OCR GCSE Computer Science

Summary:

The OCR GCSE Computer Science revision notes cover key areas of Paper 1, focusing on systems architecture and the central processing unit (CPU), memory, and storage. The CPU, which can be thought of as the computer’s brain, fetches, decodes, and executes instructions from memory. Its main components include the Control Unit (CU), which coordinates the CPU’s activities; the Arithmetic Logic Unit (ALU), responsible for arithmetic and logical operations; and the registers, which are very fast memory locations. The cache, faster than RAM but slower than registers, can store frequently used instructions, while the clock measures the CPU’s speed in MHz or GHz.

The Von Neumann Architecture (VNA) revolutionized computing by allowing different programs and data to be stored in memory (RAM), enabling the CPU to switch tasks by loading different programs. Programs and data are stored on the hard disk, and the CPU can only work with programs in RAM. While the CPU can process billions of instructions per second, it can only work on one program at a time, switching between them so quickly that it appears to multitask. Registers, such as the Program Counter, Current Instruction Register, Memory Address Register, Memory Data Register, and Accumulators, play a vital role in fetching, decoding, and executing instructions.

Memory types include RAM, ROM, virtual memory, and flash memory. RAM is volatile and stores programs and data loaded by the operating system, while ROM is non-volatile and contains unchangeable data like boot-up instructions.

Excerpt:

OCR GCSE Computer Science

Paper 1

1.1 Systems Architecture
The CPU – Brain of the Computer
• Fetches instructions from memory (RAM)
• It decodes those instructions
• It then executes (carries out) the instructions.

The main components of the CPU you need to know about are:

The Control Unit (CU)
– coordinates all of the activities taking place within the CPU, monitoring the flow of data between
CPU components
The Arithmetic Logic Unit (ALU)
– It performs standard arithmetic operations such as addition, subtraction etc
– It deals with logic and comparisons such as ‘Is this value greater than that value?’