Computer Systems Fundamentals - HNC Software Development

Central Processing Unit (CPU)

The part of the computer that executes instructions and processes data.

Arithmetic and Logic Unit (ALU)

The component of the CPU that performs arithmetic calculations and logical operations.

Program Counter (PC)

A register in the CPU that keeps track of the address of the next instruction to be executed in a program.

Instruction Register (IR)

A register in the CPU that holds the current instruction being executed.

Integrated circuits combine …

… ALU, PC and IR into 1 silicon chip

Memory Address Register (MAR)

A register in the CPU that holds the next memory location of data to be fetched

Memory Data Register (MDR)

A register in the CPU that holds the data that has been fetched from memory. (It acts as a buffer between the CPU and memory.)

Control Unit (CU)

The part of the CPU that receives, decodes, stores results and manages execution of data that flows through the CPU 

Accumulator

A register in the CPU that temporarily holds results from the ALU.

General purpose registers

Registers in the CPU that store transient (temporary) data

3 buses

Address, data, control

Address bus

Unidirectional, stores 2^(bits) unique addresses

Data bus

Bidirectional, carries data to and from memory (can store 2^(bits) unique data values)

Control bus

Signals the control of actions of the computer (e.g. read/write line)

Difference between volatile and non-volatile memory

Volatile memory is lost when not supplied with power whereas non-volatile memory retains its data even when there is no power

Example of volatile memory (power needed)

RAM

Example of non-volatile memory (no power needed)

ROM

Difference between Dynamic RAM (DRAM) and Static RAM (SRAM)

DRAM requires constant refreshing to recharge its capacitors whereas SRAM does not need refreshing (allowing for faster access times)

DRAM vs SRAM speed

SRAM is faster

DRAM vs SRAM price

DRAM is cheaper

DRAM vs SRAM quantity

DRAM has a large quantity

Difference between DRAM and Synchronous DRAM (SDRAM)

SDRAM is a type of DRAM that is synchronized with the system clock, enabling faster data processing compared to traditional DRAM. It allows for higher performance and improved efficiency in accessing data.

What is active memory?

When a block of code/data is held in memory is directly accessible to the CPU 

5 storage devices

Hard disk drives, floppy disk drives, optical media(CDs/DVDs), tape units and solid-state drives

Storage device that is fixed and non-volatile

Hard disk drive

Storage device that is removable

Floppy disk drive / tape unit / solid state drive (USB drives) / CD / DVD

Polling vs interrupts

Polling is a method where the CPU continuously checks the status of an I/O device, whereas interrupts allow devices to signal the CPU when they need attention, enabling more efficient processing.

What is a non-maskable interrupt?

A non-maskable interrupt (NMI) is an interrupt that cannot be ignored by the processor (ensuring that critical events are processed immediately, often used for system errors or emergencies)

Describe how data is read from an I/O device

Processor signals the device to be read by placing the memory address onto the address bus.

Control unit of the processor turns on the I/O read line of the control bus.

Data is placed onto the data bus from the peripheral.

CPU reads data from data bus and places it into the MDR.

Describe how data is written to an I/O device

Processor signals the I/O device to be written by placing the memory address onto the address bus.

Processor places the data to be written onto the data bus.

Control unit of the processor turns on the I/O write line of the control bus.

Data is written to the peripheral.

3 types of software

Systems software, application software, and utility software.

Name a type of operating system

Single user / multi-user / real-time systems / distributed systems

Describe a single user OS

A single user operating system allows only one user to access the computer at a time (common on personal computers)

Describe a multi-user OS

A multi-user operating system enables multiple users to access the computer resources simultaneously

Name 4 types of multi-user OS

Single processor, multi-processor, networked systems, multi-tasking systems

Describe a single processor OS

It is designed to manage one CPU, ensuring that only one process is executed at any given time

Describe a multi-processor OS

It is designed to manage multiple CPUs, allowing execution of multiple processes in parallel

Describe networked systems (OS)

It is designed connect single user PCs to form a Local Area Network (LAN), enabling multiple users to share resources

Describe multi-tasking systems (OS)

It is designed to enable multiple processes to execute on a single CPU through time-sharing, allowing users to run several applications simultaneously.

Describe real time systems (OS)

Used for direct control over electromechanical equipment such as power stations, airplanes, trains and cars. They monitor external events and react instantaneously, hence the expression “real time”

Describe distributed systems (OS)

These systems consist of multiple autonomous computers that communicate through a computer network and interact with each other in order to achieve a common goal.

Name the OS layers (inside to outside)

Kernel, memory management, I/O, file management systems, UI

Name 2 types of user interface

Command Line Interface (CLI) and Graphical User Interface (GUI)

What is the purpose of the user interface?

To allow the user to interact with the computerin a user-friendly manner and access its functionalities effectively.

What is the purpose of file management system?

To organise and manage the storage of data on permanent media (such as hard drives and SSDs)

What is the boot sequence?

The order the computer attempts to boot from the various devices (such as HDD, CD, USB)

What is file locking?

A mechanism that prevents simultaneous access to a file by multiple users or processes (to prevent inconsistencies in data).

What are file locking alternatives?

Record locking or block locking to split the file into sections and lock each section individually

What is the name for the principal directory?

The root directory (the top-level directory in a file system hierarchy)

Describe logical file structure

How data is organised and formatted within a database or file system, including the relationships

Describe physical file structure

A collection of physical storage locations organised as a linear address space

Which file structure has better interactivity with the File Management System?

Logical file structure

Why should applications not be able to access the disk by track and sector coordinates?

Accessing by track and sector coordinates can lead to data integrity issues and security risks, as it bypasses the abstraction layer provided by the file system, potentially allowing unauthorised users to manipulate raw data directly.

Blocks and clusters

TBC

What happens if the file is larger than a single cluster?

1. Files can be stored contiguously (problem if file is larger than any single area of free storage)

2. Store them in a linked list (store data in each cluster with a pointer to each byte - in a chain)

3. Use an index table (stores locations of file fragments in separate clusters with a pointer to each cluster)

What does FAT stand for?

File Allocation Table

What is the layout of a FAT?

Partition boot sector, FAT, FAT (duplicate), root folder, other folders and files

How is file space allocated on FAT volume?

In clusters

FAT file - 4 attribute bits

Read-only, hidden, system, and archive (attributes that determine file behaviour and visibility)

Where does NTFS have advantages over FAT?

Performance, reliability and compatibility

What is the layout of a MFT?

Partition boot sector, master file table, system files, file area

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