Looking inside a 1970s PROM chip that stores data in microscopic fuses

Ken Shirriff did teardown of a 1970s PROM chip:

The MMI 5300 was a memory chip from the early 1970s, storing 1024 bits in tiny fuses.1 Unlike regular RAM chips, this was a PROM (Programmable Read-Only Memory); you programmed it once by blowing fuses and then it held that data permanently.

See the full post on his blog.