HNC Computer Science

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