This document provides an overview of computer systems fundamentals including logic gates, number conversions, layers of the operating system, and components of a basic computer system. Specifically, it discusses: 1. Logic gates like AND, OR, and NOT and their truth tables. 2. Conversions between number bases like binary, decimal, and hexadecimal with examples. 3. The main layers of an operating system including the user interface, file management, input/output, memory management, and kernel. 4. Key components of a computer system including the ALU, control unit, registers, buses, and ASCII character encoding.

1. Computer Systems
Fundamentals

2. Logic Gates

3. AND Gate
A B Output
0 0 0
0 1 0
1 0 0
1 1 1
Truth Table

4. OR Gate
A B Output
0 0 0
0 1 1
1 0 1
1 1 1
Truth Table

5. NOT Gate
A Output
0 1
1 0
Truth Table

6. Conversions

7. Conversion Table
DENARY HEXADECIMAL BINARY
1 1 0001
2 2 0010
3 3 0011
4 4 0100
5 5 0101
6 6 0110
7 7 0111
8 8 1000
9 9 1001
10 A 1010
11 B 1011
12 C 1100
13 D 1101
14 E 1110
15 F 1111

8. Hex to Denary Example
Hex = 1EC
Denary = (1x16^2) + (14x16^1) + (12x16^0)
= 256 + 224 + 12
= 492

9. Denary to Hex Examples
128 64 32 16 8 4 2 1
Denary = 78
Binary = 01001110
Hex = 4E
Denary = 232
Binary = 11101000
Hex = E8

10. Hex to Binary Examples
FAB5 – 1111 1010 1011 0101
4CBF – 0100 1100 1011 1111
36ED – 0011 0110 1110 1101
9A7A – 1001 1010 0111 1010

11. Number Bases
Binary = Base 2
Denary = Base 10
Hex = Base 16

12. Use Truth Tables with Binary
NOT 10101010 = 01010101
NOT 11010110 = 00101001
10101010 AND 01010101 = 00000000
11101110 AND 00110110 = 00100110
10101010 OR 01010101 = 11111111
11010010 OR 01011001 = 11011011

13. Bits and Bytes
Byte = 8 Bits
Kilobyte = 1024 Bytes

14. Layers of the OS

15. User Interface
• User interacts directly with OS here
• Two interface types:
• Graphic User Interface (GUI)
• Command Line Interface (CLI)
• Both are examples of Human Computer Interfaces (HCI)
• Primary objectives to provide:
• Input Capability – using keyboards, mouse etc.
• Output Capability – using VDU, printers etc.
• Load and run programs

16. File Management System
• File Management System is responsible for:
• Organising files
• Managing files
• Storage of data on:
• Magnetic media (hard disks, floppy disks etc.)
• Optical media (CD-ROM, DVD etc.)
• USB Flash Drives

17. Input / Output Layer
• Deals with physical interaction of software with outside world e.g.:
• Disk drives, keyboards, printers, screens, mouse etc.
• This software is known as “device drivers”
• Layer also used for transferring the data to/from RAM/backing storage
• In Windows, a device driver will have one of the following file
extensions:
• DLL – Dynamic Link Library – uses hardware interrupts
• EXE – Executable – uses hardware interrupts
• VXD – Virtual Device Driver – uses software interrupts

18. Memory Management Layer
• Consists of loading a single program from a disk and allowing it free use
of all available memory (RAM)
• In Windows, multi-tasking is permitted; meaning more than one program
can be loaded and run at the same time
• Virtual Memory
• Virtual memory is used when there is insufficient memory to run programs
• Virtual memory is hard disk space set aside to act as memory
• To the running programs, virtual memory is invisible; treats hard disk area as RAM
• Protection
• To stop programs from clashing with each other, each is tagged with information
about which program ‘owns’ them.
• If a program attempts to access memory segments it doesn’t have authority to
access, it will be prevented.

19. Kernel
• Properties
• Most protected layer
• Deepest layer
• Most privileged layer
• Acts a timekeeper and regulator of the system
• Controls any time-slicing for multi-tasking functions
• Ensures fairness in the way resources are allocated and used
• Deals with interrupts having different properties, which arrive to
signal external and internal events

20. API (Application Programming Interface)
• An application programming interface (API) is a set of subroutine
definitions, protocols, and tools for building application software.
• In general terms, it is a set of clearly defined methods of
communication between various software components.

21. ALU (Arithmetic Logic Unit) and Control Unit
• Where data is processed and manipulated
• ALU involves arithmetic operations and logical operations
• ALU uses temporary registers to hold data
• Accumulator is a main register which holds the results of calculations
• Control Unit
• Manages execution of instructions
• Sends control signals around the computer

22. Registers
• Memory Address Register (MAR) holds the address of a location in
main memory
• Memory Buffer Register (MBR) holds the data that has just been read
from main memory or is to be written to main memory
• Instruction Register (IR) holds the current instruction being executed
• Program Counter (PC) holds the address of the next instruction to be
fetched from memory

23. Buses
• Data is transferred between memory and processors by buses:
• Address Bus
• Pinpoints memory location
• One-Way Bus
• Data Bus
• Transfers the data
• Two-Way Bus
• Control Bus
• Initiates and controls operations

24. ASCII
• American Standard Code for Information Interchange
• Represents 128 digits (0-127) (all characters on the keyboard)

25. Open-Source and Proprietary Software
Open-Source – free to use, but training isn’t free
Proprietary – paid for i.e. license agreements
- Restrictions on use, not free to share