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 stored program concept (the Von Neumann model).
• 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

• Main memory is outside the CPU. The CPU must copy instructions and data from it before using them.

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 this cycle billions of times a second: the fetch–decode–execute (FDE) cycle.

• During fetch, the instruction's address goes out on the address bus and the instruction comes back on the data bus.

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.

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.

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.

One CPU core, three apps at once. How is that possible?

The manager between apps and hardware

• The operating system (OS) is system software that manages the hardware and provides a platform for applications to run on.
• Apps don't control the hardware themselves: they ask the OS.
• The OS loads apps into memory, starts them and closes them.

Sharing the processor and memory

• Each running program is a process. One core runs one instruction at a time.
• The OS schedules processes: each gets a short time slice on the CPU, then the OS switches to the next.
• Switching is so fast that programs seem to run at once: multitasking.

Devices and files

• The OS manages peripherals (input, output and storage devices) and the data passing between them and the processor.
• A device driver lets the OS communicate with one particular device. It translates OS commands into ones that device understands.
• File management: the OS organises files into folders and lets you name, save, move, copy and delete them.

Interface, users and security

• User management: the OS creates user accounts, checks logins and gives each account access rights to particular files and programs.

Why could shuffling files around make an old hard drive faster?

Housekeeping software

• Utility software is system software that helps to maintain, protect or configure a computer.
• It carries out housekeeping tasks that the operating system may not do, or may not do well enough, on its own.
• Some utilities come with the OS; others are installed separately.
• Examples: file repair, backup, data compression, disk defragmentation and anti-malware software.

Fragmented files

• A magnetic hard disk stores files in blocks. The read/write head moves to each block to read it.
• As files are saved and deleted, free space becomes scattered gaps, so new files get split up: they are fragmented.
• Reading a fragmented file needs many slow head movements.
• A disk defragmenter moves each file's pieces next to each other (contiguous) and groups the free space together, so files load faster.

Backup, compression and encryption

• Backup software copies files to another device or the cloud, so they can be restored if lost or damaged.
• Compression software makes files smaller, so they use less storage space and transfer faster.

Protecting and repairing files

• Anti-malware software scans files and memory for malware, comparing them with known malware signatures and watching for suspicious behaviour. It quarantines or deletes what it finds.
• File repair tools fix damaged or corrupted files so they can be opened again.

Why write a 1999 hit game almost entirely in assembly?

What the processor actually runs

• A CPU can only execute machine code: instructions in binary.
• Each instruction is a simple step, such as load a value, add, store a result or jump.
• Each processor family has its own instruction set, so machine code for one family won't run on another.
• Machine code and assembly language are low-level languages: they work directly with the processor's own instructions.

Assembly language

• Assembly language replaces binary with short mnemonics such as LDA (load), ADD and STA (store).
• Each assembly instruction matches exactly one machine code instruction: a 1:1 correspondence.
• Like machine code, it is specific to one processor family.

LDA price    ; load price into the processor
ADD tax      ; add tax to it
STA total    ; store the result in total

High-level languages

• High-level languages use English-like keywords and maths-style expressions.
• One statement often becomes several machine code instructions: the line below becomes the three assembly instructions you just met.
• They are machine independent: the same source code can be translated to run on different processor families.
• A compiler or interpreter must translate the code before the CPU can run it.
• Most programs are high-level: they are quicker to write, read and debug.

total = price + tax

Choosing a level

• Use low-level for device drivers, embedded systems and code where speed or memory is critical.
• Use high-level for most applications, where quick development matters more.

Why do some languages show all your errors, and others just one?

Why translate?

• The CPU can only execute machine code, so the source code that programmers write must be translated first.
• A translator is a program that converts source code into a form the CPU can execute.
• High-level code is translated by a compiler or an interpreter.

Compiler or interpreter?

• A compiler translates the whole program before it runs, producing an executable file of machine code that runs without the compiler.
• An interpreter translates one line at a time into machine code and runs it. It is needed every time the program runs.
• Errors: a compiler gives an error report for the whole program; an interpreter stops at the first error.

Choosing a translator

• An interpreter is mostly used while developing and testing a program; a compiler translates the final program for release.

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.
• 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.
• 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.
• The OS is system software that manages the hardware and gives applications a platform to run on.
• It shares the processor between processes (multitasking) by scheduling short time slices, and manages peripherals using device drivers.
• It also manages files and user accounts.
• Utility software is system software that does housekeeping tasks to maintain, protect or configure the computer.
• A defragmenter makes files contiguous on a magnetic hard disk so they load faster; it isn't needed on an SSD.
• Backup, compression, file repair and anti-malware tools each protect, shrink or fix your files.
• The CPU only runs machine code (binary); assembly uses mnemonics with a 1:1 match to machine code. Both are low-level and tied to one processor family.
• High-level languages are easier to write, read and debug and are machine independent, but must be translated.
• Low-level suits device drivers, embedded systems and speed-critical code; high-level suits most applications.
• Source code must be translated before the CPU can run it: a compiler translates the whole program into an executable; an interpreter translates and runs it line by line.
• Interpreters suit development and testing; compilers suit the final program.