Changing ASCII case using bitwise operator

I am writing this post to share the method with my friends.

I am doing Bachelor in Computer Science and Engineering. My microprocessor course teacher gave us a task to change case from upper or lower to uppercase.

It is kind of "DO it yourself" task.

I am using emu 8086 for this task.

Here are two test cases that we have must pass:

Teas Case 1:
Input : abcd
Output : ABCD

Teas Case 2:
Input : aBCd
Output : ABCD

NOTE: Input will be always 4 character and between a-z or A-Z.

We can solve it using conditional instruction.
To make the problem more interesting, I've challenged myself to solve it without branching.

Gladly, I've found a way to solve it using bitwise operator.

We can see that the 6th bit is zero for uppercase character and one for lowercase character.

If we can set the bit to 0 no matter what is the input, the character will become Uppercase.

We can set it to zero and left rest of bits unchanged by using bitwise AND operation.

a -> 0110 0001 1101 1111 ---------------- AND -> 0100 0001 

I am going to use 11011111 (binary) or DF (hex) to set the 6th bit to zero and leave other bits unchanged.

PROC MAIN MOV AH, 01H ; input mode INT 21H ; take 1st character in AL AND AL, 0DFH ; perform AND operation MOV BL, AL ; Move result to another Register INT 21H ; same as 1st character AND AL, 0DFH MOV BH, AL INT 21H ; same as 1st character AND AL, 0DFH MOV CL, AL INT 21H ; same as 1st character AND AL, 0DFH MOV CH, AL MOV AH, 02H MOV DL, 0AH ; print newline INT 21H MOV DL, 0DH ; print cret INT 21H MOV DL, BL ; print 1st character INT 21H MOV DL, BH ; print 2nd character INT 21H MOV DL, CL ; print 3rd character INT 21H MOV DL, CH ; print 4th character INT 21H ENDP 

Case 1 :

Case 2 :

Happy Coding !!!