GCSE Computer Science

GCSE Computer Science

1. Computer Systems

   • Components of a Computer System
     • The CPU
       • Central Processing Unit
         • processes all of the data and instructions that make the system work
       • has 3 main parts
         • Control Unit
           • overall control of the CPU
           • executes program instructions by following the fetch-decode-execute cycle
           • controls the flow of data inside the CPU (to registers, ALU, cache) and outside the CPU (to main memory and input/output devices
         • Arithmetic Logic Unit
           • does all calculations - simple addition and subtractions, compares sizes of numbers, multiplications and divisions
           • performs logic operations and binary shifts
           • contains the accumulator register
         • Cache
           • very fast memory in the CPU - slower than registers, but faster than RAM
           • stores regularly used data
           • very low capacity and are expensive
           • Different levels of cache memory
             • L1 - fastest but has lowest capacity
             • L2 - slower than L1 but has higher capacity
             • L3 - slowest but has highest capacity
       • Von Neumann architecture
       • Fetch-Decode-Execute
         • Fetch
           • 1. Copy memory address from the program counter to the MAR
           • 2. Copy the instruction stored in the MAR address to the MDR
           • 3. Increment the program counter to point to the address of the next instruction, ready for next cycle
         • Decode
           • The instructions in the MDR is decoded by the CU, which may then prepare for the next step
         • Execute
           • The instruction is performed. This could be:
             • load data from memory
             • write data to memory
             • calculation or logic operation
     • Memory
       • RAM
         • main memory in a computer
         • can be read or written to
         • volatile
           • temporary memory
           • requires power to keep its data
           • non-volatile memory is permanent memory and keeps its contents even when it has no power
         • slower than the CPU cache
         • faster than secondary storage
       • Virtual Memory
         • computers have a limited amount of RAM, which fills with data as apps are opened
   • Networks
   • Issues

2. Computational Thinking, Algorithms and Programming

   • Algorithms
   • Programming
   • Design, Testing and IDEs
   • Data Representation

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