Subtract 8 bit BCD numbers

Introduction 

Binary Coded Decimal Number system uses binary numbers to represent a decimal number.

Here Each decimal digit is represented by either 4-digit or 8-digit bits.

BCD is extensively used to display the numeric values in all electronic systems containing only digital logic without microprocessors.

BCD is more powerful than the binary representations. It helps in rounding off decimal values more effectively into human readable formats. 

The Disadvantages of BDC are some operations will be more complex to implement. Also BCD consumes around 20% more space compared to binary representation.

Below is the BCD representation chart for decimal digits from 0-15

Algorithm to subtract 8-bit BCD numbers

Subtract 8 Bit BCD Numbers Program

 
.model small  
 .data                                                    
 a db 32H                              
 b db 17H  
 .code  
     mov  ax, @data      ; Initialize data section  
     mov  ds, ax                                        
     mov  al, a           ; Load number1 in al  
     mov  bl, b           ; Load number2 in bl  
     sub   al, bl          ; subtract numbers and result in al  
     das                 ; adjust result to valid BCD number  
     mov  ch, 02h        ; Count of digits to be displayed  
     mov  cl, 04h         ; Count to roll by 4 bits  
     mov  bh, al          ; Result in reg bh  
 l2:  rol    bh, cl          ; roll bl so that msb comes to lsb  
     mov  dl, bh          ; load dl with data to be displayed  
     and   dl, 0fH         ; get only lsb  
     cmp  dl, 09          ; check if digit is 0-9 or letter A-F  
     jbe   l4  
     add   dl, 07          ; if letter add 37H else only add 30H  
 l4:  add   dl, 30H  
     mov  ah, 02              ; Function 2 under INT 21H (Display character)  
     int    21H  
     dec   ch            ; Decrement Count  
     jnz   l2  
     mov  ah, 4cH        ; Terminate Program  
     int    21H  
     end