Microcomputer & Interfacing
Lecture 3
 The 8086 Instruction sets
BY: Tsegamlak Terefe
Objective
 Introduction
 Introduction to an instruction
 Instruction set of 8086
 Addressing mode
BY: Tsegamlak Terefe
Introduction
 Programs in microprocessors are stored in successive memory
locations for fetching followed by decoding and execution in general.
 There are in general three levels of programming languages.
 Machine Language
 Assembly Language
 High Level Language
BY: Tsegamlak Terefe
Introduction
Machine Language
 These are the binary codes for the instructions you want
the microcomputer to execute
11101001000000000101000
 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 programming will
Become difficult.
BY: Tsegamlak Terefe
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Introduction
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 ,ROR for rotate Wright , 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 assembler
BY: Tsegamlak Terefe
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Introduction
High-Level Languages
 These languages use program statements which are even more Englishlike 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.
BY: Tsegamlak Terefe
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Instructions
 An instruction will have two major parts
 An opcode and an address/operand to store the result after execution or to
operate on respectively.
Opcode : is the operation code which is decoded by the processor to
activate the necessary circuitry for the execution of an instruction.
Opcode
Address
(Operands)
BY: Tsegamlak Terefe
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Instructions
 The 8086 microprocessor have variable length instructions ranging from 8
bit to 48 bits.
BY: Tsegamlak Terefe
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BY: Tsegamlak Terefe
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Instructions
 For Example lets take the following 16 bit instruction 8BEC.
8BEC= 1000 1011 1110 1100
 The first six bits will tell us the opcode
100010= MOV
 The next two bits tell us
D=1=> Data will move to REG from R/M
W=1=> Data is 16 bit in size
 on the next byte
MOD=11 => data is transferred b/n two registers or R/M is register
 REG=101
The first register involved is BP
 R/M=100
The second register involved is SP
Hence the assembly equivalence of this machine code will be
MOV BP,SP
BY: Tsegamlak Terefe 10
Instructions
 Now that we got a grip of what an instruction look like in 8086 we can
generalize the instruction sets (opcode) in to six general group.
 Data transfer instructions
 Arithmetic instructions
 Bit manipulation instructions
 String manipulation instructions
 Control transfer instructions
 Processor control instructions
BY: Tsegamlak Terefe 11
Instruction Sets
 Now that we got a grip of what an instruction look like in 8086 we can
generalize the instruction sets (opcode) in to six general group.
 Data transfer instructions
 Arithmetic instructions
 Bit manipulation instructions
 String manipulation instructions
 Control transfer instructions
 Processor control instructions
BY: Tsegamlak Terefe 12
Data transfer instructions
 Used to transfer data from source operand to destination operand





Memory 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,LAHF,….
BY: Tsegamlak Terefe 13
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)
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Bit Manipulation Instruction
 Logical instructions
NOT, AND, OR, XOR
 Shift instructions
CF
Byte/Word
RCL
SHL, SHR, SAL, SAR
 Rotate instructions
ROL, ROR, RCL, RCR
RCR
CF
Byte/Word
e.g. MOV AL, 1Ch (AL←1Ch (00011100b))
ROR AL, 1 (rotate AL one bit to the right) (AL =
00001110b)
BY: Tsegamlak Terefe 15
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]
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Control Transfer Instructions
 These instructions are used to tell the processor to start
fetching instructions from some new address, rather than
continuing in sequence
 Unconditional 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;
}
BY: Tsegamlak Terefe 17
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
BY: Tsegamlak Terefe 18
Addressing Modes
 Addressing mode: Describe the types of operands and the
way they are accessed for executing an instruction
 The operand part of an instructions are accessed from a
memory location or a register in various mode of
addressing.
 The 8086 microprocessor have the following addressing
modes.
BY: Tsegamlak Terefe 19
Addressing Modes
Immediate
Direct
Register
Register Indirect
Indexed
Register Relative
Based Indexed
Relative Based Indexed
BY: Tsegamlak Terefe 20
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.
BY: Tsegamlak Terefe 21
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.
BY: Tsegamlak Terefe 22
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
BY: Tsegamlak Terefe 23
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.
BY: Tsegamlak Terefe 24
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.
BY: Tsegamlak Terefe 25
Base 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])
BY: Tsegamlak Terefe 26
Relative Base 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])
BY: Tsegamlak Terefe 27
Summary of addressing modes
BY: Tsegamlak Terefe 28
Next Lecture
 Programming the 8086
 Emu8086
BY: Tsegamlak Terefe 29
Additional Reference
 Dr Manoj’s handout, chapter 2
 The Intel Microprocessors, Barry B.
BY: Tsegamlak Terefe 30