not a very informative post...

you mean ascii codes? http://www.asciitable.com

where do you get the string. from UART? remember to have an end of string "0 hex"

i didn't understand the hole question... what i understood is you recieve a string and want to save it in RAM.
You only need to use MOV instructions.
You take each character at a time and then save it in RAM where there is a pointer is pointing at, then increment the pointer and do this until the end of string

if you are not familiar with 8051, read the tutorial and the *bible* as Erik Malund always says