Handling of Hexadecimal numbers for 8051 Assembly

Question

I am having trouble looping through the Hexadecimal numbers in 8051 Assembly for proper subtraction,

Here is the simple code that iterate 30 times and shows value from 40 to 70 (40,41,42,43,....70)

        MOV A,#40H ;first value of the loop
        MOV R0,#0H

        MOV R1,#30 ;Number of iterations


    LOOP:   

        ADD A,R0
        DA A   ;To Adjust the decimal values after each addition
        INC A

    DJNZ R1,LOOP


    END

This is how it is adjusting each value,

; 40H = 64D --(+0D)--> 64D = 40H
; 41H = 65D --(+0D)--> 65D = 41H
; 42H = 66D --(+0D)--> 66D = 42H
; 43H = 67D --(+0D)--> 67D = 43H
; 44H = 68D --(+0D)--> 68D = 44H
; 45H = 69D --(+0D)--> 69D = 45H
; 46H = 70D --(+0D)--> 70D = 46H
; 47H = 71D --(+0D)--> 71D = 47H
; 48H = 72D --(+0D)--> 72D = 48H
; 49H = 73D --(+0D)--> 73D = 49H 
; 4AH = 74D --(+6D)--> 80D = 50H

But when i want to reverse the loop i.e if i want the output in reverse order from 70 to 40 (70,69,68,67,66,65,64,.....40)

what should i have to do ?

Here is the example how it should work if the first number is 90

; 90H = 144D --(+0D)--> 144D = 90H
; 8FH = 143D --(-6D)--> 137D = 89H
; 8EH = 142D --(-6D)--> 136D = 88H
; 8DH = 141D --(-6D)--> 135D = 87H
; 8CH = 140D --(-6D)--> 134D = 86H
; 8BH = 139D --(-6D)--> 133D = 85H
; 8AH = 138D --(-6D)--> 132D = 84H

but since there is nothing for Decimal Adjust after subtraction how could i achieve the desired result ?

The desired output,

90 89 88 87 86 85....

Answer 1

You can't subtract 1, but you can add 99.

ADD A, #99H
DA A

Answer 2

What I understand from your question is that you want to display a 2-digit decimal down count starting with initial value 99d and ending with 00d.

I agree with NovaDenizen. Here is the explanation :

The limitation what you encounter is that the DA A instruction doesn't work after subtraction. Now then you have to make use of alternative logic wherein the subtraction operation is implemented as special form of addition. Immediate implication of this idea is to make use of complement's method. In 10s' complement method adding 99 is equivalent to subtracting -01. Hence this is how your code should be :

ORG 0000H   

MOV  P1,#99H

MOV  A,#99H          ; A=99h

BACK:  ACALL  DELAY  ; Call Delay Routine

       ADD A,#99H    ; Realize count down action by adding 99H 
                       (equivalent to subtracting 01)

       DA  A         ; Decimal Adjust to realize the BCD format representation

       MOV P1,A      ; Display the count on P1

       JNZ BACK      ; Continue till the count ends with 00

       SJMP  $       ; termination of the program


DELAY: MOV R1,#0FFH              ; Delay Routine to generate certain delay.

 DEC1: MOV R2,#0FFH 

 DEC2: MOV R3,#0FFH 

       DJNZ  R3,$

       DJNZ  R2,DEC2   

       DJNZ  R1,DEC1    

       RET  

END

( Run the program in Keil and check for output on Port 1. )

Hope this helps you. Regards