J277 (9-1) Computer Systems Revision

Number Bases & Shifts

• Binary (Base 2): Uses 0s and 1s. This is the language of computer hardware (On/Off). An 8-bit number is a 'byte'.
• Hexadecimal (Base 16): Uses 0-9 and A-F. It's a shorthand for binary. One Hex digit represents a 4-bit 'nibble'.
• Hex to Denary Method: Multiply the left digit by 16, then add the value of the right digit. (e.g., 2B = (2 * 16) + 11 = 43).
• Binary Shifts: Shifting bits left multiplies by 2 for each place. Shifting right divides by 2.

Sound & Images

• Sound Sampling: The amplitude of the analogue wave is measured at set intervals.
• Bit Depth: The number of bits used to store each sample. Changing it affects file size and the accuracy/range of amplitudes.
• Colour Depth: The number of bits per pixel. 7-bits can store $2^7$ = 128 colours.
• Character Sets: To represent 150 different characters, you need 8 bits ($2^7=128$ is too small, $2^8=256$ is needed).

CPU Concepts

• Fetch-Execute Cycle: The process of fetching instructions from memory, decoding them, and then executing them.
• CPU Registers: Small, fast memory locations in the CPU.
  • Program Counter (PC): Stores the address of the next instruction to be fetched.
  • Memory Address Register (MAR): Stores the address of the instruction/data to be fetched or stored.
  • Memory Data Register (MDR): Stores the data/instruction fetched from memory or waiting to be stored.
• Performance Factors: A CPU's performance is affected by Clock Speed, Cache Size, and Number of Cores.

System Software

• OS Functions: The OS manages Memory (moving data to RAM), Peripherals (input/output devices), Files (creating/deleting folders), and the User Interface.
• Encryption: A utility that changes data using a key so it cannot be understood if intercepted.
• Defragmentation: A utility that moves split files into consecutive blocks to make data access faster.

LAN vs WAN

• LAN (Local Area Network): Covers a small geographical area (e.g., one building). The organisation *owns* the hardware.
• Wired Benefits: Fast/high bandwidth, secure (unlikely to be intercepted), and reliable (little interference).
• Wireless Benefits: Allows staff/devices to be mobile, easier to add new devices, and supports devices with no wired port (e.g., phones).

Topologies & Protocols

• Star Topology: All devices connect to a central switch. Benefit: If one cable fails, the rest of the network is unaffected. Drawback: If the central switch fails, the whole network fails.
• Switch: Connects devices on a LAN, recording their MAC addresses to direct data only to the intended recipient.
• MAC Address: A 48-bit unique identifier, usually in hex, with the first half identifying the manufacturer and the second half a serial number.
• IPv4 Address: 4 groups of denary numbers (0-255) separated by dots.

Storage

• Storage Types: Magnetic (e.g., HDD) is cheap with high capacity. Solid-State (e.g., SSD) is fast and durable (no moving parts). Optical (e.g., CD) is cheap but slow and easily damaged.
• Choosing Storage: For a musician's main computer, magnetic is a good choice due to its high capacity and low cost per GB for large sound files.
• Storage Calculation: 1000 files at 3MB each = 3000MB. Since 1000MB = 1GB, this is 3GB.

Embedded Systems

• Definition: An embedded system is a computer built into a larger device to perform a specific task (e.g., a car's 'Follow Me' system).
• ROM (Read-Only Memory): Stores permanent instructions like the startup sequence (BIOS) and the system's core firmware.
• RAM (Random Access Memory): Stores temporary data, such as current sensor readings (distance, speed).
• Virtual Memory (VM): Not needed because the system runs only one dedicated program and has enough RAM for its specific, limited tasks.

Open Source vs. Proprietary

When releasing software, a programmer must choose a licence:

• Open Source Licence: Software is (usually) free. Users can access and change the source code. It can be redistributed. Benefit: Community can fix bugs. Drawback: Little financial gain for the programmer.
• Proprietary Licence: Software is purchased. Users cannot access or change the source code. Benefit: Programmer can earn money and has control. Drawback: Users cannot adapt it.

Legislation & Other Issues

• Data Protection Act (DPA): Law governing how organisations must store and process personal data securely and fairly.
• Ethical Issues: Problems that aren't illegal but are morally questionable, like planned obsolescence (designing products to fail so users must upgrade).
• Environmental Issues: The problem of e-waste as old devices are sent to landfill, and the energy used by computers.