Lecture4

Lecture 4

8086 Instruction Set

1Zelalem Birhanu, AAiT

In this lecture

• Introduction

• 8086 Instruction Types

• 8086 Addressing Modes


Introduction – recap

• There are three language levels that can be used to write a program for a microcomputer

Machine language

Assembly Language

High-Level Language


Introduction – recap…cntd

Machine Language

• These are the binary codes for the instructions you want

the microcomputer to execute

• It is hard or impossible for a programmer to write code

in machine language, because it requires memorizing all the

instructions in binary form and soon the program will

get out of control!


11101001000000000101000


Assembly Language

• Uses two, three, or four letter mnemonics to represent each instruction type

• The letters in an assembly language mnemonic are usually initials or a short form of the English word(s) for the operation performed by the instruction

e.g., SUB for subtract , XOR for Exclusive OR , etc

• Assembly language program has to be translated to

machine language so that it can be loaded into memory

and run – This is done by the assembler5Zelalem Birhanu, AAiT


High-Level Languages

• These languages use program statements which are even more English-like than those of assembly language

e.g. BASIC, C, C++, Java, ...

• Each high-level statement may represent many machine code instructions

• An interpreter (compiler) program is used to translate higher-level language statements to machine codes, which can be loaded into memory and executed.



• Elements of an instruction

• 8086 has variable-length instructions (8 bits to 40 bits)


Operation Code (Opcode)

Addresses (operands)

8086 Instruction Types

• The 8086 instructions can be grouped in to six categories

Data transfer instructions

Arithmetic instructions

Bit manipulation instructions

String manipulation instructions

Control transfer instructions

Processor control instructions


Data Transfer Instructions

• Used to transfer data from source operand to destination operandMemory to register e.g. MOV AX, [0005h] (AX←[0005h])

Register to memory e.g. PUSH AL

Immediate to memory/register e.g. MOV AH, 09h

I/O device to register e.g. IN AX, 4

Register to I/O device e.g. OUT AL, 2

• All the store, move, load, exchange, input and output instructions belong to this category

MOV , PUSH, POP , XCHG, XLAT, IN, OUT, LEA, PUSHF, POPF


Arithmetic instructions

• Perform arithmetic operations Addition e.g. ADD, ADC, INC, AAA,

Subtraction e.g. SUB, SBB, DEC, CMP

Multiplication e.g. MUL, IMUL

Division e.g. DIV, IDIV

e.g. ADD AL, 5 (AL←AL+5)

MUL BL (AX ←AL*BL)

MUL BX (DX:AX ←AX*BX)


Bit Manipulation Instructions

• Logical instructionsNOT, AND, OR, XOR

• Shift instructionsSHL, SHR, SAL, SAR

• Rotate instructionsROL, ROR, RCL, RCR

e.g. MOV AL, 1Ch (AL←1Ch (00011100b))

ROR AL, 1 (rotate AL one bit to the right) (AL = 00001110b)


Byte/WordCF

Byte/WordCF

RCL

RCR

String Manipulation Instructions

• A string is a series of bytes or a series of words in sequential memory locations. It often consists of ASCII character codes

e.g. LODSB – Load byte at DS: [SI] into AL. Update SI

STOSW – Store word in AX into ES:[DI]. Update DI

CMPSB – Compare bytes: ES:[DI] from DS:[SI]


Control Transfer Instructions

• These instructions are used to tell the processor to start fetching instructions from some new address, rather than continuing in sequenceUnconditional transfer instructions e.g. CALL, RET, JMP

Conditional transfer instructions e.g. JE, JG, JGE, JL, JLE, JZ

Iteration control instructions e.g. LOOP, LOOPE, JCXZ

Interrupt instructions e.g. INT, IRET

e.g. SUB AX, 32

JZ label

…

label:

MOV BX, 10


AX=AX-32;If(AX==0)

{BX=10;

}

Processor Control Instructions

• Set/clear flags, control the operation of the processor

Flag instructions

e.g. STC – set carry flag

External hardware synchronization instructions

e.g. WAIT - Do nothing until signal on the TEST pin is low

No operation instructions e.g. NOP


Addressing Modes

• Describe the types of operands and the way they are accessed for executing an instruction

• 8086 addressing modes: Immediate

Direct

Register

Register Indirect

Indexed

Register Relative

Based Indexed

Relative Based Indexed


Immediate Addressing

• Immediate data is a part of instruction, and appears in

the form of successive byte(s)

e.g. MOV AX, 0005H (AX←0005H)

Here, 0005H is the immediate data. The immediate

data may be 8-bit or 16-bit in size.


Direct Addressing

• In the direct addressing mode, a 16-bit memory address (offset) is directly specified in the instruction

e.g. MOV AX, [5000H ] (AX←[DS:5000H])

Here, data resides in a memory location in the data

segment, whose effective address may be computed

using 5000H as the offset address and content of DS

as segment address.


Register Addressing

• In register addressing mode, the data is stored in a register and it is referred using the particular register

• All the registers, except IP, may be used in this mode

e.g. MOV AX, BX (AX←BX)

Here, data is transferred from register BX to register AX


Register Indirect Addressing

• Sometimes, the address of the memory location, which contains data or operand, is determined in an indirect way, using the offset registers

• The offset address of data is in either BX, SI or DI registers. The default segment is either DS or ES.

e.g. MOV AX, [BX ] (AX←[DS:BX])

Here, data is present in a memory location in DS whose offset address is in BX.


Indexed Addressing

• Offset of the operand is stored in one of the index registers (SI and DI). DS and ES are the default segments for SI and DI respectively

• This mode is a special case of register indirect addressing mode

e.g. MOV AX, [SI ] (AX←[DS:SI])

Here, data is present in a memory location in DS whose offset address is in SI.


Register Relative Addressing

• The data is available at an effective address formed by adding an 8-bit or 16-bit displacement with the content of any one of the registers BX, BP, SI and DI in the default segment (DS or ES)

e.g. MOV AX, 50H[BX ] (AX←[DS:BX+50H])


Based Indexed Addressing

• The effective address of data is formed by adding content of a base register (BX or BP) to the content of an index register (SI or DI)

e.g. MOV AX, [BX ][SI] (AX←[DS:BX+SI])


Relative Based Indexed Addressing

• The effective address is formed by adding an 8 or 16-bit displacement with the sum of contents of any one of the base registers (BX or BP) and any one of the index registers (SI or DI), in a default segment

e.g. MOV AX, 50H[BX ][SI] (AX←[DS:BX+SI+50])


Emu8086

• Assembler (flat assembler)+ emulator (MS-DOS system)


More Readings

1. Dr. Manoj’s Handouts, Chapter 2

2. 8086 datasheet

3. emu8086 documentation


Lecture4

Documents

Transcript of Lecture4