8/10/2019 MPI 8086-88

1/18

8086/8088 Microprocessors

Registers and their role inmicroprocessor

8/10/2019 MPI 8086-88

2/18

Overview and Basic functions ofRegisters

Use of Register:Registers are used to store information

temporarily.

8/10/2019 MPI 8086-88

3/18

8/10/2019 MPI 8086-88

4/18

Division of registers:

1. General purpose registers (8):

Operands for logical and arithmetic operations Operands for address calculations Memory pointers

2. Segment registers (6)

3. EFLAGS register4. Instruction pointer register

8/10/2019 MPI 8086-88

5/18

General purpose Registers

There are four general registers that areAX,BX,CX and DX.

Each Can be used as a whole 16 bit/word orcan be used as two 8 bit registers.

On 8 bit half will known as upper half or AHand other half will known as lower half or AL.

8/10/2019 MPI 8086-88

6/18

General Purpose Registers and theirfunctions in System

AX (Accumulator) : favored by CPU forarithmetic operations

BX (Base): can hold the address of a procedure or

variable (SI, DI, and BP can also). Can alsoperform arithmetic and data movement. CX (count) : acts as a counter for repeating or

looping instructions. DX (Data) : holds the high 16 bits of the product

in multiply it also handles divide operations.

8/10/2019 MPI 8086-88

7/18

General Purpose

16-Bit and 8-Bit Registers

AX

BX

DX

CXAH AL

15 8 7 0

AX15 0

8/10/2019 MPI 8086-88

8/18

Segment Registers

It is used as base locations for program instructions, data andthe stack

CS Code Segment holds base location for allexecutable instructions in a program

SS - Base location of the stack DS Data Segment default base location for

variables ES Extra Segment additional base location for

memory variables.

8/10/2019 MPI 8086-88

9/18

16-BitSegment Registers

CS

SS

ES

DS

CS15 0

8/10/2019 MPI 8086-88

10/18

Index Registers

It is used to Speed up processing of strings, arrays,and other data structures containing multipleelements.

SI Source Index Used in string movementinstructions. The source string is pointed to bythe SI register.

DI Destination Index acts as thedestination for string movement instructions

8/10/2019 MPI 8086-88

11/18

Pointer Registers

It Contains the offset of data(variables, labels) andinstructions from its base segment.

BP ( Base Pointer) :contains an assumed offset fromthe SS register. Often used by a subroutine to locatevariables that were passed on the stack by a callingprogram.

SP (Stack Pointer):Contains the offset of the top ofthe stack.

8/10/2019 MPI 8086-88

12/18

16-Bit Index & pointer

Registers

BP

SP

DI

BP15 0SI

8/10/2019 MPI 8086-88

13/18

16-Bit RegistersStatus and Control

IP

Flags

IP15 0

8/10/2019 MPI 8086-88

14/18

Status and Control Registers

1. IP (Instruction Pointer): contains the offset ofthe next instruction to be executed.

2. Flags Register :individual bit positions withinregister show status of CPU or results ofarithmetic operations.

Control Flags (Direction, Interrupt, Trap)Status Flags (Carry, Overflow, Sign, Zero,Auxiliary Carry, Parity)

8/10/2019 MPI 8086-88

15/18

Status Flags Carry (CF) set when the result of an unsigned

arithmetic operation is too large to fit into thedestination.

Overflow (OF) set when the result of a signed arithmetic operation is too wide to fit into thedestination.

Sign (SF) set when the result of an arithmetic orlogical operation generates a negative result.

Zero (ZF) set when the result of an arithmetic orlogical operation is zero.

8/10/2019 MPI 8086-88

16/18

Status Flags

Auxiliary Carry (AF) set when the result of anoperation causes a carry from bit 3 to bit 4.

Parity (PF) reflects whether the number of 1bits in the result of an operation is even orodd. 1 odd, 0-even.

8/10/2019 MPI 8086-88

17/18

Control Flags

Interrupt (IF) dictates whether or not systeminterrupts can occur. 1 enabled, 0 disabled.

Trap (TF) determines whether or not the CPUis halted after each instruction. Allowsprogrammers to do tracing.

Direction (DF) affects block data transferinstructions such as MOVS, CMPS. 0 up, 1 down.

8/10/2019 MPI 8086-88

18/18

Internal structure of Microprocessorsand registers orientation

AH ALBH BLCH CLDH DL