peek into memory

is their a way to see what is stored in ROM memory locations?
if you think of a computers memory like a sub division , each house has a address and something is
stored in that address and is is different in each address it could be anything from 0 to 256 of
it could be TRUE or FALSE.
This is handy lets say you have an old usb joystick if you know the correct memory address you
could see when the button was pressed by monitoring when the value changed or with
magnets as switches with restorers you could build an alarm system computer controlled
This is something i would like to build and program.

Please see if this is of any help.