1. Morgan Kaufmann Publishers Computer Organization and Assembly Language
November 30, 2018
CS 206 D
Computer Organization and Assembly Language
Chapter 1 — Computer Abstractions and Technology

2. Chapter 6 –Symbolic Instruction and Addressing
Lecture 4
Chapter 6 –Symbolic Instruction and Addressing

3. Chapter Outline Instruction execution cycle Data Transfer Instruction
Basic Arithmetic Instruction
The INT Instruction

4. Instruction execution cycle
program counter
instruction queue
Fetch
Decode
Fetch operands
Execute
Store output
Instructions execute faster if the data is in a register

5. Registers
Information inside the microprocessor is stored in registers.
The registers are classified according to the functions they perform
In general there are fourteen 16-bit registers:
Data registers: AX, BX, CX, DX
There are four general data registers.
They hold data for an operation.
Address registers:
Segment Registers CS, DS, SS, ES
Pointer and Index Registers: SP, BP, SI, DI
They are divided into segment, pointer, and index registers.
They hold the address of an instruction or data.
Status register:
It is called the FLAGS register.
It keeps the current status of the processor.

6. Pointer and Index Registers
Data Registers
General-purpose registers
AX - the Accumulator
BX - the Base Register
CX - the Count Register
DX - the Data Register
AX --> AH,AL
BX --> BH,BL
CX --> CH,CL
DX --> DH,DL
CS - the Code Segment Register
DS - the Data Segment Register
SS - the Stack Segment Register
ES - the Extra Segment Register
SP - the Stack Pointer \ defaults to stack segment
BP - the Base Pointer /
SI - the Source Index Register
DI - the Destination Register AX AH AL BH BL BX CX CH CL DX DH DL
Segment Registers CS DS SS ES
Pointer and Index Registers SI DI SP BP IP
FLAGS Register

7. Data Registers: AX, BX, CX, DX
Low and High bytes of the data registers can be accessed separately
AH, BH, CH, DH are the high bytes
AL, BL, CL, DL are the low bytes
Data Registers are general purpose registers but they also perform special functions
AX : Accumulator Register
Preferred register to use in arithmetic, logic and data transfer instructions because it generates the shortest Machine Language Code
Must be used in multiplication and division operations
Must also be used in I/O operations
BX : Base Register
Also serves as an address register
CX : Count register
Used as a loop counter
Used in shift and rotate operations
DX : Data register
Used in multiplication and division
Also used in I/O operations

8. 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 a collection of bytes, each memory byte has an
address, starting with 0.
The processor assigns a 20-bit physical address to its memory
locations, thus it is possible to address 2 = 1,048,576 bytes
(1M byte) of memory.
The bytes in memory have addresses 00000h to 0FFFFFh. 20

9. Address Registers - Segment Registers CS, DS, SS, ES
To keep track of the various program segments, the 8086 is
equipped with four segments registers to hold segment numbers:
CS (Code Segment): contains the code segment number.
DS (Data segment): contains the data segment number.
SS (Stack Segment): contains the stack segment number.
ES (Extra Segment): is used if a program needs to access a second data segment.
one segment
linear addresses
Segmented memory addressing: absolute (linear) address is a combination of a 16-bit segment value added to a 16-bit offset

10. Pointer and Index Registers: SP, BP, SI, DI
Address Registers -
Pointer and Index Registers: SP, BP, SI, DI
SP (Stack Pointer) register is used in conjunction with SS for accessing the stack segment.
BP (Base Pointer) register is used primarily to access data on the
stack. However, unlike SP, BP can be used to access data in the
other segments.
SI (Source Index) register is used to point to memory locations in the data segment addressed by DS. By incrementing the contents of SI, we can easily access consecutive memory locations.
DI (Destination Index) register performs the same functions as SI.
There is a class of instructions, called string operations, that use DI
to access memory locations addressed by ES.

11. Address Registers - Instruction Pointer (IP or PC)
IP/PC is updated each time an instruction is executed so that it will point to the next instruction.
Unlike other registers, the IP/PC cannot be directly manipulated by an instruction (i.e. The instruction cannot contain IP as its operand).

12. Data Transfer Instruction
MOV
MOVSX-MOVZX
XCHG
LEA

13. MOV Instruction The MOV (move) instruction is used to:
Transfer data between Registers.
Transfer data between registers and memory locations.
Move a number directly into a register or memory location.
Syntax:
MOV destination, source
Example:
MOV AX, WORD1
MOV AX, BX
MOV AX, 'A'
Note: any register can be used except CS & IP
Before After
AX AX
WORD WORD1

14. Legal Combinations of operands for MOV
Destination Operand
General Segment Memory
Source operand register register location Constant
General register yes yes yes no
Segment register yes no yes no
Memory location yes yes no no
Constant yes no yes no
Illegal: MOV WORD1, WORD2 • Legal: MOV AX, WORD2
MOV WORD1, AX
Illegal: MOV DS, CS • Legal: MOV AX, CS
MOV DS, AX

