Q.Write a program to add two 8-bit number, and store it in R5. The numbers are 0F1H and 0DH

In the 8051, addition must go through the accumulator A; load the first number, add the second, then copy the result to R5. Adding $0F1H\;(241) + 0DH\;(13)$ gives $0FEH\;(254)$. …

[!TLDR]

Load $0F1H$ into A, add $0DH$, copy A to R5. $241 + 13 = 254 = 0FEH$, so R5 holds $0FEH$.

This Karnataka 2nd PUC Electronics microcontroller example uses the 8051 $ADD A, source$ instruction, whose form is $A = A + source$. The destination of an ADD is always the accumulator A, so both operands are brought through A before the result is moved to the required register R5.

8051 program:

MOV A, #0F1H   ; load F1H (241) into the accumulator
ADD A, #0DH    ; add 0DH (13); now A = A + 0DH
MOV R5, A      ; store the sum in register R5
NOP
END

Working out the addition:

  1111 0001    (0F1H = 241)
+ 0000 1101    (0DH  =  13)
-----------
  1111 1110    (0FEH = 254)
``` …