1.1 Performance of the CPU Flashcards
Factors affecting the speed of the CPU
clock speed
cache size
number of cores
What is the clock and how does it control clock speed?
a vibrating quartz crystal
the faster it vibrates the faster the instructions are processed - at least 1 per cycle
Why can’t clock speed be increased indefinitely?
the processor generates a lot of heat
the amount increases with the rate at which it processes instructions
too much heat causes it to malfunction
clock speed and heat production
the faster the clock speed, the hotter it gets, which causes it to malfunction
the processor must have a heat sink and a fan to dissipate this heat
liquid nitrogen is needed to cool supercomputers with clock speeds of 9 GHz
clock speed
the number of cycles per second
measured in GHz (billions of cycles per second)
it is the number of instructions the CPU is capable of fetching, and is controlled by the CU
the faster the internal clock ticks, the quicker the data moves around the CPU and the bigger the clock speed
cache memory
- very fast memory, usually within the processer itself
- stores recently or frequently used instructions so that they do not have to be fetched from the slower main memory
- cache memory is too expensive to use for the main memory (RAM) in personal computers
How is the cache managed to prevent it slowing down when it becomes larger?
- as the cache becomes larger it takes longer to find the data
- it is split into levels, e.g. L1, L2 with the smallest nearest to the CPU
(L1 is the smallest)
cache size and CPU efficiency
- a small amount makes a significant difference
- the more instructions are stored in the cache, the more efficient the CPU is
multi-core processors
- cores are everything inside the CPU except the CU
- contain more than one CPU
- give faster processing speeds than single cores
- can work on different tasks at the same time
- duel cores are said to double the speed
What are multiple cores controlled by?
one CU
parallel processing
multiple cores work together on the same program
multitasking
the cores work on different programs at the same time
disadvantages of more cores
not all programs will run faster
some programs may be sequential so that one task requires output from a previous task, so the second can’t be started until the first has finished
more cores can be less efficient as they have to work together
Explain why cache size affects the performance of the CPU [4]
cache is high speed memory used to store frequently used program instructions
the larger the cache size, the less often the slower main memory needs to be accessed, speeding up data transfer
the performance of the CPU will be improved as it does not have to wait as long for the data to be delivered
however if the cache becomes too large then the data access time increases and so the cache is split into units, e.g. L1, L2, L3
Explain why increasing the number of cores in a processor does not neccessarily increase the rate at which all programs are processed [2]
not all programs are written to take advantage of parallel processing and some processes may not be able to start until others have finished as they require input from them
