PROGRAMMABLE ROM (PROM):

The user can program a PROM after purchase.  Each memory bit element in the PROM contains a nichrome or silicon link that acts as a fuse.  The user can selectively 'but out' or 'blow' these fuses by applying pulses of current to the appropriate pins of the IC.  A memory element with a non-ruptured fuse stores a 1 and a ruptured fuse stores a 0.  The programming is irreversible, so it must be right first time.  Figure 11 shows the circuit for a PROM.

PROM Circuit

PROMs are capable of high operating speeds, but consume a relatively large amount of power.  However, since they are non-volatile, they can be switched off when not being accessed.