Download What is Assembly Language

~CSC159~ Lab1
Introduction
What is Assembly Language ?
Assembly Language is a machine-specific programming language with a one-to-one
correspondence between its statements and the computer’s native machine language.
There are many different types of assembly language, each specific to a processor or
processor family. Instructions in assembly language are designed to match a computer’s
machine instruction set and hardware architecture.
What is an Assembler?
An assembler is a program that converts source-code programs from assembly language
into machine language.The most popular assemblers for the Intel family are MASM
(Microsoft Assembler), TASM (Borland Turbo Assembler) and ASM86.
*** MS-DOS Command (c:\)
C:\> cd
(change directory)
C:\> dir
(display directories and files)
C:\> mkdir
(make directory)
C:\> copy
(copy files)
C:\>
Why Learn assembly Language?
You may want to learn more about the computer you work with and about the way
computer languages generate machine code.
Machine Language
Machine Language is a language made up of numbers, which can be interpreted by a
computer’s processor. A processor usually has a built-in interpreter called a
microprogram that interprets and translates machine instructions into hardware signals.
Eg: MOV AL, 5
An example of an Intel machine language instruction that moves 5 into the AL register:
1011000000000101
The first 8 bits are the operation code (opcode), which identifies it as the instruction that
moves an 8-bit number to the AL register. The second 8 bits are the operand.
The complete instruction moves the number 5 to a register called AL.
Opcode – MOV, ADD, SUB, INC, etc.
Operand can be :
a register
:
a variable
:
a memory location
:
an immediate value :
AX
count
[memory location]
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~CSC159~ Lab1
Registers
Intel 16-bit Registers.
Index
General Purpose
BP
AH
AL
BH
BL
H
CH
CL
DH
DL
SP
SI
DI
Segment
CS
Status & Control
Flags
SS
IP
DS
ES
Registers are high-speed storage locations inside the CPU that can hold 8, 16, or 32 bits.
There are 3 types of registers :
1) General-purpose/Data Register
2) Address Register
3) Status Register
1) General-purpose / Data Registers
 are used for arithmetic data movement.
 Each register can be addressed as either a 16-bit or 8-bit value.
Eg: AX register is a 16-bit register;

its upper 8 bits are called AH and its lower 8 bits are called AL.
Bit 0 in AL corresponds to bit 0 in AX and bit 0 in AH corresponds to bit 8 in AX.
Each general-purpose register has special attributes:
i)
AX (accumulator)
- is a accumulator for arithmetic operations.
ii)
BX (base)
- can hold the address of a procedure or variable.
- can also perform arithmetic and data movement.
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iii)
CX (counter)
- act as a counter for repeating or looping instructions. These
instructions automatically repeat and decrement CX.
iv)
DX (data)
- has a special role in multiply and divide operations.
2) Address Registers
 Are used to store memory address.
 There are 3 types of address registers:
i)
Segment Register
- contains four segment registers, used as base locations for program
instructions, data and the stack.
- The segment registers are as follows:
a) CS (code segment)
- holds the base location of all executable instructions (code)
in a program.
b) DS (data segment)
- is the default base location for variables. The CPU calculates
their locations using the segment value in DS.
c) SS (stack segment)
- contains the base location of the stack.
d) ES (extra segment)
- is an additional base location for memory variables.
ii)
Pointer Register
a) IP (instruction pointer)
- always contains the offset of the next instruction to be
executed within the current code segment.
b) BP (base pointer)
- contains an assumed offset from the SS register.
c) SP (stack pointer)
- contains the offset of the top of the stack.
iii)
Index Register
a) SI (source index)
- this register takes its name from the string movement
instructions, in which the source string is pointed to by the SI
register.
b) DI (destination index)
- acts as the destination for string movement instructions.
(*** ii) & iii) also known as special-purpose registers)
3) Status Registers

Also called Flag Registers or Program Status Word (PSW).
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~CSC159~ Lab1

Flag registers reflect the outcomes of arithmetic and logical operations
performed by the CPU.
Flags
Sym
bols
CF
Set
Symb
ol
CY
Clear
Symb
ol
NC
Carry
Overfl
ow
Sign
OF
OV
NV
SF
NG
PL
Zero
ZF
ZR
NZ
Interru
pt
Parity
IF
EI
DI
PF
PE
PO
Purpose
- is set when the result of an unsigned arithmetic operation is
too large to fit into the destination.
- is set when the result of a signed arithmetic operation is too
wide (too many bits) to fit into the destination.
- is set when the result of an arithmetic or logical operation
generates a negative result.
- is set when the result of an arithmetic or logical operation
generates a result of zero.
- dictates whether or not system interrupts can occur.
- reflects the number of 1 bits in the result of an operation. Odd
number -> PF=0, Even number -> PF=1
Example program:
.model small
.stack 0100h
.data
.code
main proc
mov AH, 03h
mov BH, AH
main endp
end main
To assemble your program :
C:\Users\User>cd\MASM611\BIN
C:\MASM611\BIN>md ASM_files
C:\MASM611\BIN>edit ASM_files\lab1.asm
Write this code
.model small
.stack 0100h
.data
.code
main proc
mov AH, 03h
mov BH, AH
main endp
end main
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~CSC159~ Lab1
C:\ASM_files>masm test.asm
If no error, proceed with this command:
C:\ASM_files>ml test.asm
This command will create an exe file (test.exe), you can start debug the file:
C:\ASM_files>debug test.exe
Debug Commands
Command
A
G
Q
R
T
U
Description
Starts assembling a program, placing each instruction in memory.
Execute the remainder of the program.
Quits Debug.
Displays the CPU registers.
Traces (Execute) one program instruction.
Unassemble
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