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

• The current instruction register (CIR) holds the instruction being decoded and executed.
• An address says where something is; data is what is stored there.
• A microprocessor is a CPU on a single integrated circuit (chip).

Buses

• A bus is a set of parallel wires carrying signals between the CPU, main memory and other components.
• The address bus carries memory addresses from the CPU to memory. It only goes one way.
• The data bus carries data and instructions between the CPU and memory, in both directions.
• The control bus carries control signals, such as "read" or "write" and clock pulses, to coordinate components.

The cycle that never stops

• Kept in step by the clock, the CPU repeats the fetch–decode–execute (FDE) cycle billions of times a second.

• The instruction set is the list of all the machine code commands a CPU can process.

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.

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.

Virtual memory

• When RAM is full, the operating system uses part of secondary storage as virtual memory.
• Data and programs not needed right now are moved out of RAM to virtual memory in blocks called pages, freeing space.
• When they are needed again, pages are swapped back into RAM.
• Secondary storage is much slower than RAM, so heavy swapping slows the computer down. Adding more RAM is the fix.

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.
• Flash uses NAND or NOR technology. Transistors act as control gates and floating gates; trapped electrons store the bits.
• 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.

Optical storage

• A laser reads the surface of a spinning disc. Pits and lands reflect the light differently, giving 1s and 0s.
• To write, a laser changes the surface of a recordable disc.
• Examples: CD (about 700 MB), DVD (4.7 GB), Blu-ray (25 GB per layer).
• Cheap and light to post, but low capacity, slow and easily scratched. Downloads and flash drives have largely replaced it.

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.

How does your data find one computer among billions online?

IP addresses

• An IP address identifies a device on a network, so data can be sent to it. Every packet carries a destination IP address.
• An IPv4 address is 32 bits, written as four denary numbers from 0 to 255 separated by dots, e.g. 192.168.0.12. Each number is one byte (8 bits).
• The network allocates IP addresses. A static one never changes; a dynamic one may change at each connection.

Routers

• A router connects networks together, for example a home or school LAN to the internet.
• The internet is a vast number of networks joined by routers.
• A router reads each packet's destination IP address and sends it to the next router on a route towards it. Hop by hop, the packet reaches the right network.
• A router can also assign IP addresses to devices on its network.

IPv6

• IPv4 gives about 4.3 billion addresses (2³²): too few for every phone, laptop and smart device.
• IPv6 was created to give far more. It is 128 bits, written as eight groups of four hexadecimal digits separated by colons, e.g. 2001:0db8:85a3:0000:0000:8a2e:0370:7334.

MAC addresses

• A device needs a NIC (network interface card) to join a network.
• Each NIC is given a MAC address at manufacture: 48 bits, written as 12 hex digits in six pairs, e.g. 00:1A:2B:3C:4D:5E.
• The first half is the manufacturer code; the second half is the serial code.
• MAC addresses identify devices within a LAN. A MAC address stays the same; an IP address can change.

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, CIR and ACC hold data or instructions.
• The address bus carries addresses, the data bus carries data and instructions, the control bus carries control signals.
• 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.
• When RAM is full, virtual memory on secondary storage takes the overflow, but swapping is slow.
• 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.
• Optical discs are read by a laser (pits and lands): cheap but low capacity.
• Cloud storage is on remote servers: access anywhere, but only with an internet connection.
• An IP address identifies a device on a network. Routers read each packet's destination IP address and forward it hop by hop.
• IPv4 is 32 bits (four numbers, 0–255); IPv6 is 128 bits in hex, giving far more addresses.
• A MAC address is a 48-bit hex address fixed in a NIC when it is made, used to find devices within a LAN.