CPU, Memory and Storage
The CPU (Central Processing Unit)
The CPU is the brain of the computer. It has three parts.
| Part | Job |
|---|---|
| ALU (Arithmetic Logic Unit) | Does arithmetic (+, -, ×, ÷) and logical comparisons (AND, OR, NOT, greater than). |
| CU (Control Unit) | Directs and coordinates all parts. It fetches instructions, decodes them and sends signals. It does no calculation. |
| Registers | Tiny, very fast memory inside the CPU. |
Important registers: Program Counter (PC) holds the address of the next instruction; Instruction Register (IR) holds the current instruction; Memory Address Register (MAR) holds the address being accessed; Memory Data Register (MDR/MBR) holds the data read or written; Accumulator holds results of ALU operations.
Machine cycle: Fetch → Decode → Execute → Store. Multi-core CPUs (dual, quad, octa) have several processing cores that work in parallel.
Buses
| Bus | Carries | Direction |
|---|---|---|
| Data bus | Data | Bidirectional |
| Address bus | Memory addresses | Unidirectional (CPU to memory) |
| Control bus | Control signals (read, write) | Both |
Memory hierarchy
From fastest and smallest to slowest and largest:
Registers → Cache → RAM (main memory) → Secondary storage (SSD, HDD) → Tertiary and offline storage (tape, optical). Speed and cost per byte fall as you go down; capacity rises.
Primary memory
| Memory | Facts |
|---|---|
| RAM | Random Access Memory. Volatile (data lost when power is off). Holds running programs and data. Read and write. |
| SRAM | Static RAM. Uses flip-flops. Faster, costlier, no refresh. Used for cache. |
| DRAM | Dynamic RAM. Uses capacitors. Needs constant refreshing. Slower, cheaper. Used as main memory. |
| SDRAM, DDR | Synchronous DRAM. DDR4 and DDR5 are current standards. |
| Cache | Small, very fast memory between the CPU and RAM. Levels L1 (fastest, inside the core), L2, L3. |
| ROM | Read Only Memory. Non-volatile. Stores the start-up instructions (BIOS/UEFI). |
| PROM | Programmable once by the user. |
| EPROM | Erased by ultraviolet light. |
| EEPROM | Erased electrically. Flash memory is an advanced form. |
| CMOS | Small battery-backed memory that keeps BIOS settings, date and time. |
| Virtual memory | Part of the hard disk used as extra RAM (page file or swap space). |
Secondary storage
| Device | Facts |
|---|---|
| Hard Disk (HDD) | Magnetic platters with tracks, sectors, cylinders. Speed in RPM (5,400 or 7,200). |
| SSD | Solid State Drive. Flash (NAND) memory, no moving parts, faster, quieter. |
| CD-ROM | About 700 MB |
| DVD | About 4.7 GB (single layer, single side) |
| Blu-ray | About 25 GB per layer, read with a blue-violet laser |
| Magnetic tape | Sequential access. Used for backups and archives. |
| Pen drive, SD card | Portable flash storage. |
Access: RAM and disks allow random (direct) access. Magnetic tape allows sequential access only.
Keep practising
Notes are only half the work. Test yourself with questions and discuss doubts with other aspirants.
Prepared for exam revision from standard computer textbooks, Microsoft documentation and official sources. Features differ between Excel and Word versions (2016, 2019, 2021, 365) so check which version your exam follows. Found a mistake? Tell us.