Write ability and storage permanence

Write ability

Write ability is the manner and speed with which data can be written into a memory. Its range (best to worst):

• Highest: the processor can write during normal execution, in the same way and at the same speed as it reads. RAM (SRAM, DRAM).
• Middle: the processor can write, but slowly or with a limited number of cycles/block erase: Flash (block erase, then write) and EEPROM (electrically erasable byte by byte, slower than reads).
• Lower: writing needs a special programmer device outside the system: EPROM (UV erasable), OTP ROM / PROM.
• Lowest: written only during fabrication, using a mask, by the manufacturer: mask-programmed ROM.

Storage permanence

Storage permanence is the ability of a memory to hold its stored bits after they are written: for how long, and under what conditions. Its range:

• Essentially permanent: the bits stay for the life of the chip, even with power off: mask ROM, OTP ROM (blown fuses cannot be restored).
• Non-volatile, but changeable: data is retained for years after power off (about 10 years): EPROM, EEPROM, Flash.
• Battery-backed: SRAM kept alive by a battery (NVRAM).
• Volatile, static: SRAM keeps data as long as power is on.
• Volatile, needs refresh: DRAM loses data within milliseconds unless it is refreshed periodically, even with power on.