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