Memory and storage

Computers use a hierarchy of memory technologies, each with different cost, speed and persistence trade-offs. AQA expects you to distinguish RAM from ROM, volatile from non-volatile, and the major secondary-storage technologies.

Primary memory

RAM (Random Access Memory)

• Volatile — contents lost when power is removed.
• Read/write — programs and data can be loaded and modified.
• Fast but more expensive per byte than secondary storage.
• Holds the running OS, currently-open apps and their data.
• Typical size: 8-32 GB on modern PCs.

ROM (Read Only Memory)

• Non-volatile — contents survive power loss.
• Pre-programmed at manufacture; usually cannot be changed (read-only) or only with special procedures.
• Used for firmware and BIOS — the boot-up code that starts the computer and loads the OS into RAM.
• Typical size: a few MB.

Volatile vs non-volatile (key term)

• Volatile — needs power to retain data. RAM, cache, registers.
• Non-volatile — retains data without power. ROM, hard drives, SSDs, flash memory.

Cache

A small, very fast volatile memory between the CPU and RAM (covered in CS4.7). L1/L2/L3 caches reduce average memory access time.

Virtual memory

When RAM fills up, the OS uses a portion of secondary storage (the page file or swap file) as if it were extra RAM. Pages of memory not currently being used are swapped out to disk; brought back when needed.

• Allows more processes to run than physical RAM allows.
• Slow because disk is much slower than RAM — system feels sluggish ("thrashing" if too much swapping).
• Adding more RAM is a common upgrade to reduce reliance on virtual memory.

Secondary storage

Larger, cheaper, non-volatile storage. Three main technologies:

Magnetic (HDD — Hard Disk Drive)

• Spinning platters coated in magnetic material; read/write heads on a moving arm.
• Capacity: cheap, very large (1-20 TB).
• Speed: moderate (~100 MB/s read).
• Reliability: mechanical parts can fail.
• Use: bulk storage, backups, archives.

Solid-state (SSD)

• No moving parts; stores data in NAND flash chips.
• Capacity: 256 GB-8 TB on consumer PCs.
• Speed: fast (~500 MB/s SATA, 3-7 GB/s NVMe).
• Reliability: limited write cycles, but no mechanical failure.
• Power: low, silent, light.
• Use: OS drive, laptops, gaming, anywhere speed matters.

Optical (CD, DVD, Blu-ray)

• Pits and lands on a reflective disc, read by a laser.
• Capacity: 700 MB (CD) to 100 GB+ (Blu-ray).
• Speed: slow.
• Use: distribution media (films, software installs), archive (rare today).

Memory hierarchy

From fastest+smallest+most expensive to slowest+largest+cheapest:

Registers          (~1 ns;     <1 KB)
L1 cache           (~1 ns;     ~64 KB)
L2 cache           (~3 ns;     ~256 KB)
L3 cache           (~10 ns;    ~10 MB)
RAM                (~80 ns;    8-32 GB)
SSD                (~50 µs;    256 GB-2 TB)
HDD                (~5 ms;     1-20 TB)
Optical / Tape     (~100 ms+;  arbitrary)

The cost per byte drops by orders of magnitude as you move down the hierarchy.

Choosing storage

• Speed-critical (OS, games): SSD.
• Massive bulk data (backups, video archives): HDD.
• Distribution / archival: optical.
• Tiny embedded: flash memory chips on-board.