IB Computer Science Paper 1 Topic 2

Components in the CPU

• Control Unit (CU)

• Arithmetic Logic Unit (ALU)

• Memory Address Register (MAR)

• Memory Data Register (MDR)

• Address Bus

• Data Bus

Arithmetic Logic Unit (ALU)

Performs all basic arithmetic and logical operations

Control Unit (CU)

• Obtains data from memory and decodes it into instructions

• Coordinates all the units of the computer

Memory Address Register (MAR)

• Holds the memory of the address data to be used by the ALU

• Holds address of where processed data will need to be stored

Memory Data Register (MDR)

• Holds the data to be used by the ALU

• Holds a copy of the contents of the memory to be transported to other components of the CPU

Address Bus

* Connects MAR to primary memory (RAM)

Data Bus

* Connects MDR and primary memory (RAM)

Two types of primary memory

Random Access Memory (RAM) and Read Only Memory (ROM)

Random Access Memory (RAM)

• Stores data and instructions currently in use - can be overwritten

• Volatile - whenever power is lost its memory is wiped

• Can be written to

Read Only Memory (ROM)

• Stores permanent instructions and data - info is embedded cannot be overwritten

• Non-volatile

• Much smaller than ROM

• Holds the Basic Input / Output System (BIOS)

Cache

• Stores frequently used instructions

• Is used to save time in accessing the RAM, as can be accessed faster than the RAM

• Situated between RAM and CPU, so closer to CPU

• Very small

• Static

Components of the machine instruction cycle

Fetch, decode, execute

Fetch

• CPU sends address from MAR of instruction through the address bus to the primary memory (RAM)

• Instruction in that particular address is sent through the data bus to the CU, stored in MDR

Decode

* CU decodes instruction, fetches any required data from the primary memory

Execute

* ALU executes instruction

Store

* The results of the executed instruction are stored in the accumulator

Secondary memory

• Relatively slow memory that can be written to

• Non-volatile

• Hard drives, USB sticks, Floppy disk

Operating system

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

Roles of an operating system

• Peripheral communication - communicating directly with hardware (keyboards, mice, printers)

• Memory management - manages how memory is used by applications, ensures one doesn’t interfere with the memory of another

• Data management - accessing and storing data

• Security

Benefits / execution of memory management

• Allocates and deallocates specific sections of memory to each process

• Ensures that a program has sufficient memory to run

• Ensures that the memory assigned to one program is not overwritten

How colour can be represented in a computer

* Split into three components (e.g RGB), and each component is given a specific number of bytes