Hi Ben,
here is my first idea:
decode the value you get if a key is pressed to the corresponding
number of the key (from 0 to 79).
the organization ofthe lookup table is as following:
five blocks of 80 bytes(first block is first byte of each key and so on...).
to fetch the five bytes you just have move the decoded number of the
key to ACC and save a copy to stack or one reg, setup DPTR with the base address of the first block and fetch the first byte with the movc instruction. repeat this for the next byte but setup DPTR with the base address of the second block.
if you don't know how to decode your value to the number, tell me the
output values of the keypad when a special key is pressed(e.g. row 4
line 6).
Christian