12.4 IO Speed Hierarchy

12.4 I/O Speed Hierarchy

Different peripheral devices have different data transfer rates. Some devices, like keyboards, are extremely slow when compared to CPU speeds. Other devices, like disk drives , can actually transfer data faster than the CPU can process it. The appropriate programming technique for data transfer depends strongly on the transfer speed of the peripheral device involved in the I/O operation. Therefore, in order to understand how to write the most appropriate code, it first makes sense to invent some terminology to describe the different transfer rates of peripheral devices.

• Low-speed devices Devices that produce or consume data at a rate much slower than the CPU is capable of processing. For the purposes of discussion, we'll assume that low-speed devices operate at speeds that are two or more orders of magnitude slower than the CPU.

• Medium-speed devices Devices that transfer data at approximately the same rate as, or up to two orders of magnitude slower than, the CPU.

• High-speed devices Devices that transfer data faster than the CPU is capable of handling using programmed I/O.

The speed of the peripheral device will determine the type of I/O mechanism used for the I/O operation. Clearly, high-speed devices must use DMA because programmed I/O is too slow. Medium- and low-speed devices may use any of the three I/O mechanisms for data transfer (though low-speed devices rarely use DMA because of the cost of the extra hardware involved).

With typical bus architectures, personal computer systems are capable of one transfer per microsecond or better. Therefore, high-speed devices are those that transfer data more rapidly than once per microsecond. Medium-speed transfers are those that involve a data transfer every 1 to 100 microseconds. Low-speed devices usually transfer data less often than once every 100 microseconds. Of course, these definitions for the speed of low-, medium-, and high-speed devices are system dependent. Faster CPUs with faster buses allow faster medium-speed operations.

Note that one transfer per microsecond is not the same thing as a 1-MB-per-second transfer rate. A peripheral device can actually transfer more than one byte per data transfer operation. For example, when using the 80x86 in( dx, eax ); instruction, the peripheral device can transfer four bytes in one transfer. Therefore, if the device is reaching one transfer per microsecond, the device can transfer 4 MB per second using this instruction.