, this become complex and several research problems are arise due to these complexity.