Dear mike
Erik and Craig ways are correct
And this is a new one
see

;20h,21h are reserved for reversing data

mov 20h,a ;a is the first value
mov c,00h
mov 0fh,c
mov c,01h
mov 0dh,c
mov c,02h
mov 0eh,c
mov c,03h
mov 0ch,c
mov c,04h
mov 0bh,c
mov c,05h
mov 0ah,c
mov c,06h
mov 09h,c
mov c,07h
mov 08h,c
mov a,21h ;a is the reversed data
...
...
...

Best
Mehdi