RAM is fast but volatile — kill the power and everything vanishes. Yet a computer must remember something even before an OS loads: how to boot, how to talk to hardware. So we need non-volatile storage: memory that retains data with no power.
The ROM family answers one question: "How permanently do we fix the bits, and who gets to change them?" As we move ROM → PROM → EPROM → EEPROM, writing gets progressively more flexible (and usually the chip gets more complex/expensive per bit).
ROM — the diode/transistor grid.
Think of a grid of word-lines (rows) and bit-lines (columns). At each crossing the manufacturer either places a transistor (reads as one logic value) or leaves it out (reads as the other). The presence/absence is etched by a photomask — hence Mask ROM. The pattern is literally part of the chip's geometry, so it can never change.
PROM — the fuse.
Every cell starts connected through a thin fuse (or antifuse). Programming pushes a high current through selected cells to melt the fuse, permanently breaking the connection. A blown fuse = one bit value, an intact fuse = the other. You can only melt each fuse once → OTP.
EPROM & EEPROM — the floating gate.
This is the clever idea behind all modern non-volatile memory.
Writing (both): use a high voltage to inject electrons onto the floating gate (hot-electron injection / Fowler–Nordheim tunneling).
Erasing EPROM: UV photons give trapped electrons enough energy to escape back through the oxide — but light hits the whole chip, so it erases everything. Needs the quartz window.
Erasing EEPROM: a reverse electric field tunnels electrons off the gate — done selectively, per byte, in-circuit.
It retains stored data even with no power supplied.
In ROM, how is data stored?
Permanently masked into the silicon (transistor present/absent) during manufacture — never changeable.
What does the "P" in PROM add over ROM?
Programmable once by the user (by burning fuses), instead of only at the factory.
Why is PROM called OTP?
One-Time Programmable — fuses blow irreversibly, so it can be written only once.
How is EPROM erased?
By exposing the chip to ultraviolet (UV) light through a quartz window (whole-chip erase).
How is EEPROM erased and at what granularity?
Electrically, byte-by-byte, in-circuit — no UV, no chip removal.
What single letter distinguishes EEPROM from EPROM and what does it mean?
The extra 'E' = Electrically (erasable).
What device stores the bit in EPROM/EEPROM?
A floating-gate transistor holding trapped electrons that shift its threshold voltage.
Why does a floating gate keep its charge without power?
It's surrounded by insulating oxide; electrons are trapped and leak only by extremely slow tunneling (~10-year retention).
Difference between EEPROM and Flash?
EEPROM erases per byte; Flash erases in large blocks (denser, cheaper).
Why does EEPROM have finite endurance?
Each write stresses/degrades the oxide, so cells wear out after ~10^4–10^6 cycles.
What is wear leveling and why use it?
Spreading writes across many cells so no single cell hits its endurance limit — extends usable life.
When is Mask ROM the best choice?
High-volume, fixed firmware — lowest cost per bit and immune to accidental rewrite.
Recall Feynman: explain to a 12-year-old
Think of four kinds of notebooks. ROM is a book printed at a factory — you can only read it, never write in it. PROM is a blank book where you can write once with permanent ink you press really hard — but you get only one chance, no eraser. EPROM is written in special ink that fades only under a strong flashlight (UV) — shine the light on the whole book to blank it, then rewrite. EEPROM is a little whiteboard: wipe any word with a fingertip anytime and write a new one, even while it's still on the shelf. All four keep their words when you close them (power off) — that's what "non-volatile" means. The catch: whiteboards wear out if you scrub the same spot thousands of times.
Dekho, ROM family ka pura funda ek hi sawaal pe tika hai: bits ko kitni permanently fix karna hai, aur badalne ki permission kiske paas hai? Ye sab non-volatile memory hai — matlab power off karo tab bhi data safe rehta hai (RAM ke opposite, jo power jaate hi khaali). Isliye computer ka boot code, firmware waghera yahin rakha jaata hai.
Char members: ROM — factory mein hi mask se bits daal diye jaate hain, kabhi change nahi hote, but bulk mein sabse sasta. PROM — blank aata hai, aap ek baar fuse jala ke program karte ho, phir permanent (One-Time). EPROM — floating gate transistor mein electrons trap karke bit store hota hai; erase karne ke liye chip pe UV light maarni padti hai (pura chip ek saath saaf), quartz window ke through. EEPROM — wahi floating gate, par erase electrically, byte-by-byte, circuit mein hi ho jaata hai, na UV chahiye na chip nikalni. Flash bhi EEPROM ka cousin hai, bas woh bade blocks mein erase karta hai (isliye sasta aur dense).
Ek important cheez: EEPROM/Flash ki endurance limited hoti hai (~10^4 se 10^6 writes per cell), kyunki har write se oxide thoda degrade hota hai. Isliye engineers wear leveling karte hain — same byte pe baar-baar likhne ki jagah writes ko kai cells pe ghumate hain, taaki koi ek cell jaldi na marey. Exam mein yaad rakhna: extra E matlab Electrical erase, aur "better" hamesha zyada rewritable ka matlab nahi — high volume fixed firmware ke liye sasta Mask ROM hi best hai.
