ROM Cell Features (2 Marks)

ROM (Read Only Memory) cell stores a fixed bit permanently:

1. Non-volatile: Retains contents without power — used for bootstrap, firmware, microprogram.

2. Mask-programmed: Contents written at fabrication via contact mask / transistor presence (1 = transistor present, 0 = absent). Not field-writable.

3. Non-destructive read: Reading does not erase cell; access via address decoder → word line → bit line sense.

4. High density, low cost/bit for large volumes; slower write-once but fast read comparable to RAM.

5. Variants: PROM (fusible link, OTP), EPROM (floating gate, UV erase), EEPROM/Flash (electrically erasable) retain ROM cell principle but allow reprogramming.

Single ROM cell = 1 transistor + contact/via; array is OR plane of word lines.