Lecture 4
Chapter 6 –Symbolic Instruction and Addressing
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Chapter Outline
Data Transfer Instruction
Basic Arithmetic Instruction
The INT Instruction
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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:
• There are four general data registers.
• They hold data for an operation.
• Address registers:
• 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.
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Registers
Data Registers
AX
BX
CX
DX
AH
AL
BH
BL
CH
CL
DH
DL
Segment Registers
CS
DS
SS
ES
Pointer and Index Registers
SI
DI
SP
BP
IP
FLAGS Register
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Data Registers: AX, BX, CX, DX
• These four registers, in addition to being general-purpose registers,
also perform special functions.
•The high and low bytes of these registers can be accessed separately.
• Ex. The high byte of AX is called AH, and the low byte is called AL.
• This arrangement gives us more registers to use when dealing
with byte-size data.
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Data Registers: AX, BX, CX, DX
• AX (Accumulator Register) is the preferred register to use in
arithmetic, logic, and data
transfer instructions
• BX (Base Register) also serves as an address register.
• CX (Count Register) Program loop constructions are facilitated by
the use of CX, which
serves as a loop counter.
•DX (Data Register) is used in multiplication and division.
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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
20
locations
thus it is possible to address 2 = 1,048,576 bytes
(one megabyte) of memory.
• The bytes in memory have addresses 00000h to FFFFFh.
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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.
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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.
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Address Registers - Instruction Pointer (IP)
• IP is updated each time an instruction is executed so that it will point
to the next instruction.
• Unlike other registers, the IP cannot be directly manipulated by an
instruction (i.e. The instruction cannot contain IP as its operand).
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Data Transfer Instruction
MOV
MOVSX-MOVZX
XCHG
LEA
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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.
Note: any register can be used except CS & IP
• Syntax:
MOV destination, source
• Example:
MOV AX, WORD1
MOV AX, BX
MOV AX, 'A'
Before
0006
AX
After
0008
AX
0008
WORD1
0008
WORD1
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Legal Combinations of operands for MOV
Source operand
General register
Segment register
Memory location
Constant
Destination Operand
General Segment Memory
register register location
yes
yes
yes
yes
no
yes
yes
yes
no
yes
no
yes
Constant
no
no
no
no
• Illegal: MOV WORD1, WORD2 • Legal: MOV AX, WORD2
MOV WORD1, AX
• Illegal: MOV DS, CS
• Legal: MOV AX, CS
MOV DS, AX
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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
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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,10110000B
. CX= 11111111 10110000
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MOVE-and-Fill Instruction (MOVSX-MOVZX)
• 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,10110000B
. CX= 00000000 10110000
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XCHG Instruction
• 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
Before
1A
00
AH
AL
After
05
00
AH
AL
00
BH
00
BH
05
BL
1A
BL
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Legal Combinations of operands for XCHG
Source Operand
General register
Memory location
Destination Operand
General Memory
register location
yes
yes
yes
no
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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.
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Data Definition + Basic Instructions
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Basic Arithmetic Instruction
ADD
SUB
INC
DEC
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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
01BC
AX
0523
WORD1
After
01BC
AX
06DF
WORD1
Before
0000
AX
0001
DX
After
FFFF
AX
0001
DX
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Legal Combinations of operands for ADD & SUB
Source Operand
General register
Memory location
Constant
Destination Operand
General Memory
register location
yes
yes
yes
no
yes
yes
• Illegal: ADD BYTE1, BYTE2
• Legal: MOV AL, BYTE2
ADD BYTE1, AL
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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
0002
WORD1
After
0003
WORD1
Before
FFFE
BYTE1
After
FFFD
BYTE1
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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.
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:
Function number
1
2
9
Routine
single-key input
single-character output
character string output
• INT21h functions expect input values to be in certain registers and
return output values in other registers.
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
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
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)
07H
08H
09H
0AH
0DH
Symbol
BEL
BS
HT
LF
CR
Function
beep (sounds a tone)
backspace
tab
line feed (new line)
carriage return (start of current line)
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 AX, @DATA
A program containing a data segment should
begins with these two instructions
MOV DS, AX
MOV AH, 9
LEA DX, MSG
INT 21H
; display string function
; get message (Load Effective Address)
; display string
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