Internal and External Memory

Computer Organization & Embedded System — Computer Arithmetic and Memory System, NEC licence examination syllabus (Nepal Engineering Council).

Internal and External Memory: the technology behind each layer

Not all RAM is the same, and not all ROM is permanent.

When your phone's storage says "256GB Flash" and your laptop's spec sheet brags about "16GB RAM," those two numbers are describing completely different technologies, built for completely different jobs. Here's why.

⚡ SRAM vs DRAM

SRAMStores each bit in a flip-flop-like circuit. Fast, no need to refresh, but uses more transistors per bit (expensive). Used for CPU cache.

DRAMStores each bit as a charge on a tiny capacitor. Cheaper and denser, but the charge leaks, so it must be refreshed thousands of times per second. Used for your computer's main RAM.

📀 The ROM Family

ROMProgrammed at manufacture, can never change again.

PROMProgrammed once by the user (with a special device), but not reprogrammable afterward.

EPROMCan be erased with UV light and reprogrammed multiple times.

EEPROMErased and rewritten electrically, byte by byte — no UV light needed.

FlashLike EEPROM but erases/writes in larger blocks, making it faster for bulk storage. This is what's actually inside your phone's storage and every USB drive.

💽 External Storage & RAID

Magnetic disksStore data on spinning platters; access time = seek time + rotational latency + transfer time.

RAID 0Striping — data split across disks for speed. No redundancy: one disk dies, all data is lost.

RAID 1Mirroring — the exact same data written to two disks. Full redundancy, but you pay double the storage cost.

RAID 5Striping + parity — balances speed and fault tolerance, the most common choice for servers that need both performance and safety.

💡 Know the RAID levels 0, 1, and 5 well — a comparison question ("which RAID level for which use case") is common.

Syllabus points

Internal: SRAM vs DRAM; ROM family (ROM/PROM/EPROM/EEPROM/Flash)
External: magnetic disk (+ access time), RAID levels, optical, SSD, tape

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