8086-8088 Microprocessor BIU Unit

8086-8088 Microprocessor BIU Unit generates the system central signal and accept these signals. It keeps the pre-fetch queue filled with instructions. It works as a window between E.U and memory.

Parts of BIU

  1. Pre-fetch Queue
  2. Segment Register
  3. Instruction Pointer
  4. Bus Controller

Pre-fetch Queue

When the EU is busy to decode or execute an instruction, during that time the EU does not require buses. So these buses are used by the BIU to fetch or pick up 6 instructions for next use. These instructions are stored in a FEFO register which is called pre-fetch queue or instruction queue. When the EU completes an execution operations it fetches the next instruction directly from pre-fetch queue and does not pick up form memory. So in this way the speed of microprocessor is greatly increased.

  1. Pipe lining

    The fetching of next instruction during the execution of the current instruction is called pipe lining.

  2. Bus Controller

    It is a section that is related to all the buses which means that all the buses are controlled by this section. This is a circuit which works for all the three buses that are control bus, address bus and data bus.

  3. Addressing Modes

    The different ways or methods through which a microprocessor can address the data or access the data for its use are called addressing modes i.e. mov, destination, source.

    When executed this instruction copies a word or byte from the specified source location to the specified destination location.

    MOV B, C

    In this instruction the data will be copied from C register and will be stored in B register.

    The source can be a direct number written directly in the instruction.

    MOV C, 04H

    The source can be a memory location

    MOV C, [0004H]

    This instruction indicates that the microprocessor will first identify or address (0004H) memory location and the data stored in it will be copied to C register.

    In an instruction one destination and source can be both a memory location i.e.

    MOV [03H], [04H]

    The source cannot be changed while the destination is always and almost change.


Types of Addressing Mode

  1. Register addressing
  2. Immediate Addressing
  3. Direct Addressing
  4. In Direct Addressing
    • Register indirect Addressing
    • Base Plus Index
    • Register Relative
    • Base Relative Index

Register Addressing

Copy of a byte or word is transferred from the source register to the destination register i.e.

MOV C, B (8085)

MOV Cx, Bx (8086)

In 8085 microprocessor instruction the data is copied from B register to C register.

Immediate Addressing

In this mode the data is directly stored in the register for example

MOV D, 03H       or            MOI D, 03H

After this instruction the data 03H will be directly stored in the D register or in the destination D register.

Direct Addressing

Moves a byte or word between a memory location directly i.e.

MOV Dx, [0124H]

After this instruction the data of the memory location 0124H will be stored in the destination register Dx. The microprocessor first identifies the data location and then the data will be picked up and will be stored in the Dx register.

The microprocessor will add 0H to this location [0124H] and makes the real location [00124H]

Indirect Addressing Modes

  1. Register Indirect Addressing

    Transfer a byte or word between a register and memory location addressed by an index register or base register. The index register are SI (source index) and DI (destination Index). The base register is base pointers and Bx. The example will be as follows.
    MOV Cx, [Bx].
    This instruction copies the data from the memory location [Bx] to [Cx] register. In that memory location will be address by Bx register.

  2. Base Plus Index Addressing

    Transfer a byte or word between a register and memory location address by a base register and index register i.e.
    MOV Cx, [SI + Bx]

  3. Register Relative Addressing

    In this type of addressing mode the copy of a byte or word is transferred between a register and a memory location which is addressed by base register and displacement i.e.
    MOV Cx, [Bx + 03H]

  4. Register Relative Plus Index

    In this type of addressing mode the copy of a byte or word is transferred between a register and a memory location. This is addressed by a base register, index register and displacement. It means that the memory location is made by adding these three elements which are in brackets i.e.
    MOV Cx, [Bx + DI + 03H]


Instruction Pointer

A component of a task descriptor that designates the next instruction to be executed by the task. An element of the control component of the stack model of block structure execution, which points to the current instruction.

Addressable Memory RAM and ROM

That part of memory which is address by a specific microprocessor is the part of architecture of microprocessor. There are two types of addressable memories that are addressable RAM and addressable ROM.

Those instructions of a program which are never change are stored in ROM memory. Which defines that it only can be read and cannot be change or write. That instruction of a program which are changed and used according to our wishes and requirements are stored in RAM memory.

The special purpose computer which performs some specific functions and tasks uses ROM memory much greater then RAM memory.

General purpose computers use RAM memory which is much greater than ROM memory. This is because of a large RAM memory is required which can be programed according to our wishers and requirements.