SUBWF MyFile, W ;subtract W from the file
BTFSS STATUS, C ;Test for borrow (NOT carry)
GOTO WBigger ;W was bigger then file
;W was smaller or equal to file
This sample subtracts W from the content of MyFile. If W is bigger then MyFile then there will be an underflow of MyFile and the borrow flag will be set. On a pic borrow is opposite of carry (so carry is clear on borrow and set on no borrow).
We test for the flag and jump accordingly