8086 program to Subtract 8 bit BCD numbers

03:20


ØØ       Explanation :
·           Consider that a byte of data is present in the AL register and a second byte of data is present in the BL register. We have to subtract byte in BL with byte in AL. Using SUB instruction, we will subtract the contents. The result is stored in the AL register. Using DAS instruction we will make sure that the result is valid BCD.

eg. :
AL = 32 BCD


0
0
1
1

0
0
1
0


BL = 17 BCD

­–














0
0
0
1

0
1
1
1





0
0
0
1

1
0
1
1
® invalid BCD, so subtract 6.



–











DAS






0
1
1
0





0
0
0
1

0
1
0
1
= 15 BCD
·            Invalid BCD, means if digit is greater than 9. If digit is invalid we subtract 6 to make it valid. DAS instruction (decimal adjust after subtraction), automatically adjusts to valid BCD result.
ØØ       Algorithm :
Step I       :    Initialize the data memory.           
Step II     :    Get the first BCD number in AL.
Step III   :    Get the second BCD number in BL.
Step IV    :    Subtract the two BCD numbers.
Step V     :    Adjust result to valid BCD number.
Step VI    :    Display the result.
Step VII  :    Stop.
ØØ       Flowchart : Refer flowchart 14.

ØØ       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



ØØ       Result :
C:\programs>tasm sub-bcd8.asm
Turbo Assembler  Version 3.0  Copyright (c) 1988, 1991 Borland International
Assembling file:   sub-bcd8.asm
Error messages:    None
Warning messages:  None
Passes:            1
Remaining memory:  438k
C:\programs>tlink sub-bcd8
Turbo Link  Version 3.0 Copyright (c) 1987, 1990 Borland International
Warning: No stack
C:\programs>sub-bcd8
15
C:\programs>