What Is a Computer?; Machine Cycle: Fetch–Decode–Execute; CPU Organization

What Is a Computer?

A computer is an electronic machine that performs tasks by executing a stored sequence of instructions. At the architectural level, the central idea is simple: fetch an instruction, decode it, execute it and repeat until the program halts or an interrupt redirects control.

Modern computers are built from:

Block	Role
CPU	Executes instructions, performs arithmetic/logic
Memory	Stores programs and data
I/O	Communicates with humans and other systems
Buses	Connect CPU, memory and I/O

Understanding architecture means understanding how these blocks cooperate in time (cycles, pipelining, interrupts) and how data is stored (RAM, ROM, cache, auxiliary stores).

Machine Cycle: Fetch–Decode–Execute

Every instruction passes through a machine cycle (also called the instruction cycle). The classic three phases taught in standard textbooks are:

┌─────────┐ ┌─────────┐ ┌──────────┐
│ FETCH │───▶│ DECODE │───▶│ EXECUTE │───▶ (next instruction)
└─────────┘ └─────────┘ └──────────┘

2.1 Fetch

The Program Counter (PC) holds the address of the next instruction.
That address is placed in the MAR (Memory Address Register).
The memory returns the instruction opcode/operands via MDR into the CIR (Current Instruction Register).
PC is incremented (or updated by branch) to point to the following instruction.

2.2 Decode

The Control Unit (CU) interprets the opcode in CIR.
Control signals are generated to select ALU operation, register paths and memory/I/O access.

2.3 Execute

The operation runs: e.g. ADD places result in Accumulator (AC), STORE writes AC to memory, JMP loads PC.

2.4 Worked Timeline (Conceptual)

Suppose memory contains:

Address	Content	Meaning
2001	LOAD A	Load operand A
2002	LOAD B	Load operand B
2003	ADD	Add A + B
2004	STORE C	Store sum to C

Cycle 1: PC=2001 → Fetch LOAD A → Decode → Execute (A loaded to AC). Cycle 2: PC=2002 → Fetch LOAD B → Execute (B in register). Cycle 3: PC=2003 → ADD → result in AC. Cycle 4: PC=2004 → STORE → memory[C] updated.

This sequential story is exactly what non-pipelined machines do: one full cycle completes before the next instruction begins.

CPU Organization

Computer architecture design consists of:

Control Unit (CU)
Arithmetic and Logic Unit (ALU)
Memory Unit (conceptually; physically separate chips)
Registers
Input/Output devices

The CPU (Central Processing Unit) is the electronic circuit that executes program instructions. It is often called the microprocessor or processor when implemented on a single chip.

 ┌──────────────────────────────────────┐
 │ CPU │
 │ ┌────┐ ┌────┐ ┌──────────────┐ │
 │ │ CU │───│ALU │───│ Registers │ │
 │ └────┘ └────┘ └──────────────┘ │
 └───────────┬────────────┬─────────────┘
 │ Address │ Data
 │ Control │
 ┌───────────▼────────────▼─────────────┐
 │ Memory / I/O │
 └──────────────────────────────────────┘