IB Computer Science SL: Computer Organisation

Arithmetic and Logic Unit (ALU)

Handles all arithmetic (addition / subtraction) and logical (AND/OR) calculations

Functions of the Control Unit

1. Handle loading of new instructions into the CPU and decode them

2. Direct data flow and ALU

Registers

Are small, very fast circuits that store intermediate values from calculations or instructions inside the CPU

Examples are the Memory Address Register (MAR) and the Memory Data Register (MDR)

Memory Address Register (MAR)

Contains RAM address of the instruction the CPU wants next

Memory Data Register (MDR)

Holds data that will be written to or was read from the RAM

Busses

The connecting wires that connect the CPU to other devices as they carry instructions.

1) Data bus (RAM → CPU via MDR)

2) Control bus (RAM → via CPU CU)

3) Memory bus (RAM → CPU via MAR)

Primary memory

Random Access Memory (RAM) and Read-only Memory (ROM).

• RAM: “Short term”, volatile - loses its contents when power shuts off

• ROM: Stores permanent instructions to boot up computer, holds the Basic Input Output System, non-volatile

Use of cache memory

High-speed memory in the CPU that stores instructions from the RAM. It’s checked by the processor first, if data is found, = cache hit. If data isn’t found = cache miss and RAM is searched. It is closer to the CPU than the RAM, which is why it’s high speed.

CPU <=> L1 Cache <=> L2 Cache <=> L3 Cache <=> RAM

Fetch-Decode-Execute Cycle

1. Address to be checked appears in the Program Counter (PC).

2. The address of the next instruction is copied onto the MAR from PC.

3. MAR sends address to RAM and is then sent to the MDR.

4. The instruction, stored in the MDR, is then copied into the CIR.

5. Instructions are sent to the CU to be executed by the ALU.

PC → MAR → RAM → MDR → CIR → CU → ALU

The computer only requires one cycle to reach a result.

The need for persistent storage (secondary storage)

The processing done in CPU has no permanent storage has the RAM is volatile therefore HDD (Hard disk drives) and SSD (Solid state drives) allows for persistent storage. They are slower compared to primary storage but are cheaper.

Operating systems

Set of software that controls computer’s hardware and resources and provides services for computer programs

Functions of an operating system

User interface: link between hardware and user

• Graphical User Interface (GUI): menus, icons, point & click

• Command Line Interface (CLI): user type in commands and code

• Natural Language Interface (NLI): user can speak to computer e.g. Siri

• Menu Based Interface (MBI): similar to CLI but only menu options

Memory management: tracks storage devices and controls which applications have access to RAM

• Provide file management services

• Notifies user when memory is full

• User can copy and delete files

Peripheral management: controlling keyboards, mouses, monitors, and printers through device drivers

• Coordinate with BIOS

• Use device drivers to interface with peripherals

Multitasking: allocating CPU cycles to programs based on prioriy and time

Security: prevents unauthorised access through usernames and passwords

The range of application software

• Word processor stores, manipulates, and formats text entered from a keyboard to make a printout

• Spreadsheet programs arranges data in a grid table to be manipulated and used in calculations

• Database management systems provides users and programmers a systematic way to create, retrieve, update, and manage data

• Email clients help in accessing and managing user’s emails.

• Web browsers retrieve and navigate information resources on the World Wide Web

• Computer Aided Design programs use computer systems to create, modify, or optimise a design.

• Graphic Processing Software enables users to manipulate visual images on a computer

Programming features

• Consistent grammar and syntax

• Can define basic data types and operations

• Can handle input and output

• Can provide a conditional loop

• Must have a compiler or interpreter

High-level vs low-level programming languages

1. High-level provide greater abstraction from hardware whereas low-level languages require interaction with hardware specifics e.g. memory addresses and registers

2. High-level languages are generally easier to learn as they are more similar to the natural human language

3. Low-level language have finer control and faster performance due to their proximity to machine code

Compiler

Translates code written in high-level language into machine code (takes it chunk by chunk)

Interpreter

Directly executes instructions in a programming language (line-by-line)

Differences between high-level programming languages

• Method of translation - compiler and/or interpreter

• Loosely/strongly typed - whether data types are specified or not

• Compatibility with different environments

• Syntax differences

ASCII vs Unicode

Unicode can represent multiple languages

ASCII uses 8 bits for each character

Logic gates

NOT, AND, OR, XOR, NAND, XOR

NOT logic gate

A | NOT A

0 | 1

1 | 0

AND logic gate

A | B | A AND B

0 | 0 | 1

0 | 1 | 0

1 | 0 | 0

1 | 1 | 1

OR logic gate

A | B | A OR B

0 | 0 | 0

0 | 1 | 1

1 | 0 | 1

1 | 1 | 1

XOR logic gate

A | B | A XOR B

0 | 0 | 1

0 | 1 | 1

1 | 0 | 1

1 | 1 | 0

NOR logic gate

A | B | A NOR B

0 | 0 | 1

0 | 1 | 0

1 | 0 | 0

1 | 1 | 0

XOR logic gate

A | B | A XOR B

0 | 0 | 0

0 | 1 | 1

1 | 0 | 1

1 | 1 | 0