1. Microcomputer & Interfacing Lecture 3
The 8086 Instruction sets
BY: Tsegamlak Terefe

2. Introduction to an instruction
Objective
Introduction
Introduction to an instruction
Instruction set of 8086
Addressing mode
BY: Tsegamlak Terefe

3. 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

4. Introduction 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 programming will
Become difficult.
BY: Tsegamlak Terefe

5. 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

6. Introduction 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.
BY: Tsegamlak Terefe

7. 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

8. Instructions
The 8086 microprocessor have variable length instructions ranging from 8 bit to 48 bits.
Note: The first six bit of the opcode will tell you which operations to perform for E.g. Addition , subtraction, division , move,…. The D bit will tell you the direction of data flow i.e. if D=1 data will flow to REG from R/M. similarly if D=0 data will flow to R/M from REG that is shown in the second byte of instruction . The W bit will tell you the data is a word or a byte i.e. if W=1 the data is a word similarly if W=0 the data is a byte.
BY: Tsegamlak Terefe

9. Note: most instructions involving data manipulations are performed between a register-register, register-memory. The second byte of the instruction will tell you which choice is chosen for the instruction at hand. The mode field will tell you the various way of calculating an address for memory locations when the register-memory choice is taken which we are going to talk about in the next slides.
BY: Tsegamlak Terefe

10. Instructions
For Example lets take the following 16 bit instruction 8BEC.
8BEC=
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
Note: Imagine yourself writing codes in such binary format how many days would it take you to write a simple code?
BY: Tsegamlak Terefe

11. Instructions Data transfer instructions Arithmetic 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

12. Instruction Sets Data transfer instructions Arithmetic 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

13. 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

14. 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)
BY: Tsegamlak Terefe

15. Bit Manipulation Instruction
Logical instructions
NOT, AND, OR, XOR
Shift instructions
SHL, SHR, SAL, SAR
Rotate instructions
ROL, ROR, RCL, RCR
e.g. MOV AL, 1Ch (AL←1Ch ( b))
ROR AL, 1 (rotate AL one bit to the right) (AL = b)
Byte/Word CF
RCL
Byte/Word CF
RCR
BY: Tsegamlak Terefe

16. 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]
BY: Tsegamlak Terefe

17. 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

18. 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

19. 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

20. Relative Based Indexed
Addressing Modes
Immediate
Direct
Register
Register Indirect
Indexed
Register Relative
Based Indexed
Relative Based Indexed
BY: Tsegamlak Terefe

21. 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

22. 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

23. 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

24. 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

25. e.g. MOV AX, [SI ] (AX←[DS:SI])
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

26. 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

27. 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

28. Summary of addressing modes
Note : This addressing modes are used for data. There is also an addressing mode for program (Instructions) i.e. when an instruction of type Jump, call, branch are used. You can refer this modes on chapter 3 of Barry B. starting at page 91.
BY: Tsegamlak Terefe

29. Next Lecture
Programming the 8086
Emu8086
BY: Tsegamlak Terefe

30. Additional Reference Dr Manoj’s handout, chapter 2
The Intel Microprocessors, Barry B.
BY: Tsegamlak Terefe