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 Today, PROM in computers has been replaced by EEPROM.
Tip: A programmable ROM is also referred to as a FPROM (field programmable read-only memory) or OTP (one-time programmable) chip.
When the PROM is created, all bits read as "1" and during the programming, any bit needing to be changed to a "0" is etched or burned into the chip using 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, replacing the old chip. A variation of the PROM is an EPROM, which is a PROM that can be erased and reprogrammed without being replaced.