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Assembly Instructions

Assembly language instructions may involve mnemonics, labels,

variables, constants, and directives. Examples are as follows.

here1 mov5 ax,23 ;set ax value

msg2 db6 "help$" ;ASCII bytes assigned to msg

var3 db6 10 ;var assigned a value of 10

num4 equ6 20 ;num has the value 20

_____________________________________________________________1 label 4 constant2 variable of ASCII bytes 5 mnemonic3 one-byte numeric variable 6 directive

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Define Byte, Word

Define Byte Variables (memory locations)

Define Word Variables (memory locations)

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Define Byte, Word

• list db 10,20,30,40

will store the four values in consecutive locations. If the offset of list is 0000, they will be stored in 0000, 0001, 0002, 0003, respectively. list refers to 0000, list+1 refers to 0001, etc.

• Value1 dw 2AB6h

will place two bytes in consecutive locations with the low order byte in the lower memory location as: B6 2A

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Character or String Constants

• ‘ABC’• “This is a test.”• ‘This is my file.’

• Define a variable called message stored at offset 0000 for any of the above. For example: message db ‘ABC’.

• Then “B” of “ABC” is at offset 0001 and “i” of “This …” is at offset 2

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Pointer

Given:

message db ‘ABC’ ;define 3 bytes

P db message ;P points to message

The variable P contains the offset of message

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ASCII Table

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MOV Instruction

• Allowable MOVs with scratch registersMOV reg,reg

MOV mem,reg

MOV reg,mem

MOV mem,immed

MOV reg,immed

• Allowable MOVs with segment registers (except CS)MOV segreg,reg16

MOV segreg,mem16

MOV reg16,segreg

MOV mem16,segreg

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Direct Operands

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Illegal Moves

Label Directive

.data

countB label byte ;byte attribute,

;no storage allocated

countw dw 0020h ;word attribute

.code

mov al,countB ;retrieve low byte of count

mov cx,countW ;retrieve all of count

20 00

countBcountW

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Addressing Modes

Addressing Mode Example Description

Direct mov ax,bx Effective address (EA) are

the registers

Register Indirect mov ax,[bx] EA is the contents of a register

Based mov ax,[bp + 1] EA is the sum of a base register

and a displacement

Indexed mov [di + 2],ax EA is the sum of an index

register and a displacement

Based Indexed mov ax,[bx + si] EA is the sum of a base register

and an index register

Based Indexed with mov ax,[bx + si + 2] EA is the sum of a base

Displacement register, an index register and a

displacement

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Based Index Addressing ExampleAdding Bytes

In the following example of based indexed addressing, the contents of

16d bytes beginning at ds:1000h and ending with ds:100Fh are

accumulated in the al register.

cs:100 mov al,0 ;initialize AL register

cs:102 mov cx,10h ;set loop counter to 10h=16d

cs:105 mov si,0 ;set si=0

cs:108 mov bx,1000h ;set bx=1000h as offset address

cs:10b add al,[bx + si] ;accum. contents of mem. loc. [bx+si]

cs:10d inc si ;increment si

cs:11e1 loop 10bh ;return to add

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Based Index Addressing ExampleAdding Words

In the following example of based indexed addressing, the contents of

16d words beginning at ds:1000h and ending with ds:101Dh are

accumulated in the ax register.

cs:100 mov ax,0 ;initialize AX register

cs:103 mov cx,10h ;set loop counter to 10h=16d

cs:106 mov si,0 ;set si=0

cs:109 mov bx,1000h ;set bx=1000h as offset address

cs:10c add ax,[bx + si] ;accum. contents of mem. loc. [bx+si]

cs:10e add si,2 ;increment si by 2

cs:111 loop 10ch ;return to add

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Stack Operation

To save register contents before calling a subroutine:

;save register contents before calling subroutine

push ax

push bx

push cx

push dx

;restore register contents after returning from subroutine

pop dx

pop cx

pop bx

pop ax

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Assemble-Link-Execute Cycle

.asm

.obj

.lst

.exe

.map

MASM

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Hello World .lst File

title Hello World Program (hello.asm);This program displays "Hello, world!"

directive ->.model small <-code and data each < 64K directive -> .stack 100h <- 256 bytes reserved

0000 directive -> .data 0000 48 65 6C 6C 6F 2C message db "Hello, world!",0dh,0ah,'$' 20 77 6F 72 6C 64 21 0D 0A 24 0000 directive ->.code 0000 main proc <-begin procedure 0000 B8 ---- R mov ax,@data <-assigns seg. Addr. 0003 8E D8 mov ds,ax to DS 0005 B4 09 mov ah,9 0007 BA 0000 R mov dx,offset message 000A CD 21 int 21h 000C B8 4C00 mov ax,4C00h 000F CD 21 int 21h 0011 main endp <-end procdure

end main <-end assembly

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Hello World .map File

Start Stop Length Name Class

00000H 00010H 00011H _TEXT CODE

00012H 00021H 00010H _DATA DATA

00030H 0012FH 00100H STACK STACK

Origin Group

0001:0 DGROUP

Program entry point at 0000:0000

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XCHG Instruction

Problem: move bx to ax and ax to bx

mov cx,ax ;ax stored temporarily in cx

mov ax,bx ;move bx to ax

mov bx,cx ;move cx (really ax) to bx

or use:

xchg ax,bx

Allowed:

xchg reg,reg

xchg reg,mem

xchg mem,reg

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XCHGing Two Variables

Cannot do: xchg mem1,mem2, but

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Arithmetic InstructionsINC and DEC Instructions

inc destination ;add 1 to destination

dec destination ;subtract 1 from destination

where destination is reg or mem

Examples:

inc al

dec ax

dec byte ptr membyte;dec 8-bit memory operand

dec memword ;dec memory operand

inc word ptr memword;inc 16-bit memory operand

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Arithmetic InstructionsADD Instruction

add destination, source

Example: add ax,bx

Contents of Registers

Before After

add ax,bx AX |0FFF |1000 |

BX |0001 |0001 |

add ax,bx AX |0002 |0001 | plus a

BX |FFFF |FFFF | carry

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Arithmetic InstructionsADD Instruction

Consider the way in which add and adc, add with a carry,

deal differently with the carry flag. Both the AX and BX

registers contain 0000 and the carry flag is set, CY.

add ax,bx yields AX=0, BX=0 and NC (no carry)

adc ax,bx yields AX=1, BX=0 and NC

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Arithmetic InstructionsSUB Instruction

sub destination, source

Example: sub ax,bx

Contents of Registers

Before After

sub ax,bx AX |00A0 |009F |

BX |0001 |0001 |

sub ax,bx AX |0005 |FFFF |

BX |0006 |0006 |

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Arithmetic InstructionsMUL Instruction

mul multiplier ;multiplicand in ax

;product in dx,ax

Example: mul bx

Contents of Registers

Before After

mul bx AX |FFFF |FFF0 |

BX |0010 |0010 |

DX |0000 |000F |

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Arithmetic InstructionsDIV Instruction

div divisor ;dividend in dx,ax: quotient in ax

;remainder in dx

Example: div bx

Contents of Registers

Before After

div bx AX |FFF1 |FFFF |

BX |0010 |0010 |

DX |000F |0001 |

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Memory Models

Produces .com files

What we will use

Linear addressing

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Overlapping Segments

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