🔧 What is PROM?
PROM stands for Programmable Read-Only Memory.
It is a type of non-volatile memory that is initially blank and can be programmed only once by the user or manufacturer. Once written, it cannot be erased or modified.
🧠 Core Characteristics:
| Feature | Description |
|---|---|
| 🧪 Programmable | Yes, but only once |
| 🔄 Rewritable | ❌ No (write-once) |
| ⚡ Volatile | ❌ No (data is retained without power) |
| 🔧 Use Case | Permanent firmware storage that doesn’t need updates |
| 🔥 Programming Method | Done using a special device called a PROM programmer or burner |
🏭 How PROM Works (Simplified):
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PROM has fuses inside it.
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When you “burn” the PROM, a high voltage permanently blows specific fuses.
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Blown fuse = 0, intact fuse = 1 (or vice versa depending on design)
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This defines the bit pattern of the program/data.
🔥 Once a fuse is blown, it can’t be restored—the data is permanent.
📦 Real-World Use Cases:
| Device | PROM Use |
|---|---|
| 🔌 Early TVs, calculators | Storing fixed logic or settings |
| 🚗 Automotive systems | Permanent firmware |
| 🧪 Embedded devices | When cost is key and updates are unnecessary |
| 🛠️ Industrial machines | Reliable, tamper-proof logic |
🧠 Why PROM Exists:
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Cheaper than mask ROM for small production runs.
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Easier than hard-coding data at manufacturing.
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Reliable for firmware that will never need updates.
❌ Limitations:
| Limitation | Impact |
|---|---|
| 🧯 One-time programmable | Mistakes = scrap |
| ❌ Not updateable | No support for versioning or firmware updates |
| ⚠️ Needs external burner | Extra hardware required to program it |
🧠 Interview-Ready Definition:
PROM (Programmable Read-Only Memory) is a type of ROM that is initially blank and can be permanently programmed once using a PROM burner. It’s non-volatile and used to store fixed programs or data that won’t need updating, such as device firmware or configuration tables.