1. BIC 10503: COMPUTER ARCHITECTURE
Chapter 6 Instruction Sets

2. Overview
6.1 Characteristics and Functions
6.2 Addressing Modes and Formats
6.3 Assembly Language Programming

3. 6.1 Characteristics and Functions

4. What is an Instruction Set?
The collection of different instructions that the processor can execute is referred to as the processor’s instruction set
The operation of the processor is determined by the instructions it executes, referred to as machine instructions or computer instructions
Each instruction must contain the information required by the processor for execution 2

5. Elements of an Instruction
Operation code (Op code)
Specifies the operation to be performed (LOAD,ADD)
Source Operand reference
Source for the operation
Result Operand reference
Source to store the result
Next Instruction Reference
Tells the processor where to fetch next instruction 3

6. Instruction Cycle State Diagram

7. Instruction Representation
In machine code each instruction has a unique bit pattern
Each unique bit pattern has a symbolic representation to ease programmers in reading/writing codes:
e.g. ADD, SUB, LOAD
These representation is also called as assembly code
Operation Code are normally followed by Operands :-
e.g. ADD A,B 5

8. Simple Instruction Format

9. Some of the most commonly used:-
Opcodes
Also called mnemonics
Some of the most commonly used:-
ADD – add
SUB – subtract
MUL – multiply
DIV – divide
LOAD – load data from memory
STOR – store data to memory
MOV – move data
* operand can also be represented symbolically

10. Number of Addresses (a)
Result, Operand 1, Operand 2
a = b + c;
May be a forth - next instruction (usually implicit)
Not common
Needs very long words to hold everything 7

11. Number of Addresses (b)
One address act as operand and result
a = a + b
Reduces length of instruction
Requires some extra work
Temporary storage to hold some results 8

12. Number of Addresses (c)
Implicit second address
Usually a register (AC)
Common on early machines 9

13. How Many Addresses? More addresses Fewer addresses Pro: Cons:
Fewer instructions per program
Cons:
More complex instructions
More registers
Fewer addresses
Less complex instructions
Faster fetch/execution of instructions
More instructions per program 11

14. Types of Operand Addresses Numbers Characters Logical Data
Integer/floating point
Characters
ASCII etc.
Logical Data
AND NEG NOT OR 14

15. Data storage (main memory) Data movement (I/O)
Instruction Types
Data processing
Data can be processed by using Arithmetic and Logic instructions, e.g. ADD
Data storage (main memory)
Data can be stored to/from memory/registers
Data movement (I/O)
I/O instructions are meant to move data and instructions to/from external devices
Program flow control
Test and Branch instructions are used to control program flow 6

16. Types of Operation Transfer of Control I/O Data Transfer Arithmetic
Logical
Conversion
Data Processing
Data Storage
Program Flow Control
Data Movement 18

17. Types of Operation 1. Arithmetic20

18. Types of Operation 2. Logical21

19. Types of Operation 3. Conversion21

20. Types of Operation 4. Data Transfer19

21. Types of Operation 5. Input/Output23

22. Types of Operation 6. Transfer of Control25

23. 6.2 Addressing Modes and Formats

24. The operand of an instruction is representing either:
Addressing Modes
The operand of an instruction is representing either:
The actual memory address of the operand, OR
A pointer to a physical memory address, OR
Other representation.
Addressing modes allow us to specify where the operands are located. 2

25. Addressing Modes Seven most basic Addressing Modes:- Immediate Direct
Indirect
Register
Register Indirect
Displacement (Indexed)
Stack
Notation used:
A = contents of an address field in the instruction
R = contents of an address field in the instruction that refers to a register 2

26. Addressing Modes 1. Immediate Addressing
Operand is part of instruction
Operand value stored in address field
Operand = A
e.g. ADD 5
Add 5 to contents of accumulator
5 is operand
No memory reference to fetch data
Fast
Limited range 3

