Bus Interface Unit (BIU) | Internal Architecture of 8086 Microprocessor


Internal architecture of 8086 microprocessor divided into two independent functional units. They are Bus Interface Unit (BIU), and Execution Unit (EU). In this article we will mainly talk about the function of BIU. Later in this article we will see the function of Execution Unit of 8086 microprocessor.

Bus Interface Unit (BIU)

BIU is basically an interface or mediator between external devices such as (RAM, ROM, I/O devices) and Execution Unit. So the BIU is basically a bridge between the outer devices and to the processor. The functions of the bus interface unit as follows:

  • It fetches instruction or data from the memory or I/O devices.
  • It also writes back the executed date or the result to the memory or i/o devices.
  • The function of BIU also include writing back the data to i/o ports, and
  • Reading data from the ports.
Bus Interface Unit (BIU) | Internal Architecture of 8086 Microprocessor | CS School
Fig: Internal Architecture of 8086 Microprocessor

Functional Parts of the BIU

Now we will see the internal parts that involve executing those function of the BIU. There are three functional parts of the BIU which execute those above four works or functions.

  • Instruction Pointer (IP)
  • Segment Register, and
  • Instruction Queue

These three functional units of BIU performs above four operation of BIU. Now we will discuss details about these three functional unit.

Instruction Pointer (IP)

Instruction pointer in BIU is a 16 bit register which keeps the address of memory location of coming next instruction to be executed.

Segment Register

The Segment Register holds 1 Megabyte of Memory Space. This 1 MB of memory segmented into four blocks. Each block consists of register with max size 64 KB. The four blocks or segment registers are:
– Code Segment (CS)
– Data Segment (DS)
– Stack Segment (SS)
– Extra Segment (ES)

Each segment consists of register of max-size 64 KB, and the total memory hold by these four segments is 1 MB.

Instruction Queue

Instruction Queue is the third functional part of BIU. As we know that the bus interface unit performs its operation parallel with execution unit. So when the BIU fetches the instruction byte while EU in executing operation, the pre-fetched instruction is saved in a group of high speed registers. This group of high speed registers are the instruction queue.

In the next article here we will discuss about the details of EU of 8086 Microprocessor.

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