CPU — Central Processing Unit; Memory Hierarchy — Computer Fundamentals

CPU — Central Processing Unit

What is CPU?

The CPU (Central Processing Unit) is the "brain" of the computer — it processes all instructions and data.

Also called: Processor, Microprocessor

Components of CPU

+-------------------------------------------+
|                   CPU                     |
|   +------------------+  +---------------+ |
|   |       ALU        |  |   CU          | |
|   | Arithmetic Logic |  | (Control Unit)| |
|   | Unit             |  |               | |
|   |  + - × ÷ AND OR  |  | Fetch-Decode  | |
|   |  NOT XOR CMP     |  | -Execute      | |
|   +------------------+  +---------------+ |
|   +---------------------------------------+ |
|   |         Registers                    | |
|   | PC, IR, MAR, MDR, Accumulator, Flags | |
|   +---------------------------------------+ |
+-------------------------------------------+

Component	Full Form	Function
ALU	Arithmetic Logic Unit	Performs arithmetic (+,-,×,÷) and logical (AND OR, NOT, XOR, compare) operations
CU	Control Unit	Directs flow of data; fetches, decodes, executes instructions
Registers	—	Fast, small storage within CPU for temporary values

CPU Registers

Register	Purpose
PC (Program Counter)	Holds address of next instruction to execute
IR (Instruction Register)	Holds currently executing instruction
MAR (Memory Address Register)	Holds address for memory access
MDR (Memory Data Register)	Holds data read from/written to memory
Accumulator	Holds results of ALU operations
Flags/Status Register	Holds condition codes (zero, overflow, carry, etc.)

CPU Fetch-Decode-Execute Cycle

1. FETCH:   PC → MAR → Memory → MDR → IR  (load instruction)
2. DECODE:  CU decodes instruction in IR
3. EXECUTE: ALU performs operation
4. WRITEBACK: Store result → register/memory
5. INCREMENT: PC++ (point to next instruction)
Repeat forever

CPU Performance Metrics

Metric	Description	Formula
Clock Speed	How many cycles per second	Hz (GHz for modern CPUs)
CPI	Clock cycles per instruction	CPI = Cycles / Instructions
IPC	Instructions per clock	Higher = better
CPU Time	Time for a program	Instructions × CPI / Clock Speed
MIPS	Million instructions per second	Clock Speed / (CPI × 10^6)

CPU Cache

Cache is small, fast memory inside/near CPU that stores frequently used data.

Level	Size	Speed	Location
L1 Cache	32-128 KB	Very fast (1-4 cycles)	Inside CPU core
L2 Cache	256 KB - 4 MB	Fast (4-12 cycles)	Inside CPU package
L3 Cache	4-64 MB	Moderate (30-50 cycles)	Shared between cores
RAM	4-128 GB	Slow (100-300 cycles)	On motherboard

Memory Hierarchy

Memory Hierarchy Pyramid

         /\
        /  \  CPU Registers (fastest, smallest)
       /    \
      /      \  L1 Cache
     /--------\
    /          \  L2 Cache
   /------------\
  /              \  L3 Cache
 /----------------\
/                  \  RAM (Main Memory)  ← Primary Storage
/--------------------\
        SSD/HDD         ← Secondary Storage
        NAS/SAN         ← Tertiary Storage
        Tape/Cloud      ← Off-line Storage

As you go down the pyramid:

Capacity increases (Bytes → GB → TB → PB)
Speed decreases (nanoseconds → milliseconds → seconds)
Cost per byte decreases

Memory Types

Memory	Volatile?	Speed	Description
CPU Registers	Yes	Fastest	Inside CPU; accessed in <1 ns
Cache (L1/L2/L3)	Yes	Very Fast	SRAM; stores frequently used data
RAM	Yes	Fast	Main memory; program execution
SSD	No	Medium	Solid-state disk; fast permanent storage
HDD	No	Slow	Hard disk; spinning magnetic disk
Flash Drive	No	Medium	USB storage; portable
CD/DVD	No	Slow	Optical storage; read-only or writable

Primary vs Secondary Storage

Feature	Primary (RAM)	Secondary (HDD/SSD)
Speed	Very fast	Slower
Volatile	Yes (lost on power off)	No (permanent)
Capacity	4-128 GB	256 GB - 20 TB
Cost	Higher per GB	Lower per GB
Purpose	Running programs	Storing programs/data

RAM Types

Type	Full Form	Description
DRAM	Dynamic RAM	Must be refreshed; slow but cheap; used for main RAM
SRAM	Static RAM	No refresh needed; fast; expensive; used for cache
DDR4	Double Data Rate 4	Modern RAM standard; 2133-3200 MHz
DDR5	Double Data Rate 5	Latest RAM standard; up to 8400 MHz
GDDR6	Graphics DDR6	Used in GPUs for graphics memory

CPU — Central Processing Unit; Memory Hierarchy