Print the HEX ASCII equivalent of a Alphanumeric character on the screen using Assembly language

Question

So my doubt is why are we using ADD AL,07H if AL contains something greater than 10? What's the explanation for ADD AL,07. Here is the code.

MOV AH,01H  ;TAKE INPUT 
INT 21H

MOV BL,AL   ; SAVE VALUE OF AL, SO THAT IT CAN BE USED LATER
MOV CL,04H  
SHR AL,CL   ;SHIFT AL TOWARDS RIGHT BY 4 BITS

CMP AL,0AH  ;COMPARE IF AL HAS 10
JB DIGIT
ADD AL,07H
DIGIT: 
    ADD AL,30H  ;Add 30 to make HEX equivalent ASCII code
    MOV RES,AL

    AND BL,0FH
    CMP BL,0AH
    JB DIGIT1
    ADD BL,7H

DIGIT1: 
    ADD BL,30H
    MOV RES+1,BL

LEA DX,RES    ; display the result
    MOV AH,9
    INT 21H

Thank you.

Remove it and you will see why. Have an ASCII chart near by for reference. — Jongware, May 25 '15 at 11:37

score 2 · Accepted Answer · answered May 31 '15 at 14:30

Your program does (2 times in a row) the conversion of a 4-bit value into a displayable character. Results will obey the following table:

 0 -> "0"   "0" has ASCII 48 =  0 + 48
 1 -> "1"
 2 -> "2"
 3 -> "3"
 4 -> "4"
 5 -> "5"
 6 -> "6"
 7 -> "7"
 8 -> "8"
 9 -> "9"   "9" has ASCII 57 =  9 + 48
10 -> "A"   "A" has ASCII 65 = 10 + 48 + 7
11 -> "B"
12 -> "C"
13 -> "D"
14 -> "E"
15 -> "F"   "F" has ASCII 70 = 15 + 48 + 7

From this table you can see that the ASCII's don't nicely follow each other when changing from 9 to 10. To compensate for this 7 character gap (it holds the characters :;<=>?@) you used the instructions add al,7 and add bl,7

Print the HEX ASCII equivalent of a Alphanumeric character on the screen using Assembly language

1 Answers1