Reply 40 of 42, by rpocc
Couple of notes about burning ROMs, chips, etc
1. In 74LS138P, the P just indicates the plastic DIP package.
2. Cloning many copies in a large chip is required only in one situation: when you have to install a chip of superior capacity with backward-compatible pin-out into a socket with the same number of pins, e.g. 27C512 into a socket dedicated for 27C64/128/256, because it's hard to obtain 2764 and lower capacity chips. When you insert such a chip, some of its address pins often become floating because there is no physical connection either to ground or Vcc. so, any fluctiation in electric fields around may switch most significant bits of address to different states, so copying images guarantees that with any combination of MS bits the right data will come out. So, if you're putting 27512 chip in a socket marked "27512", each address pin will be controlled by the circuit and there will be no floating pins and there's no difference if ROM is copied or not, anyway larger addresses will never be read unless he code will set instruction pointer specifically to higher areas.