COMPUTER SYSTEMS ORGANIZATION CHAP. 2
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Memory bus
CPU PCI
bridge
SCSI
controller
SCSI
disk
Network
controller
Video
controller
Printer
controller
Sound
card ISA Modem
bridge
SCSI
scanner
Main
SCSI memory
bus
PCI bus
ISA bus
cache
Figure 2-3. A typical modern PC with a PCI bus and an ISA bus. The modem
and sound card are ISA devices; the SCSI controller is a PCI device.
Keyboards
Keyboards come in several varieties. The original IBM PC came with a keyboard
that had a snap-action switch under each key that gave tactile feedback and
made a click with the key was depressed far enough. Nowadays, the cheaper keyboards
have keys that just make mechanical contact when depressed. Better ones
have a sheet of elastometric material (a kind of rubber) between the keys and the
underlying printed circuit board. Under each key is a small dome that buckles
when depressed far enough. A small spot of conductive material inside the dome
closes the circuit. Some keyboards have a magnet under each key that passes
through a coil when struck, thus inducing a current that can be detected. Various
other methods, both mechanical and electromagnetic, are also in use.
On personal computers, when a key is depressed, an interrupt is generated and
the keyboard interrupt handler (a piece of software that is part of the operating
system) is started. The interrupt handler reads a hardware register inside the keyboard
controller to get the number of the key (1 through 102) that was just
depressed. When a key is released, a second interrupt is caused. Thus if a user
depresses the SHIFT key, then depresses and releases the M key, then releases the
SHIFT key, the operating system can see that the user wants an upper case ‘‘M’’
rather than a lower case ‘‘m.’’ Handling of multikey sequences involving SHIFT,
CTRL, and ALT is done entirely in software (including the infamous CTRLALT-
DEL key sequence that is used to reboot all IBM PCs and clones