1.3 · Memory Types

Goal: distinguish RAM, ROM, cache and storage by speed, volatility and use case.

Memory pyramid

            ▲   small, fast, expensive
            │
   ┌────────┴──────────┐  registers       (inside CPU,   <1 ns)
   ├───────────────────┤  cache (L1/L2/L3) (CPU die,     1-30 ns)
   ├───────────────────┤  main memory (RAM)               (~80 ns)
   ├───────────────────┤  SSD                            (~50 μs)
   ├───────────────────┤  HDD                            (~5 ms)
   ├───────────────────┤  Tape, optical archive          (seconds)
            │
            ▼   large, slow, cheap

The CPU works best when the data it needs is high in the pyramid.

RAM — Random Access Memory

Property	Value
Volatility	Volatile (loses data when power off)
Speed	Fast (~80 ns)
Use	Holds running programs and active data
Examples	DDR4 / DDR5 modules in a PC, LPDDR in phones

When you double-click an app, the OS copies its code from storage into RAM so the CPU can fetch instructions quickly.

ROM — Read Only Memory

Property	Value
Volatility	Non-volatile (retains data without power)
Speed	Comparable to RAM for reads
Writable	Limited / not at all (in classic ROM)
Use	Firmware (BIOS/UEFI, embedded device boot code)

ROM stores instructions that must survive a power cut — the very first code the CPU runs on power-up.

Modern variants:

• PROM (Programmable ROM) — written once.
• EPROM / EEPROM / Flash — erasable and rewritable, but with limited cycles.

Cache

Property	Value
Volatility	Volatile
Speed	Fastest after registers (1–30 ns)
Size	Small (KB–MB on modern CPUs)
Use	Holds frequently-used data near the CPU

The CPU first checks the cache; only on a cache miss does it go to RAM. Good cache hit rates dramatically improve performance.

Levels:

Level	Speed	Size	Notes
L1	~1 ns	tens of KB per core	split into instruction + data
L2	~3 ns	hundreds of KB	per core or shared
L3	~10 ns	a few MB	shared across cores

Relationship to performance

• Bigger memory → can hold more open tabs / open files.
• More memory addresses the CPU can support → larger usable memory (32-bit OS caps at ~4 GB; 64-bit OS supports terabytes).
• Wider word length → more bits transferred per cycle.

Capacity units — the famous "1 KB" question

Unit (binary)	Bytes
1 byte	8 bits
1 KB	1,024 bytes
1 MB	1,024 KB
1 GB	1,024 MB
1 TB	1,024 GB

The SI (decimal) version uses 1,000 instead of 1,024. Hard-disk vendors advertise SI-style ("1 TB" = 10¹² bytes), so Windows shows less than the box says.

Worked example · Why my PC slows down

• 8 GB RAM, you open 25 Chrome tabs + a game + Photoshop.
• Combined usage exceeds RAM. The OS uses paging to swap chunks to the SSD ("virtual memory").
• SSD access is ~1000× slower than RAM.
• Apps stutter as the CPU waits for paging.

The cure: more RAM or fewer open apps.

Common student mistakes

• Saying ROM is volatile (it's not).
• Saying cache is the same as RAM (it's smaller, faster, on the CPU die).
• Mixing firmware (software stored in ROM) with operating system (loaded from disk into RAM).
• Claiming SSD is a kind of RAM (SSDs are non-volatile mass storage; only RAM holds running data).

Exam-style question

Q (4 marks): Distinguish RAM and ROM in terms of volatility, typical use, and one example of data each holds. Then explain how cache memory improves CPU performance.

Sample answer:

• RAM: volatile, used as the working area for running programs (e.g. open documents, active variables).
• ROM: non-volatile, used to store firmware that must survive power loss (e.g. the BIOS/UEFI that boots the computer).
• Cache: a small, very fast memory between the CPU and RAM. By keeping recently or frequently used data and instructions close to the CPU, the cache avoids slow RAM access and reduces the average memory latency, allowing the CPU to execute more instructions per second.

Key takeaways

• RAM = volatile workspace.
• ROM = non-volatile firmware store.
• Cache = tiny, fastest memory on the CPU.
• Capacity prefixes use 1024 in the IT sense; decimal SI uses 1000.

➡️ Next: 1.4 Storage Devices