15. Type Agreement of Operands
The operands of any two-operand instruction must be of the same
type (i.e. Both bytes or words).
Illegal: MOV AX, BYTE1
However, the assembler will accept both of the following:
MOV AH, 'A' moves 41H into AH
MOV AX, 'A' moves 0041H into AX

16. MOVE-and-Fill Instruction (MOVSX-MOVZX)
The MOVSX (move) instruction is used to:
Move a byte or word source to a word or doubleword destination.
Use with signed arithmetic values
Syntax:
MOVSX destination, source
Example:
MOVSX CX, B .
; CX=
The MOVZX (move) instruction is used to:
Move a byte or word source to a word or doubleword destination.
Use with unsigned arithmetic values
Syntax:
MOVZX destination, source
Example:
MOVZX CX, B . ;CX=

17. XCHG Instruction Destination Operand General Memory
The XCHG (exchange) operation is used to exchange the contents
of:
Two registers.
A register and a memory location.
Syntax:
XCHG destination, source
Example:
XCHG AH, BL
XCHG AX,WORD1
Destination Operand
General Memory
Source Operand register location
General register yes yes
Memory location yes no
Before After
AH AL AH AL
BH BL BH BL 1A A

18. LEA Instruction
LEA (Load Effective Address) puts a copy of the source offset
address into the destination.
Syntax:
LEA destination, source
Where destination is a general register and source is a memory
location
Example:
MSG DB 41H, 42H, 43H
LEA DX, MSG
puts the offset address of the variable MSG into DX.
Data Definition + Basic Instructions 17

19. Basic Arithmetic Instruction
ADD
SUB
INC
DEC

20. ADD and SUB Instructions
The ADD (add) and SUB (subtract) instructions are used to:
Add/subtract the contents of:
Two registers.
A register and a memory location.
Add/subtract a number to/from a register or memory location.
Syntax:
ADD destination, source SUB destination, source
Examples:
ADD WORD1, AX SUB AX, DX
Before After
01BC BC
AX AX DF
WORD WORD1
Before After
FFFF
AX AX
DX DX

21. Legal Combinations of operands for ADD & SUB
Destination Operand
General Memory
Source Operand register location
General register yes yes
Memory location yes no
Constant yes yes
Illegal: ADD BYTE1, BYTE2
Legal: MOV AL, BYTE2
ADD BYTE1, AL

22. INC and DEC Instructions
INC (increment) is used to add 1 to the contents of a register or
memory location.
DEC (decrement) is used to subtract 1 from a register or memory
location.
Syntax:
INC destination DEC destination
Examples:
INC WORD1 DEC BYTE1
Before After
WORD WORD1
Before After
BYTE BYTE1
FFFE FFFD

23. INT Instruction
To invoke a DOS or BIOS routine, the INT (interrupt) instruction is
used.
Format:
INT interrupt_number
where interrupt_number is a number that specifies a routine.

24. INT 21h INT 21h may be used to invoke a large number of DOS functions.
A particular function is requested by placing a function number in
the AH register and invoking INT 21h.
Some of the functions are:
INT 21h functions expect input values to be in certain registers and
return output values in other registers.
Function number Routine
1 single-key input
2 single-character output
9 character string output

25. INT 21h Function 1: Single-Key Input Input: AH = 1
Output: AL = ASCII code if character key is pressed
= 0 if non-character key is pressed
To invoke the routine, the following instructions should be executed:
MOV AH,1 ; input key function
INT 21H ; ASCII code in AL

26. INT 21h Function 2: Display a character or execute a control function
Input: AH = 2
DL = ASCII code of the character
Output AL = ASCII code of the character
To invoke the routine, the following instructions should be executed:
MOV AH, 2 ; display character function
MOV DL, '?' ; character is '?' (or any other character)
INT 21H ; display character

27. INT 21h Function 2 may be used to perform control functions.
If DL contains the ASCII code of a control character, INT 21h
causes the control function to be performed.
The principal control characters are :
ASCII code (Hex) Symbol Function
07H BEL beep (sounds a tone)
08H BS backspace
09H HT tab
0AH LF line feed (new line)
0DH CR carriage return (start of current line)

28. INT 21h Function 9: Display a string Input: AH = 9
DX = offset address of string.
The string must end with a '$' character
To invoke the routine, the following instructions should be executed:
MOV
MOV DS, AX
MOV AH, 9 ; display string function
LEA DX, MSG ; get message (Load Effective Address)
INT 21H ; display string
A program containing a data segment should
begins with these two instructions

29. INT 21h Function 4CH: Returning control to DOS Input: AH = 4CH
To invoke the routine, the following instructions should be executed:
MOV AH, 4CH ; DOS exit function
INT 21H ; exit to DOS