DATA SEGMENT
NUM1 DB ?
NUM2 DB ?
RESULT DB ?
MSG1 DB 10,13,"ENTER FIRST NUMBER TO ADD : $"
MSG2 DB 10,13,"ENTER SECOND NUMBER TO ADD : $"
MSG3 DB 10,13,"RESULT OF ADDITION IS : $"
ENDS
CODE SEGMENT
ASSUME DS:DATA CS:CODE
START:
MOV AX,DATA
MOV DS,AX
LEA DX,MSG1
MOV AH,9
INT 21H
MOV AH,1
INT 21H
SUB AL,30H
MOV NUM1,AL
LEA DX,MSG2
MOV AH,9
INT 21H
MOV AH,1
INT 21H
SUB AL,30H
MOV NUM2,AL
ADD AL,NUM1
MOV RESULT,AL
MOV AH,0
AAA
ADD AH,30H
ADD AL,30H
MOV BX,AX
LEA DX,MSG3
MOV AH,9
INT 21H
MOV AH,2
MOV DL,BH
INT 21H
MOV AH,2
MOV DL,BL
INT 21H
MOV AH,4CH
INT 21H
ENDS
END START
Explanation :
In this Assembly Language Programming, A single program is divided
into four Segments which are 1. Data Segment, 2. Code Segment, 3. Stack
Segment, and 4. Extra Segment. Now, from these one is compulsory i.e.
Code Segment if at all you don’t need variable(s) for your program.if
you need variable(s) for your program you will need two Segments i.e.
Code Segment and Data Segment.
CODE SEGMENT
CODE SEGMENT is the starting point of the Code Segment in a Program
and CODE is the name given to this segment and SEGMENT is the keyword
for defining Segments, Where we can write the coding of the program.
ASSUME DS:DATA CS:CODE
In this Assembly Language Programming, their are Different Registers
present for Different Purpose So we have to assume DATA is the name
given to Data Segment register and CODE is the name given to Code
Segment register (SS,ES are used in the same way as CS,DS )
START:
START is the label used to show the starting point of the code which
is written in the Code Segment. : is used to define a label as in C
programming.
MOV AX,DATA
MOV DS,AX
After Assuming DATA and CODE Segment, Still it is compulsory to
initialize Data Segment to DS register. MOV is a keyword to move the
second element into the first element. But we cannot move DATA Directly
to DS due to MOV commands restriction, Hence we move DATA to AX and then
from AX to DS. AX is the first and most important register in the ALU
unit. This part is also called INITIALIZATION OF DATA SEGMENT and It is
important so that the Data elements or variables in the DATA Segment are
made accessable. Other Segments are not needed to be initialized, Only
assuming is enhalf.
LEA DX,MSG1
MOV AH,9
INT 21H
The above three line code is used to print String or Message present
in the character Array till $ symbol which tells the compiler to stop.
Now, lets understand line by line
LEA DX,MSG1 in this LEA stands for LOAD EFFECTIVE ADDRESS and it
loads the effective address of second element into the first element.
This same code can be interchangably written as MOV DX, OFFSET
MSG1 where OFFSET means effective address and MOV means move second
element into the first element.
MOV AH,9
INT 21H
The above two line code is used to PRINT the String or Message of the address present in DX register.
Standard Input and Standard Output related Interupts are found in INT
21H which is also called as DOS interrupt. It works with the value of
AH register, If the Value is 9 or 9h, That means PRINT the String or
Message of the address present in DX register.
MOV AH,1
INT 21H
SUB AL,30H
MOV NUM1,AL
The above Four line code is used to Read a Character from Console and
save the value entered in variable NUM1 in its BCD form. This can be
done by subtracting 30H i.e. SUB AL,30H. The value coming from Console
is Basically in ASCII form. eg. When you enter 5 we see 35H,So by
subtracting 30H we get back to value as 5.
Standard Input and Standard Output related Interupts are found in INT
21H which is also called as DOS interrupt. It works with the value of
AH register, If the Value is 1 or 1h, That means READ a Character from
Console, Echo it on screen and save the value entered in AL register.
SUB AL,30H means subtracting 30H from AL.