27. Immediate Addressing Diagram
Instruction
Opcode
Operand 4

28. Addressing Modes 2. Direct Addressing
Address field contains address of operand
Effective address (EA) in address field
EA = A
e.g. LDA 100b
Look in memory at address 100b for operand
Copy/load contents of memory address 100b to accumulator
Single memory reference to access data
No additional calculations to work out effective address
Limited address space 5

29. Direct Addressing Diagram
Instruction
Opcode
Address A
Memory
Operand 6

30. Addressing Modes 3. Indirect Addressing
Memory cell pointed to by address field contains the address of (pointer to) the operand
EA = (A)
Look in A, find address (A) and look there for operand
e.g. ADD (M)
Add contents of cell pointed to by contents of M to accumulator 7

31. Indirect Addressing (cont.)
Large address space
May be nested
e.g. EA = (((A)))
Multiple memory accesses to find operand
Hence slower 8

32. Indirect Addressing Diagram
Instruction
Opcode
Address A
Memory
Pointer to operand
Operand 9

33. Addressing Modes 4. Register Addressing
Operand is held in register named in address filed
EA = R
Limited number of registers
Shorter instructions
Faster instruction fetch 10

34. Register Addressing (cont.)
No memory access
Very fast execution
Very limited address space
Multiple registers helps performance
Requires good assembly programming or compiler writing 11

35. Register Addressing Diagram
Instruction
Opcode
Register Address R
Registers
Operand 12

36. Addressing Modes 5. Register Indirect Addressing
EA = (R)
Operand is in memory cell pointed to by contents of register R 13

37. Register Indirect Addressing Diagram
Instruction
Opcode
Register Address R
Memory
Registers
Pointer to Operand
Operand 14

38. Addressing Modes 6. Displacement Addressing
EA = A + (R)
Address field hold two values
A = base value
R = register that holds displacement
or vice versa 15

39. Displacement Addressing Diagram
Instruction
Opcode
Register R
Address A
Memory
Registers
Pointer to Operand +
Operand 16

40. Addressing Modes 7. Stack
EA = top of stack
e.g.
POP AX ;Pop top item from stack and load into register AX 15

41. Stack Addressing Diagram
Instruction
Implicit
Top of stack
register 12

42. Addressing Modes Summary

43. 6.3 Assembly Language Programming

44. Assembly Language Assembly Language Assembler
A symbolic representation of the machine language of a specific processor.
Augmented by additional types of statements that facilitate program writing and that provide instructions to the assembler.
Assembler
A program that translates assembly code into machine code.

45. Assembly Language Statement Structure
Example 1 (without label):
ADD R1, 5 ; R1 = R1 + 5
mnemonic operands comment
Example 2 (with label):
MYLABEL: INC R1 ; increment R1
label mnemonic operands comment

46. Programming Language
Learning any programming language involves mastering a number of common concepts:
Variables: Declaration/definition
Assignment: Assigning values to variables
Input/Output: Displaying messages/displaying variable values
Control flow: Loops, JUMPs
Subprograms: Definition and Usage
Programming in assembly language involves mastering the same concepts and a few other issues.

47. Registers have predefined names and do not need to be declared.
Variables
Here we will simply use the x86 registers’ name as the variables in our programs.
Registers have predefined names and do not need to be declared.

48. Registers 1. 8-bit general registers: AL, BL, CL, DL, AH, BH, CH, DH
bit general registers: AX, BX, CX, DX,
SP, BP, SI, Dl
3. 32-bit general registers: EAX ; Accumulator
EBX ; Base
ECX ; Counter
EDX ; Data
ESP ; Stack pointer
EBP ; Base pointer
ESI ; Source index
EDI ; Destination Index

49. Registers
Each of the registers is 8/16/32 bits long i.e. can contain a 8/16/32-bit binary number.
The main four registers are sometimes referred to as data registers. They are the AX, BX, CX and DX registers.
The data registers can be treated as 16-bit registers or they can each be treated as two 8-bit registers.
Each 8-bit register can be used independently.

