Riyadh Philanthropic Society For Science

Prince Sultan College For WomanDept. of Computer & Information Sciences

CS 251Introduction to Computer Organization

& Assembly Language

Lecture 10

8086 Register Set

8086 Register Set 2

From Lecture notes only

Organization of the 8086/8088 Microprocessors

Registers Data Registers: AX, BX, CX, DX

Address Registers

Segment Registers: CS, DS, SS, ES

Pointer and Index Registers: SP, BP, SL, DL

Instruction Pointer: IP

Flag Register

Outline

8086 Register Set 3

Organization of 8086/8088 Microprocessors

The IBM personal family are all based on the Intel 8086 family of microprocessors.

The 8086/8088 processors have the simplest structure.

Most of the instructions we will study are 8086/8088 instructions.

Because the 8086 and 8088 have essentially the same internal structure the

name 8086 will be applied to both 8086 and 8088.

Registers in 8086 are 16 bits in size ( means 16 bits can be saved in them)

8086 Register Set 4

Registers Information inside microprocessors are stored in registers

The classification of the registers is done according to the function they perform

In general there are fourteen 16-bit registers

Data Registers

There are 4 general purpose data registers

They hold data for the operation

Address Registers

They are divided into segment, pointer, and index registers

They hold the address of the data or instruction in memory

Status Registers

It is called the FLAGS register

It keeps the current status of the processor

8086 Register Set 5

Registers (Cont.)AH AL

BH BL

CH CL

DH DL

AXBXCXDX

CSDSSSES

SIDISPBPIP

Data Registers

Segment Registers

Pointer and Index Registers

FLAGS Register

8086 Register Set 6

Data Registers: AX, BX, CX, DX These four registers are available to the programmer for general data manipulation.

Even thought the processor can operate on data stored in memory, the same instruction is

faster if data are stored in registers.

This is why modern processors tend to have a lot of registers.

The high and low bytes of these registers can be accessed separately.

Ex. The high byte of AX is called AH, and the low byte is called AL.

This arrangement gives us more registers to use when dealing with byte-size data.

In addition to being general purpose, each of these registers have a specific operation as well.

8086 Register Set 7

Data Register : AX (Accumulator)

AX is the preferred register to use in the logic, arithmetic, and data transfer instructions (its

use will generate the shortest machine code)

In multiplication and division operations, one of the numbers should be in AX or AL

Input and Output operations also require the use of AL and AX

8086 Register Set 8

Data Register : BX (Base Register)

Also serves as an address register

Ex. Table look-up instruction XLAT (used for translation)

8086 Register Set 9

Data Register : CX (Count Register)

Program Loop constructions are facilitated by the use of CX, which acts as a loop counter

CL is used as a count in instructions that shift and rotate bits

8086 Register Set 10

Data Register : DX ( Data Register)

DX is used in multiplication and Division

It is also used in I/O operations

8086 Register Set 11

Address Registers – Segment Registers CS, DS, SS, ES

Address registers store addresses of instructions and data in memory

These values are used by the processor to access memory locations

In the 8086 processor (16-bit processor)

Memory is composed of bytes, each memory byte has an address starting with 0.

The processor assigns a 20-bit physical address to its memory locations

It is possible to address 220 =1,048,576 bytes (1 megabyte of memory)

The bytes in memory have addresses from 00000h to FFFFFh

8086 Register Set 12

Address Registers – Memory Segments

It is a direct consequence of using a 20-bit address in a 16-bit processor

The addresses are too big to fit in a 16-bit register or memory word

8086 gets around this by partitioning its memory into segments

8086 Register Set 13

Memory Segments

A memory segment is a block of 216 (64 K) consecutive memory bytes.

