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). The CPU reads instructions from there and executes them.
• 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

• A bus is a collection of wires that carries data or signals from one component to another, for example between the CPU and main memory.
• Main memory is outside the CPU. The CPU must copy instructions and data from it before using them.

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.

Pull the plug and unsaved work is gone. So how does it boot?

Primary storage

• Primary storage (main memory) is memory the CPU can access directly.
• It holds the instructions and data the CPU is using now. Secondary storage is too slow for the CPU to work from directly.
• Main memory is made up of RAM and ROM.
• Registers and cache are also memory the CPU uses directly, but they are not counted as main memory.

RAM

• RAM (random access memory) holds the operating system, the programs that are open and the data they are using.
• The CPU can read and write RAM, so its contents change all the time.
• RAM is volatile: its contents are lost when the power is switched off. That is why unsaved work disappears.
• More RAM lets more programs and data be open at once without slowing down.

ROM

• ROM (read-only memory) is non-volatile: it keeps its contents with the power off.
• It can only be read. Its contents cannot normally be changed.
• ROM holds the start-up instructions (the BIOS or boot program). They load the operating system from secondary storage into RAM.

Registers and cache

• Registers are tiny memory locations inside the CPU holding the instruction, data or address in use right now.
• Cache is small, very fast memory on or near the CPU. It holds copies of frequently used instructions and data, so fewer slow trips to RAM are needed.
• Both are volatile. The faster the memory, the more it costs per byte, so fast memory is kept small.

One storage firm runs 300,000+ spinning hard disks. Why not SSDs?

Why secondary storage?

• Secondary storage is non-volatile: it keeps the operating system, programs and files with the power off.
• It holds far more than RAM, for much less money per gigabyte.
• The CPU does not access it directly. Programs and data are copied into RAM (main memory) first.
• To choose a device, compare its capacity, speed, portability, durability, reliability and cost.

Magnetic storage

• A hard disk drive (HDD) stores data on spinning magnetic platters.
• Platters are divided into circular tracks, split into sectors.
• A moving read/write head uses an electromagnet to magnetise tiny areas (one way for 1, the other for 0) and to read them.
• Moving parts: slower, and easily damaged if dropped. But huge and cheap per gigabyte.

Solid-state storage

• Solid-state storage uses flash memory: electrical circuits that keep data with the power off. No moving parts.
• Examples: SSD (solid-state drive), SD card, USB flash drive.
• Fast, light, low power and robust. But it costs more per gigabyte, and cells wear out after many rewrites.

Cloud storage

• Cloud storage keeps your files on servers at a remote location, reached over the internet. The servers use magnetic and/or solid-state storage.
• Advantages: access from any device, anywhere; easy sharing; the provider handles backups and maintenance; buy more space when needed.
• Disadvantages: no internet, no access; slow connections mean slow transfers; ongoing fees; you rely on the provider to keep data secure and private.

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.
• 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.
• RAM is volatile and holds what is in use now; ROM is non-volatile and holds the start-up instructions.
• Registers and cache are small, fast, volatile memory close to the CPU.
• Secondary storage is non-volatile and keeps data for the long term; choose it by capacity, speed, portability, durability, reliability and cost.
• Magnetic: huge and cheap per gigabyte, but moving parts. Solid state: fast and robust, but dearer per gigabyte.
• Cloud storage is on remote servers: access anywhere, but only with an internet connection.