50. Registers
The AX register may be accessed as ah and al (H and L refer to high-order and low-order bytes).
BX may be accessed as BH, BL
CX may be accessed as CH, CL
DX may be accessed as DH, DL
If you use a data register as an 8 bit register, you cannot use its 16 bit parent at the same time.

51. Assignment
In programming languages such as C/C++/Java, assignment takes the form:
x = 42 ;
y = 24;
z = x + y;
In assembly language we carry out the same operation but we use an instruction to denote the assignment operator (“=”). The above assignments would be carried out in x86 assembly language as follows:
MOV AX, 42
ADD AX, 24
MOV BX,AX

52. The MOV instruction carries out assignment.
Assignment (cont.)
The MOV instruction carries out assignment.
It allows us place a number in a register or in a memory location (a variable) i.e. it assigns a value to a register or a variable.
e.g.
MOV BX, ‘A’ ; To store the ASCII code for the letter A in register BX.
MOV BX, 2 ;set the value 2 into BX

53. Input/Output
In assembly language we must have a mechanism to call the operating system subprogram to carry out I/O.
In addition we must be able to tell the operating system what kind of I/O operation we wish to carry out, e.g. to read a character from the keyboard, to display a character or string on the screen, etc…...

54. In x86 assembly language, we do not call operating system subprograms by name, instead, we use a software interrupt mechanism
The x86 INT instruction generates a software interrupt.
It uses a single operand which is a number indicating which MS-DOS subprogram is to be invoked.
For I/O, the number used is 21h. Thus, the instruction INT 21h transfers control to the operating system, to a subprogram that handles I/O operations.
This subprogram handles a variety of I/O operations by calling appropriate subprograms.
This means that you must also specify which I/O operation (e.g. read a character, display a character) you wish to carry out. This is done by placing a specific number in a specific register.

55. The AH register is used to pass this information.
For example, the subprogram to display a character is subprogram number 2h.
This number must be stored in the AH register.
When the I/O operation is finished, the interrupt service program terminates and our program will be resumed.

56. Character Output
There are three elements involved in carrying out this operation using the INT instruction:
We specify the character to be displayed. This is done by storing the character’s ASCII code in a specific x86 register. In this case we use the DL register, i.e. we use DL to pass a parameter to the output subprogram.
We specify which of MS-DOS’s I/O subprograms we wish to use. The subprogram to display a character is subprogram number 2h. This number is stored in the AH register.
We request MS-DOS to carry out the I/O operation using the INT instruction. This means that we interrupt our program and transfer control to the MS-DOS subprogram that we have specified using the AH register.

57. Character Output
Example : Write a code fragment to display the character ’a’ on the screen:
MOV DL, ‘a’
; DL = ‘a’
MOV AH, 2h
; 2h is a subprogram to display character
INT 21h
; call ms-dos, output character

58. Character Input
There are also three elements involved in performing character input:
As for character output, we specify which of MS-DOS’s I/O subprograms we wish to use, i.e. the character input from the keyboard subprogram. This is MS-DOS subprogram number 1h. This number must be stored in the AH register.
We call MS-DOS to carry out the I/O operation using the INT instruction.
The MS-DOS subprogram uses the AL register to store the character it reads from the keyboard.

59. Example: Write a code fragment to read a character from the keyboard
Character Input
Example: Write a code fragment to read a character from the keyboard
MOV AH, 1h
; keyboard input subprogram
INT 21h
; character input
; character is stored in AL

60. Example: Reading and displaying a character:
Character Input
Example: Reading and displaying a character:
MOV AH, 1h
; 1h is keyboard input subprogram
INT 21h
; read character into AL
MOV DL, AL
; copy character to DL
MOV AH, 2h
; 2h is character output subprogram
; display character in dl

61. Reference List of Intel x86 most basic instructions

62. THANK YOU