Chapter 4: Operating System & Computer Architecture (Computer Science IGCSE)

Control Unit (CU)

- Controls operations of the CPU's internal components (memory, processor & input/output). - Sends signals to other components telling them 'what to do'. - Fetches each instruction in turn & manages the execution of each instruction. - CU = where instructions are interpreted.

Features of an OS

- Human Computer interface (HCI) - Multitasking - Multi-programming - Batch processing; using Job Control Language (JCL) - Error handling - Loading & running of applications/software - management of user accounts - File utilities (e.g. copy/save/sort/delete) - Processor management - Memory management - Real-time processing - Interrupt handling routines - Security (manages log on/passwords) - Input & output control - Spooling

Read Operation

1) Address of location of data is read from & first written into MAR. 2) Read signal sent to computer memory using the control bus. 3) Contents of memory location are put into the MDR.

Write Operation

1) Data to be stored is first written into MDR. 2) Address of the memory location the data is to be written into is written into MAR. 3) Write signal sent to computer memory using the control bus. 4) Data value is written into the correct memory location.

Steps of Fetch Cycle

1) PC holds address/location of the instruction. 2) Address held in PC is sent to MAR via address bus. 3) PC is incremented. 4) Instruction is sent from address in memory to MDR. 5) Instruction is transferred via data bus to CIR.

Memory Management

A process of assigning blocks of memory to programs running in a computer and managing the transfer of programs in/out of memory.

Von Neumann Architecture

A processor where data and instructions are stored in the same memory and accessed via buses. 1) program initially stored on a secondary storage device (e.g. hard drive) 2) When the program is run, data & instructions are both moved to RAM. 3) Data & instructions are moved to registers to be executed & fetched one at a time.

Utility

A program that performs a specific task required for the operation of a computer system.

Interrupt

A signal sent by a device or program to the processor requesting its attention. Causes the processor to temporarily stop what it is doing & service the interrupt. Once the signal is received the processor either: - carries on what it was doing; or - stops to service the device/program that generated the interrupt according to interrupt priority. Interrupt handler used to save status of current task whenever an interrupt is serviced. Once interrupt if fully serviced, the task is initiated, continuing from point prior to the interrupt being sent.

Interrupt

A signal that causes the OS to take a specified action.

Data Bus

Carries data between the processor, the memory unit and the input/output devices. Bidirectional.

Control Bus

Carries signals relating to the control and coordination of all activities within the computer (e.g. read/write functions). Unidirectional/Bidirectional

Address Bus

Carries the address of memory locations used to store data and program instructions. Unidirectional.

Busses

Connect together the internal components of the CPU.

Control Unit (CU)

Contains Current Instruction Register (CIR) & Program counter (PC). 1) CU reads an instruction from memory. 2) The address of the location where the instruction can be found is stored in the PC. 3) The instruction is interpreted. During that process, signals are sent along the control bus to tell the other computer components what to do.

Memory Data Register (MDR)

Contains the data value being fetched or stored.

Input Devices

Convert external data into a form the computer can understand & then process. (e.g. keyboards/touchscreens/microphones)

Read Only Memory (ROM)

Data can be read but not altered. Non-volatile (permanent) memory. Stores bootstrap loader & BIOS.

Spooling

Data temporarily held in a buffer waiting for an output device to access it.

Register

High-speed memory built into the CPU that temporarily stores data during processing.

Memory Address Register (MAR)

Holds the address of the memory location to be accessed.

Immediate Access Store (IAS)

Holds the instructions and data that is ready to be used by the CPU before they are processed.

Decode

Instruction in CIR is decoded so that it can be interpreted in the next part of the cycle.

Fetch

Instruction is retrieved from the memory address currently stored in the PC and is then stored in the CIR. The PC is then incremented (increased by 1) so that the next instruction can be processed.

Multitasking

Many processes appear to run simultaneously.

Processor

Short for microprocessor or CPU, the component of a computer that follows instructions. Contains Arithmetic & Logic Unit (ALU) & the Control Unit (CU).

Output devices

Show the results of computer processing in a humanly understandable form. (e.g. printers/monitors/speakers)

Random Access Memory (RAM)

Stores part of OS & data currently in use. Volatile memory.

Program Counter (PC)

Stores the address of the next instruction to be executed.

Current Instruction Register (CIR)

Stores the current instruction being executed.

Computer Architecture

The internal, logical structure and organisation of the computer hardware.

Arithemtic-Logic Unit (ALU)

The part of the CPU that performs arithmetic and logical operations. Contains register called Accumulator (Acc) to hold the results of calculations.

Fetch-Decode-Execute Cycle

The process that the CPU uses to retrieve and execute instructions.

Execute

The processor passes the decoded instruction as a set of control signals to the appropriate components w/in the computer system. This allows each instruction to be carried out in its logical sequence.

Operating System (OS)

The system software that controls the way in which a computer system functions, including the management of hardware, peripherals, and software.

Buffer

Used as temporary memory area. Filled from the processor/memory unit & whilst these are emptied to the hardware device, the processor carries on w/ other tasks.

Register

Used to temporarily hold data & instructions during processing.