Memory and Storage

The two primary-storage chips you must know are RAM and ROM.

RAM (Random Access Memory)

RAM is primary storage that holds whatever data and instructions a running program needs right now. The CPU reads and writes RAM directly during the fetch-decode-execute cycle.

Key properties:

• Volatile. Everything stored in RAM disappears the moment power is removed.
• Read/write. The CPU can both read from and write to any RAM location.
• Fast. Much faster than any secondary storage, though slower than cache.
• Large compared to ROM. A modern PC has 8 GB to 64 GB of RAM.

When you launch a program, its instructions are copied from secondary storage into RAM so the CPU can run them. When you save a file, the data is copied from RAM out to secondary storage so it survives the power being switched off.

ROM (Read-Only Memory)

ROM is primary storage that holds the very first instructions the computer needs at start-up, including the bootstrap loader and the BIOS.

Key properties:

• Non-volatile. Contents survive the power being turned off, which is essential because the computer needs ROM's contents in order to boot.
• Read-only. The contents are written once at the factory and (in basic ROM) cannot be changed by the running computer.
• Fast (faster than secondary storage but slower than RAM).
• Small. Only a few hundred kilobytes; just enough to hold the start-up code.

ROM contains the bootstrap: the small program that runs the moment the computer is switched on. Its job is to test the hardware, find the operating system on secondary storage, and load enough of it into RAM for the system to take over. After that point, the running computer mostly uses RAM rather than ROM.

Side-by-side