1 6 Further System Fundamentals (HL) 6.5 Computer – Peripheral Communication

6.5.1 DMA and Buffers

3 Buffers The CPU is usually the fastest component in the system. Peripherals such as printers may not be able to deal with data as fast, so areas of RAM called buffers are used as temporary stores, allowing the CPU to take on other tasks while waiting for more data.

4 Buffers When reading or writing to hard disk, the data to be saved are first saved in a buffer, then written to disk when convenient, then the buffer is flushed. In a command line interface, keystrokes are buffered until the Enter key is pressed which generates an interrupt for the CPU to flush the buffer and attempt to interpret the command.

5 Double buffering Two (or more) buffers can speed up data transfer – while one is filling, the other can be being read. Also useful if two jobs (e.g. a read and a write) are occurring at the same time.

6 Direct Memory Access Recent trend devolve control of data transfer out to peripherals. To allow faster access (without the direct intervention of the CPU), peripherals such as hard disks can be accessed directly. Effectively, the disk has a memory address and is treated as if it were part of RAM.

7 Direct Memory Access Such DMA is useful for tasks involving transfer of a lot of data e.g. background backups. Compared with interrupts and polling (see next section), DMA is under the control of the device controller chip rather than the CPU, so the CPU is freed for other tasks.