Now we will write another Assembly program to find the sum of two BCD numbers stored in memory.
Let’s identify variables needed for this program.
First variables will be the one which will hold the values present in the variables to be Added and it will be NUM1 and NUM2. Other variables will be holding the Output or Result of the Addition and it will be RESULT So in all Three variables.
The identified variables are NUM1, NUM2 and RESULT.
First Line – DATA SEGMENT
DATA SEGMENT is the starting point of the Data Segment in a Program and DATA is the name given to this segment and SEGMENT is the keyword for defining Segments, Where we can declare our variables.
Next Line – NUM1 DB 9H
NUM2 DB 7H
RESULT DB ?
We are initializing NUM1 to 9H (H or h stands for Hexadecimal), NUM2 to 7H (H or h stands for Hexadecimal), RESULT to ? (? stands for blank value). Detailed explanation is given below.
Next Line – DATA ENDS
DATA ENDS is the End point of the Data 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 Data Segment.
Now, Selection of data type is DB data type the numbers which we are adding will be integers so DB is sufficient.
DATA SEGMENT NUM1 DB 9H NUM2 DB 7H RESULT DB ? ENDS
In Assembly programming, the variable are all defined by bytes only.
DB – Define Byte (Size – 1 Byte)
DW – Define Word (Size – 2 Byte)
DD – Define Double word (Size - 4 Bytes)
DQ – Define Quad word (Size – 8 Bytes)
DT – Define Ten Bytes (Size – 10 Bytes)
NUMBER SYSTEM in Assembly Programming is Decimal, Octal, Hexadecimal, Binary.
In the Program, We are entering the values for the variables and Do arithmetical Operations like Addition, Subtraction, Multiplication and Division So the Computer should understand which kind of Number is entered. Hence there is a different letters for different Number Systems. O or o stands for Octal, H or h stands for Hexadecimal, B or b stands for Binary, D or d stands for Decimal. By default type of numbering system is Decimal. If you do not specify any letter then the number is understood to be Decimal (By default).
DATA SEGMENT NUM1 DB 05 NUM2 DB 06 RESULT DB ? ENDS CODE SEGMENT ASSUME DS:DATA CS:CODE START: MOV AX,DATA MOV DS,AX MOV AL,NUM1 ADD AL,NUM2 MOV RESULT,AL MOV AH,0 AAA MOV AH,4CH INT 21H ENDS END START
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.
Next Line –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.
Next Line – 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 )
Next Line – 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.
Next Line – MOV AX,DATA
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.
Next Line – MOV AL,NUM1
The above three 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
Next Line – MOV AH,0
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.
Lets understand with results found in variables and registers.
here we can see two BCD numbers having 05 and 06 respectively. Before execution RESULT is 0 (zero) and After execution RESULT is B (B means Eleven in HEXadecimal).
The Result of Addition is only in AL register. AH contains 07 which is garbage value, Hence we clear the value by assigning zero to it, Before using AAA which coverts B in AL to 01 in AH and 01 in AL MAKING it one one (11) eleven in BCD form.
Next Line – MOV AH,4CH
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.
Next Line – 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.
Last Line – 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.
Note :- In this Assembly Language Programming, We have Com format and EXE format. We are Learning in EXE format only which simple then COM format to understand and Write. We can write the program in lower or upper case, But i prepare Upper Case.
Screen Shots :-
Before Execution :-
After Execution :-
Note :- To see the variable and its value you have to click vars button in the emulator.