ENIAC took days to reprogram. Your phone swaps apps instantly. Why?

Programs live in memory

• A program is a list of instructions in binary (machine code), processed by the CPU.
• A running program's instructions and data are both held in main memory (RAM). This is the Von Neumann architecture.
• New program? Load new instructions into memory. Nothing is rewired.
• Input devices send data in; output devices present results; secondary storage keeps programs when they are not running.

Inside the CPU

• An address says where something is; data is what is stored there.
• Cache is a small, very fast memory in the CPU for frequently used instructions and data.

The cycle that never stops

• Kept in step by the clock, the CPU repeats this cycle billions of times a second: the fetch-execute cycle.

Embedded systems

• An embedded system is a computer built into a larger device to do one dedicated job.
• Examples: washing machine, microwave, thermostat, car engine management, traffic lights, vending machine.
• Usually small, cheap, low power and reliable. Its program is stored permanently, often in ROM.
• A general-purpose computer, such as a PC or laptop, runs many different programs.

Double the cores. Why isn't the computer twice as fast?

Clock speed

• The clock sends out regular electrical pulses that keep the CPU's parts in step.
• Clock speed is the number of clock cycles per second, measured in hertz (Hz).
• 1 GHz = 1,000,000,000 cycles per second, so 3.5 GHz is 3.5 billion.
• Higher clock speed: more fetch–decode–execute cycles each second, so more instructions are processed.

Cores

• A core is a complete processing unit. Each core can fetch, decode and execute instructions on its own.
• A dual-core CPU can process two instructions at the same time; a quad-core CPU, four.
• Doubling the cores rarely doubles the speed: some tasks cannot be split, and one core may wait for another's result.
• Software must be written to use several cores.

Cache

• Cache is a small amount of very fast memory on or very near the CPU.
• It holds frequently used instructions and data, saving slower trips to RAM.
• A larger cache holds more, so the CPU waits less. But cache is expensive, so it stays small.

Big ideas

• Instructions and data are both stored in main memory; the CPU fetches, decodes and executes them, one after another, kept in step by the clock.
• Registers do specific jobs: the PC and MAR hold addresses; the MDR and ACC hold data or instructions.
• An embedded system is a computer inside a larger device that does one dedicated job.
• Higher clock speed means more cycles, and so more instructions, every second.
• More cores help only when the work can be split; a larger cache means fewer slow trips to RAM.