Alternatively referred to as a FPROM (Field Programmable Read Only Memory) or OTP (one-time programmable) chip, PROM or Programmable ROM (Programmable Read Only Memory) is a computer memory chip that can be programmed once after it has been created. Once the PROM has been programmed, the information written is permanent and cannot be erased or deleted. PROM was first developed by Wen Tsing Chow in 1956 and a good example of a PROM is a computer BIOS in early computers. Today, PROM in computers has been replaced by EEPROM.
When the PROM is created it ships with all bits reading as "1" and during the programming any bit that needs to be changed to a 0 is etched or burned into the chip using a device known as a gang programmer. Below is an example of a gang programmer from Advin that programs multiple ROM chips at one time.
If a PROM has been programmed and has an error or needs to be updated the chip is discarded, and a new PROM is created and replaces the old chip. A variation of the PROM is an EPROM, which is a PROM that can be erased and reprogrammed without being replaced.