Transcript Assembly Language

Assembly Language – 1
1. Machine Language
This is what the computer actually sees and deals with. Every
command the computer sees is given as a number or sequence of
numbers.
2. Assembly Language
This is the same as machine language, except the command numbers
have been replaced by letter sequences which are easier to
memorize. Other small things are done to make it easier as well.
3. High-Level Language
They are there to make programming easier. Assembly language
requires you to work with the machine itself. High-level languages
allow you to describe the program in a more natural language. A
single command in a high-level language usually is equivalent to
several commands in an assembly language.
To study… - SELF-study
• Syntax
• Variables
• Basic data movements and arithmetic
instructions
• Program organization – comprising code, data & stack
• Assembly lang prog MUST be converted to a machine
lang prog before it can be executed  by an
assembler
1. Syntax
• Assembly lang – low-level programming
language
• An assembly language is specific to a
certain computer architecture, in contrast to
most high-level programming languages,
which generally are portable to multiple
systems.
Assembler
• Assembly language programs are converted
into executable machine code by a utility
program referred to as an assembler, the
conversion process being referred to as
assembly or assembling the program.
• Assembler  Microsoft Macro Assembler
[MASM]
Assembly lang
• A program written in assembly language consists of a
series of (mnemonic) processor instructions and metastatements (known variously as directives, pseudoinstructions and pseudo-ops), comments and data.
• Assembly lang. instructions usually consist of
 an opcode mnemonic
 followed by a list of data, arguments or parameters.
These are translated by an assembler into machine
language instructions that can be loaded into memory
and executed.
name
operation
START: MOV
operand(s)
CX, 5
;
;comment
;
যা ইচ্ছা লিখ! ‘ ’ লিও!
Example
• The instruction that tells an x86 processor
to move an immediate 8-bit value into a
register.
• The binary code for this instruction: 10110
• followed by a 3-bit identifier for which
register to use. The identifier for the AL
register is 000,
• so the following machine code loads the AL
register with the data 01100001.
10110 000
Instruction
In HEX 
AL
B0
Instruction
AL
01100001
data
61
data
-Intel assembly language provides the mnemonic MOV
(an abbreviation of move) for instructions such as this
-so the machine code above can be written as follows in
assembly language, complete with an explanatory
comment if required, after the semicolon.
MOV
AL, 61h
; Load AL with 97 decimal (61 hex)
• The Intel opcode 10110000 (B0h) copies an
8-bit value into the AL register;
• while 10110001 (B1h) moves it into CL.
• 10110010 (B2) does so into DL.
MOV
AL, 1h
; Load AL with immediate value 1
MOV
CL, 5h
; Load CL with immediate value 5
MOV
DL, 3h
; Load DL with immediate value 3
-MOV
EAX, [EBX]
;Move the 4 bytes in memory at the address contained in EBX into EAX
MOV
[ESI+EAX], CL
;Move the contents of CL into the byte at address ESI+EAX
• Transforming assembly language into machine
code is the job of an assembler, and
• The reverse can at least partially be achieved by a
disassembler.
PC to PC: vary…
• Each computer architecture has its own machine
language.
• Computers differ
– in the number and type of operations they support,
– in the different sizes and numbers of registers, and
– in the representations of data in storage.
• While most general-purpose computers are able to
carry out essentially the same functionality,
the ways they do so differ;
the corresponding assembly languages reflect these
differences.
.section .data
• Anything starting with a period isn’t directly
translated into a machine instruction.
• Instead, it’s an instruction to the assembler itself.
• …called assembler directives, or pseudo-operations
because they are handled by the assembler & are not
actually run by the computer.
• .section command breaks your program up into
sections.
• This command starts the data section, where you list
any memory storage you will need for data.
.section .text
which starts the text section.
The text section of a program is where the
program instructions live.
.globl _start
• This instructs the assembler that _start is
important to remember.
• _start is a symbol, which means that it is going to
be replaced by something else either during
assembly or linking.
• Symbols are generally used to mark locations of
programs or data, so you can refer to them by
name instead of by their location number
_start:
• It defines the value of the _start label.
• A label is a symbol followed by a colon.
• Labels define a symbol’s value. When the assembler
is assembling the program, it has to assign each data
value and instruction an address. Labels tell the
assembler to make the symbol’s value be wherever
the next instruction or data element will be.
• This way, if the actual physical location of the data
or instruction changes, you don’t have to rewrite any
references to it - the symbol automatically gets the
new value.
Next is instruction!
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MOV
ADD
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