ROMs - Read only memory used to stored data that does not change and
  needs to be there when system first comes up.
  
  Most ROMs organized as collections of bytes or words (single chip).

  ROM-BIOS (basic I/O system) - sets the basic behavior of a system.
  Other ROMs - on I/O devices to make it behave in a standard way.


Mask ROM - manufactured with the data embedded on it.
  Produced by photo-lithography.  

  Expensive to set up manufacturing.

  Very inexpensive to produce in large quantities.

  Highest bit density available (many bits in small space).

  Bits extremely stable.

  If error found, 
    Chips lost.
    New masks required.

PROM - Burn-able or one-time "programmable read-only memory" 
  "blank" ROM  
    Programmed by blowing fuses on chip.

  Cheap to manufacture "blank".

  All bits set to 1 (usually) 
    and contain a fuse which if blown changes the bit to 0.

  Slightly less bit density than Mask ROM.

  Chip individually programmed.
    Devices available to program several chips in parallel.

  Changes can be done only once.

  If error found, 
    Only finished chips lost.

  Programming is software controlled so easy to 'fix' problem.

  PLAs use same technology just tasked for different purpose.