Memory Units

Memory units hold the data and instructions a computer works with. This section covers primary memory (RAM and ROM), which the CPU uses directly, and secondary memory, which keeps data permanently.

This section has two sub-lessons. Primary Memory (Main Memory) explains volatile RAM, which holds the programs and data in use, and non-volatile ROM, which holds firmware and the start-up (bootstrap) program. Secondary Memory (Storage) explains how hard disk drives, solid-state drives, optical discs, pen drives and cloud storage keep data even when the power is off, and how they differ in capacity, speed, durability and cost.

Memory Units: What This Section Covers

A computer needs two kinds of memory: fast memory for the instructions and data it is using right now, and permanent memory for files that must survive when the power is switched off. The two sub-lessons cover these two needs in turn.

1. Primary Memory (Main Memory)

• Primary memory is the memory the CPU reads and writes directly while it runs programs.
• RAM (Random Access Memory) is volatile: it holds the programs and data currently in use and loses them when the power goes off. More RAM lets more programs run smoothly at the same time.
• ROM (Read-Only Memory) is non-volatile: it keeps firmware and the bootstrap program that starts the computer, even without power.
• The sub-lesson also names the types of ROM: PROM, which can be programmed once; EPROM, which can be erased with ultraviolet light and reprogrammed; and EEPROM, which can be erased electrically.

2. Secondary Memory (Storage)

• Secondary memory keeps data permanently, so it holds saved files, installed applications and the operating system.
• Its main types are magnetic storage (hard disk drives: large capacity, but moving parts), optical storage (CDs and DVDs: read by a laser, slower), solid-state storage (SSDs and pen drives: no moving parts, faster) and cloud storage (kept on remote servers and reached over the internet).
• Choosing secondary memory means balancing capacity, speed, durability and cost.

How the two work together: when you open a file, it is copied from secondary memory into RAM, and the CPU works on it there; when you save it, the changes are written back to secondary memory. Primary memory is faster but smaller, and RAM is temporary; secondary memory is slower but much larger and permanent. The next section, Storage Devices, looks at secondary storage devices in more detail.