Describe the bus interface unit (BIU) of 8086 microprocessor.

BIU (Bus Interface Unit): BIU takes care of all data and addresses transfers on the buses for the EU like sending addresses, fetching instructions from the memory, reading data from the ports and the memory as well as writing data to the ports and the memory. EU has no direction connection with System Buses so this is possible with the BIU. EU and BIU are connected with the Internal Bus.

Figure: BIU (Bus Interface Unit)

It has the following functional parts –

Instruction queue: BIU contains the instruction queue. BIU gets up to 6 bytes of next instructions and stores them in the instruction queue. When EU executes instructions and is ready for its next instruction, then it simply reads the instruction from this instruction queue resulting in increased execution speed. Fetching the next instruction while the current instruction executes is called pipelining.

Segment register: BIU has 4 segment buses, i.e. CS, DS, SS & ES. It holds the addresses of instructions and data in memory, which are used by the processor to access memory locations it also contains 1 pointer register IP, which holds the address of the next instruction to executed by the EU.

(i) CS: It stands for Code Segment it is used for addressing a memory location in the code segment of the memory, where the executable program is stored.

(ii) DS: It stands for Data Segment. It consists of data used by the program and is accessed in the data segment by an offset address or the content of other register that holds the offset address.

(iii) SS: It stands for Stack Segment. It handles memory to store data and addresses during execution.

(iii) ES: It stands for Extra Segment. ES is additional data segment, which is used by the string to hold the extra destination data.

Instruction pointer: It is a 16-bit register used to hold the address of the next instruction to be executed.