DATA SEGMENT
MESSAGE DB "ENTER CHARACTER :$"
NUM DB ?
BCD DB ?
ENDS
CODE SEGMENT
ASSUME DS:DATA CS:CODE
START:
MOV AX,DATA
MOV DS,AX
LEA DX,MESSAGE
MOV AH,9
INT 21H
MOV AH,1
INT 21H
MOV NUM,AL
SUB AL,30H
MOV BCD,AL
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,MESSAGE
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,MESSAGE 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
MESSAGE 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
MOV NUM,AL
The above three line code is used to Read a Character from Console and save the value entered in variable NUM in its ASCII form.
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.
MOV NUM,AL means move value in AL register into variable NUM.
Now, lets understand line by line
SUB AL,30H
MOV BCD,AL
The above Two line code is used to convert the value entered in
variable NUM from ASCII form to 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.
SUB AL,30H means subtracting 30H from AL.
MOV BCD,AL means move value in AL register into variable BCD.
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.
MOV AH,4CH This same code can be interchangably written as
MOV AX,4C00H where AX register is initialized with 4C00H which means 4C
gets saved in AH register and 00 gets saved in AL register. different
books follow different forms.
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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