Computer Science Component 1

Computer Science Component 1

1. The CPU

   • The Control Unit
     • In overall contoll of the CPU
     • Its main job is to execute program instructions by following the Fetch-decode-execute cycle
   • The Cache
     • The Cache is very fast memory in the CPU. It's slower than the registers but faster than RAM.
     • It stores regularly used data so that the CPU can access it quickly
     • Caches have very low capacity and are expensive compared to RAM and Secondary Storage
     • There are different levels of cache memory
   • The Arithmetic Logic Unit
     • The ALU does all the calculations
     • Does simple addition and subtraction, compares the size of numbers and can do multiplications and divisions using repeated addition and subtraction
     • It performs logic operations such as AND, OR, and NOT and binary shifts
     • It contains the accumulator

2. Von Neumans design for the CPU

   • The program counter holds the memory address of the instruction for each cycle
   • The accumulator stores immediate results of calculations in the ALU
   • The memory address register holds any memeory address about to be used by the CPU. The address might pont to data or a CPU instruction
   • The memory data register holds the actual data or instruction. This may have been fetched from memory, or be waiting to be written to memory
   • The memory holds the program instructions and the program data

3. Fetch - Decode - Execute Cycle

   • Fetch
     • Copy memory address form the program counter to the MAR
     • Copy instruction stored in the MAR address to the MDR
     • Increment the program counter ready for the next cycle
   • Execute
     • The instruction is then performed
   • Decode
     • The instruction in the MDR is decoded by the CU. The CU may then prepare for the next stage.

4. RAM and ROM

   • RAM
     • Random Access Memory is used as the main memory. It can be read and written to
     • The main memory is where all the data, files, and programs are stored while they're being used
     • It is volatile
     • When a computer boots up, the operating system is copied from secondary storage to RAM
     • When software applications, documents and files are opened they are copied from secondary storage to RAM, they stay in RAM until closed
     • RAM is slower than the CPU cache but way faster than secondary storage
   • ROM
     • Read only memory
     • Comes on a small factory made chip built into the motherboard
     • It contains all the instructions a computer needs to properly boot up
     • As soon as the computer is powered on the CPU reads the instructions from ROM
     • Tells the computer to do self checks and set up the computer
     • Although the CPU can only read ROM, it is possible to update the BIOS on a ROM chip

5. Virtual memory

   • Computers have limited RAM
   • When RAM is full the computer uses secondary storage for data that hasn't been used recently
   • Virtual memory may be needed if there are too many applications open at once or if a memory intensive program is being used
   • If the CPU needs to read data stored in virtual memory then it must move it back to RAM which is a fairly slow process
   • Using the virtual memory can make a computer slow to respond when switching between apps.

6. CPU performance

   • Clock speed
     • The number of instructions a single processor can do in a second. For most computers it is around 3.5GHz (3.5 billion)
     • The higher the clock speed the greater number of instructions the computer can carry out per second
     • Some CPUs can be overclocked to make them run at a higher clock speed than the factory set rate. Risky if not done properly as it can overheat
   • Number of cores
     • The more cores a CPU has, the more instructions it can carry out at once
     • Each core in a CPU can process data independently
     • Most Pcs and smartphones have 4 or more cores
   • Cache size
     • The cache is data storage inside the CPU
     • Much faster than RAM
     • A larger CPU cache gives the CPU faster access to more data it needs to process

7. GPUs

   • Specialised circuits for handling graphics and image processing. they relieve the processing load on the CPU, freeing it to do other things
   • Computers have basic GPUs integrated onto the motherboard or the CPU
   • For better graphics more dedicated graphics card is used
   • Using high-end graphics cards can greatly improve performance in graphics intensive applications

8. The operating system

   • Communicate with internal and external hardware
   • Provide a user interface allowing a user to interact with the computer and vice versa
   • Provide a platform for different applications to run
   • Allow the computer to multi-task by controlling memory resources and the CPU

9. Secondary Storage

   • HDDs
     • HDDs are cheaper
     • Both are high capacity, but HDDs are higher
     • HDDs have a longer read/write life than SSDs. SSDs can only be written a certain number of times
   • SSDs
     • SSDs are faster
     • SSDs dont need defragmenting
     • SSDs are more shock-proof than HDDs