MOV NUM1,AL means move value in AL register into variable NUM1.
LEA DX,MSG2
MOV AH,9
INT 21H
The above two line code is used to PRINT the String or Message of the address present in DX register i.e. for MSG2.
MOV AH,1
INT 21H
SUB AL,30H
MOV NUM2,AL
The above Four line code is used to Read a Character from Console and save the value entered in variable NUM2 in its BCD form.
Next Line – ADD AL,NUM1
The above line code is used to add the two variables and save the result in another variable.
As we know the programs work only with the instructions in the instruction set. Instruction ADD is used to add to numbers in the following permutations above. REG stands for Registers (Eg. AX, BX, CX, DX ). memory stands for Variable or Address. immediate stands for Numbers or Values. Let us understand the meanings of the above permutations.
First permutation :- REG , memory means Register can be added with memory.
Second permutation :- memory , REG means memory can be added with Register.
Third permutation :- REG, REG means Register can be added with Register.
Fourth permutation :- memory , immediate means memory can be added with immediate.
Fifth permutation :- REG, immediate means Register can be added with immediate.
Note :- In the permutations above it will work only in the order
mentioned above and not by interchanging the first to second and second
to first.
Now, we have understood part of it to add to number we can write ADD NUM1, NUM2, But there is no permutation for ADD m emory, memory,
Hence we have to send one number to AL or AX depending on DB or DW. AX
Register is called Accumalator. and is used for holding the result of
Addition in it After Addition. Now we are taking DB, So we have t0
instruction MOV AL,NUM1 move NUM1 variable value to AL Register. After moving NUM1 to AL, We can Add REG to memory, So we have ADD AL,NUM2 or (We can Add memory to REG , So we have ADD NUM2,AL)
Both are allowed as per permutations so use one from two. Now, the
Resultant Value is saved in Accumalator AL for DB and AX for DW, So move
Resultant value to RESULT variable by instruction MOV RESULT,AL
MOV AH,0
AAA
The first line is just to clear the unwanted garbage value present
in AH register as the AH register is going to be used later. The above
two line code is used to corrects result in AH and AL after addition
when working with BCD values . AAA means ASCII Adjust after Addition. AAA
Instruction has NO Operands or values attached to it. If we are adding
two BCD numbers the Result is saved in AL register in HEXadecimal form.
What AAA exactly does is it converts the result into BCD form and first digit is saved in AH register and second digit is saved in AL register.
ADD AH,30H
ADD AL,30H
Since the Result of Multiplication is in AH and AL register in BCD
form After using AAM. Now we want to print the result on screen or
console. the BCD form value will not show us the Result But will print
the Coresponding Ascii Codes of the number, Hence By adding 30H to BCD
will Convert it to ASCII code which will print the digit (number) on
screen.
MOV BX,AX
Since the AH register is used again and again. We cannot lose the
result in AH and AL register, So to save the to be printed value in BH
and BL register. By moving AX to BX.
LEA DX,MSG3
MOV AH,9
INT 21H
The above two line code is used to PRINT the String or Message of the address present in DX register i.e. for MSG3.
MOV AH,2
MOV DL,BH
INT 21H
The above Three line code is used to Write a Character on Console present in BH register.
Standard Input and Standard Output related Interupts are found in INT
21H which is also called as DOS interrupt. It works with the value of
AH register, If the Value is 2 or 2h, That means WRITE a Character on
Console present in DL register hence the value to be printed is moved to
DL register. Here we are printing BH register.
MOV AH,2
MOV DL,BL
INT 21H
The above Three line code is used to Write a Character on Console present in BL register.
MOV AH,4CH
INT 21H
The above two line code is used to exit to dos or exit to operating
system. Standard Input and Standard Output related Interupts are found
in INT 21H which is also called as DOS interrupt. It works with the
value of AH register, If the Value is 4ch, That means Return to
Operating System or DOS which is the End of the program.
CODE ENDS
CODE ENDS is the End point of the Code Segment in a Program. We can
write just ENDS But to differentiate the end of which segment it is of
which we have to write the same name given to the Code Segment.
END START
END START is the end of the label used to show the ending point of the code which is written in the Code Segment.
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