Each segment is identified by a segment number (16 bits)

The range for the segment number is 0000h to FFFF h

Within the segment, the memory location is specified by giving an offset

Offset: number of bytes from the beginning of a segment (16 bits)

Also the offset range is 0000h to FFFF h

8086 Register Set 14

Segment:Offset A memory location can be specified by providing a segment number and an offset, written

in the form Segment:Offset

The Segment: Offset is known as the logical Address

Ex. A4FB:4872h

segment offset

To obtain a 20-bit physical address, the 8086 microprocessor first shifts the segment number by 4 bits to the left and then adds the offset

The physical address of A4FB:4872h is: A4FB0h + 4872h

20-bit physical address A9822h

8086 Register Set 15

Segment Locations

For memory location whose physical address is specified by 1256Ah, give the address is

segment:offset form for segments 1256h and 1240h

Physical address = segment x 10h + offset offset = physical address – segment x 10h

For segment 1256:offset = 1256Ah – (1256h x 10h) = Ah 1256:000Ah

For segment 1240:offset = 1256Ah – (1240h x 10h) = 16Ah 1240: 016Ah

Thus 1256Ah = 1256:000Ah = 1240:016Ah

8086 Register Set 16

Segments Location (Cont.)

A memory location has physical address 80FD2h. In what segment does it have offset

BFD2h?

Physical address = segment x 10h + offset

segment = (physical address - offset) / 10h

segment = (80FD2h - BFD2h) / 10h = 7500h

8086 Register Set 17

Segment Registers : CS, DS, SS, ES

A machine language program consists of instructions (code) and data

The stack is a data structure used by the processor to make procedure calls

The data, instructions, & stack parts of the program are loaded into different memory

segments

They are called: data segment, code segment, and stack segment

8086 Register Set 18

Segment Registers: CS, DS , SS, ES

There are four segment registers that the 8086 processor uses to keep track of the

segment numbers in the program.

CS: Code Segment: holds the code (or instruction) segment number

DS: Data Segment: holds the data segment number

SS: Stack Segment: contains the stack segment number

ES: Extra Segment: it is used in case the program needs to access another data segment

8086 Register Set 19

Segment Registers (Memory view)

8086 Processor Address Memory

CS 0F8Ah 0F8A:0000 code segment begins

DS 0F89h 0F89:0000 data segment begins

SS 0F69h 0F69:0000 stack segment begins

ES

8086 Register Set 20

Segment Registers – Active segments

The memory locations addressed by the segment registers are accessible

i.e. only four segments are active at a time

However, the content of the segment register could be modified in the program to address

different segments

8086 Register Set 21

Pointer & Index Registers: SP, BP, SI, DI

These registers usually point to (contains the offset addresses) of memory locations

Unlike the Segment registers, the pointer and index registers can be used for arithmetic

and other operations as well.

Each of these registers, in addition to the general purpose functionality, has a specific task

8086 Register Set 22

Pointer & Index Registers: SP, BP, SI, DI

SP (Stack Pointer) register: used in conjunction with SS to access the stack segment

BP (Base Pointer) register: used primarily to access data on the stack. However, unlike

SP, the BP can be used to access data on other segments

SI (Source Index) register: used to point to memory locations in the data segment

addressed by DS. By incrementing the contents of SI, we could easily access consecutive

memory locations

DI (Destination Index) register: performs the same operation as SI. There is also a class

of instructions, called string operations, that use DI to access memory locations addressed

by ES.

8086 Register Set 23

Address Register – IP

Instruction Pointer

To access instructions, the processor 8086 uses the registers CS and IP.

The CS register holds the segment number of the next instruction

The IP register holds the offset

IP is updated each time an instruction is executed so that it will point to the next instruction

Unlike other registers, the IP can’t be directly manipulated by an instruction (i.e. the

instruction can’t have IP an operand)

8086 Register Set 24

FLAGS Register

Its function is to indicate the status of the processor

It sets individual bits called FLAGS

There are two kinds of flags:

Status Flags: Reflects the result of an instruction executed by the processor (ex. When a

subtraction results in a 0, the ZF “Zero Flag” is set to 1, a subsequent instruction can examine

the ZF and branch to some code that handles a zero result)

Control Flags: enable or disable certain operations of the processor (ex. If the IF “Interrupt

Flag” is set to 0, inputs from the keyboard are ignored by the processor).