Main memory provides random access to data and instructions. Today all manufacturers measure memory
with a count of 8-bit bytes. Most machines, regardless of 16-bit, 32-bit, or 64-bit word size, also offer byte
addressability.
Since access to memory takes longer than access to registers on the CPU itself, modern designs incorporate
cache memory near the CPU to provide a copy of the contents of a section of main memory in order to obviate
the need to read from main memory so frequently. Cache memory entails complexity to manage cache
coherency, but it typically results in speedup of average memory access time by 50 percent.
Input and output functions today are based on I/O controllers, which are small special-purpose computers
built to control the details of the I/O device, and provide a local memory buffer for the information being
transferred in or out. Computers today use an interrupt system to allow the CPU to process other work while
I/O occurs under the supervision of the I/O controller. When the transfer is complete, the I/O controller notifies
the CPU by generating an interrupt.
A further improvement in I/O efficiency is direct memory access (DMA). A DMA controller is another
special-purpose computer within the computer, and it shares access to main memory with the CPU.
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