ASSEMBLY LANGUAGE FUNDAMENTALS

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ASSEMBLY LANGUAGE FUNDAMENTALS

• ICS33

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DATA DEFINITION DIRECTIVES

• A variable is a symbolic name for a location in
  memory where some data are stored.
• In assembly language, variables are identified by
  labels. Each label shows the starting location of
  a variable. We say that the labels offset is the
  distance from the beginning of the segment to the
  beginning of the variable.

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DATA DEFINITION DIRECTIVES

• A label however, does not indicate how many bytes
  of storage are allocated for a variable. For
  example, if we declare an array of 14 characters,
  the label aList identifies only the offset of the
  first character (A)
• Declaration Stored As

aList db ABCD
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DATA DEFINITION DIRECTIVES

• So if the first letter were at offset 0, the next
  one would be at offset 1, the next at 2, and so
  on. The label aList would equal 0, the offset of
  the first letter.
• We use data definition directives to allocate
  storage, based on the ff. predefined types

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DATA DEFINITION DIRECTIVES
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Define Byte (DB)

• DB directive allocates storage for one or more
  8-bit values.
• Syntax name DB initialvalue ,initialvalue
• name is optional
• initialvalue can be one or more 8-bit numeric
  values, a string constant, a constant expression,
  or a ?.
• Examples
• char db A ASCII character
• signed1 db -128 smallest signed value
• signed2 db 127 largest signed value

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DB

• unsigned1 db 0 smallest unsigned value
• Unsigned2 db 255 largest unsigned value
• Multiple values.
• A list of 8-bit numbers may be grouped under a
  single label, with the values separated by
  commas.
• list db 10,20,30,40
• Characters and integers are one and the same, as
  far as the assembler is concerned.

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DB

• The following variables contain exactly the same
  value and may be processed the same way
• char db A a character (ASCII 41h)
• hex db 41h hexadecimal
• dec db 65 decimal
• bin db 01000001b binary
• oct db 101q octal

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DB

• Each constant may use

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Loops

• mov cx,9
• nameLabel2
• inc si
• inc di
• loop nameLabel2
• This code will increment si and di nine times.

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Conditional Loops

• Before you do any conditions you need to do a cmp
  operation. It compares the first to the second
  operand then sets some flags that you can use to
  determine the relation between the two. Then you
  can use operations like je, jne, jle, jg. They
  are short for jump equal, not equal, less than or
  equal, and greater than. If the condition is true
  it will jump to the operand you supplied. Here is
  an example.

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Conditional Loops

• mov cx,9
• nameLabel2
• cmp si,di
• je anotherLabel
• inc si
• dec di
• loop nameLabel2
• anotherLabel

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Conditional Loops

• Here is an example of an if then else statement.
• cmp bx,9
• je label1
• mov ax,5
• jmp endLabel
• label1
• mov ax,4
• endLabel

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• One thing that is different is that is backwards.
  The else part is first then the then part comes
  next.

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While Loop

• A while loop is done like this.
• label1
• cmp bx,4
• je label2
• inc bx
• loop label1
• label2
• This while loop will continue until bx is equal
  to 4.

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Nested Loops

• This would be a good time to learn how to use the
  stack. Here is an example of a nested loop.

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Nested Loops

• mov cx,5
• nameLabel
• push cx
• mov cx,9
• nameLabel2
• mov ax,0
• mov ax,si
• mov di,ax
• inc si
• inc di
• loop nameLabel2
• add di,23
• pop cx
• loop nameLabel

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Nested Loops

• As you can see we have two loops. The problem is
  that both of these loops use cx as their counter.
  To remedy this when we enter the loop we push the
  value of cx onto the stack and then when the
  inner loop is done we pop the top of the stack
  into cx.

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Arrays

• Here are some examples of different kinds of
  arrays.
• someStrings db "hello world","hello 2 ","hello 3
  ",0
• The first variable is an array of strings. One
  thing you should do is make all the strings the
  same length. It makes it a lot easier to use
  them.

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Arrays

• someNumbers dw 10 dup(?)
• The second variable uses the dup command to make
  ten empty locations which you can set later on.
  You can put a number in those parentheses and it
  will put that number in each location.

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Arrays

• moreNumbers dw 2,1,2,5,5,6,3,2
• The third variable is just an array of numbers.
  Each number needs to separated by a comma.

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Arrays

• someChars db 'adsf',0
• The last variable is an array of characters.

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Arrays

• To use these arrays you must load the offset of
  one of them into a register. Here is an example.
• mov si,offset someStrings
• Once the offset is in the register you can access
  the values like this.
• mov ax,si
• That will put whatever is in the first position
  of si which would be the letter h not the whole
  string because a string is an array itself. To
  get to the next character do this.
• inc si

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Procedures and Macros

• Procedures and macros are very similar to
  functions in c. They can be used to make
  repetitive tasks easier.

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Procedures

• Procedures
• Do you remember the main proc that is in every
  program, well that is a procedure. A procedure
  can be created by giving it a name then putting
  proc after it. Put your code there. Then end it
  with a ret, then the name you chose then endp.
  You should put all this outside of the main
  procedure.

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Procedures

• Procedures
• To use this procedure you use the call command
  and then the name of the procedure. Here is an
  example
• .code
• main proc
• mov ax,_at_data
• mov ds,ax
• call myProc
• mov ax,4c00h
• int 21h
• main endp
• myProc proc
• inc bx
• ret
• myProc endp

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Procedures

• This is how it works. When the procedure is
  called it goes down to the location of the
  procedure. It does it stuff then ret is called
  and it returns to where it was in main.

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MACRO

• A macro is very similar syntactically but it's
  inner workings is different. First it can be
  passed a parameter. Second instead of the program
  going to where the macro is located it pastes a
  copy of it into the program wherever it is
  called. For this reason it is not a good idea to
  use a macro in a loop. Here is an example of a
  macro.

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MACRO

• .code
• main proc
• mov ax,_at_data
• mov ds,ax
• aMacro 100
• mov ax,4c00h
• int 21h
• main endp
• aMacro macro theParameter
• mov ax,theParameter
• endm

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MACRO

• This macro is passed the value 100 and the macro
  puts that into ax.