8051 code to use R4(LSB) and R5(MSB) as a single 16bit counter, and increment the pair untill number is FFFFh

Logic is too simple. For better understanding i would suggest you somthing.

Use a scientific calci and in hex mode go on adding 0x01 each time. And then study my code. You will have a better understanding.

ORG 000h

MOV R4,#00h

MOV R5,#00h

main:

         INC R4

         CJNE R4,#0FFh,main

         CJNE R5,#0FFh,end  ; Observe this line carefully and think after it 

         INC R5

         MOV R4,#00h

end: NOP

END

Enjoy coding.....XD