When information is sent out to a device, the information is passed to the device controller, and the controller
is given a command by the computer to transfer the information to the device. The controller then does so. When
the transfer is complete, the controller signals the computer that the task has been accomplished.
The controller itself represents a significant off-loading of detail for the computer. For example, the
computer can tell the disk controller that it wants to write a set of bytes at a certain address (track and sector),
and the controller will take care of commanding the disk read/write heads to move, and of actually causing the
bits to be written in the right place.
In general, devices are classified as ???character??? or ???block??? devices. Roughly, these terms equate to ???slow???
or ???fast,??? respectively. A character device like a keyboard or printer transfers one character, or byte, at a time.
A block device like a disk or a tape drive transfers a whole buffer, some multiple number of bytes, at a time.
On the other hand, some devices don??™t fit in this classification, such as a monitor with ???memory mapped video.???
CHAP. 6] OPERATING SYSTEMS 99
100 OPERATING SYSTEMS [CHAP.
Pages:
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269