What Is a Computer?

The five roles

Every computer, from a phone to a data-center server, organizes work into five roles.

The distinction that matters most is between working memory (RAM) and storage (disk/SSD). RAM is fast but volatile, its contents vanish when power is lost. Storage is slower but persistent, it keeps your files between sessions.

The fetch–decode–execute cycle

The CPU does not "understand" your whole program at once. It grinds through one instruction at a time, in a loop that repeats billions of times per second.

1. 1FetchThe CPU reads the next instruction from RAM, at the address held in its program counter.
2. 2DecodeThe control unit interprets that bit pattern: which operation to run, and on which data.
3. 3ExecuteThe right sub-unit acts, for arithmetic, the ALU (arithmetic logic unit) does the math.
4. 4AdvanceThe program counter moves on, and the cycle repeats with the next instruction.

Because the instruction was itself just bits fetched from memory, a program is not a special kind of thing: a program is data that tells the CPU what to do.

Worked example: computing 2 + 3

Watch the five roles hand the work to each other.

1. 1InputYou type 2 and 3. Each keypress is converted into bits and enters the machine.
2. 2Into RAMThe two values are stored as bit patterns in working memory: 2 is 00000010, and 3 is 00000011.
3. 3Fetch and decodeThe CPU fetches an ADD instruction (also just bits in RAM) and decodes it as 'add these two numbers'.
4. 4ExecuteThe ALU adds the two patterns.
5. 5Back to RAMThe result, 5 (00000101), is written back into working memory.
6. 6OutputThe bits for 5 are sent to the screen, where they are drawn as the character '5'.

The binary addition itself:

  00000010    (2)
+ 00000011    (3)
= 00000101    (5)

Common mistakes

Further reading

• nand2tetris, build a working computer from logic gates up to a running program.
• Code: The Hidden Language of Computer Hardware and Software by Charles Petzold, bits to CPUs, no prerequisites.
• Computer Organization and Design by Patterson & Hennessy, the standard university text on how CPUs execute instructions.
• CrashCourse Computer Science (YouTube), a short, visual video series covering these foundations.