"Tea or coffee?" Would a computer's OR let you have both?

1 or 0, and NOT

• A Boolean value has only two states: 1 (TRUE) or 0 (FALSE).
• A truth table lists every possible combination of inputs with the output for each.
• NOT has one input and reverses it: NOT 0 = 1, NOT 1 = 0.
• Write it as NOT A.

AND

• AND has two inputs. The output is 1 only when both inputs are 1.
• If either input is 0, the output is 0.
• Write it as A AND B.

OR

• OR has two inputs. The output is 1 when at least one input is 1.
• The output is 0 only when both inputs are 0.
• This is wider than everyday "or", which often means "one or the other, not both".
• Write it as A OR B.

Three sensors, one alarm: how many cases must you test?

Combining operators

• One logic statement (expression) can use several operators, e.g. (A AND B) OR NOT C.
• Work out brackets first.
• NOT applies only to the input or bracket just after it.
• Example: (1 AND 0) OR (1 AND 1) = 0 OR 1 = 1

Every combination of three inputs

• Each input can be 0 or 1, so each new input doubles the number of rows.
• 1 input: 2 rows. 2 inputs: 4 rows. 3 inputs: 8 rows.
• Fill the input columns by counting up in binary from 000 to 111. Then no row is missed or repeated.

Working columns

• For a long statement, add a column for each part.
• Example: P = (A AND B) OR NOT C.
• Column 4: A AND B. Column 5: NOT C. Column 6: P = column 4 OR column 5.
• Working columns are optional, but they stop you mixing up steps.

From a problem to a logic statement

• Give each condition a letter (1 = true, 0 = false).
• "and" → AND; "or" / "either" → OR; "not" / "is off" → NOT.
• Use brackets to group conditions that go together.
• Example: the alarm X sounds if the door is open (D) and the system is armed (S), or if the panic button (P) is pressed.
• X = (D AND S) OR P

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.
• System software manages the computer's resources, e.g. the OS and utility software. Application software does tasks for the user, e.g. a web browser or a game.

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

• The user interface lets you control the computer, e.g. a graphical user interface (GUI) with windows, icons, menus and a pointer, or a command-line interface where you type commands.

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 managing, repairing and converting tools, compression, defragmentation, backup 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 (antivirus and anti-spyware) 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.
• Other utilities manage files (copy, rename, organise) and convert files from one format to another.

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.
• An assembler translates assembly language into machine code.

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.

SET total TO 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.
• Assembly is translated by an assembler: each assembly instruction becomes one machine code instruction.

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 and runs the code one line at a time. 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.
• An assembler is used when a program is written in assembly, e.g. for embedded systems or device drivers.

Big ideas

• Every input and output is 1 (TRUE) or 0 (FALSE). A truth table lists every combination of inputs.
• AND: 1 only if both inputs are 1. OR: 1 if at least one input is 1. NOT: reverses its one input.
• Work out brackets first; NOT applies only to what comes straight after it.
• 3 inputs give 8 rows: count in binary from 000 to 111, and add working columns for each part.
• Turn a problem into a statement by giving each condition a letter and mapping "and", "or", "not".
• 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 the user interface.
• 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; assemblers translate assembly language.