COMPUTER SYSTEMS ORGANIZATION - Structured Computer Organization - page 109

for the politically correct). The motherboard contains the CPU chip, some slots
into which DIMM modules can be clicked, and various support chips. It also con-
tains a bus etched along its length, and sockets into which the edge connectors of
I/O boards can be inserted.
SCSI controller
Sound card
Modem
Card cage
Edge connector
Figure 2-29. Physical structure of a personal computer.
The logical structure of a simple personal computer is shown in Fig. 2-30.
This one has a single bus used to connect the CPU, memory, and I/O devices; most
systems have two or more buses. Each I/O device consists of two parts: one con-
taining most of the electronics, called the controller , and one containing the I/O
device itself, such as a disk drive. The controller is usually integrated directly onto
the motherboard or sometimes contained on a board plugged into a free bus slot.
Even though the display (monitor) is not an option, the video controller is some-
times located on a plug-in board to allow the user to choose between boards with
or without graphics accelerators, extra memory, and so on. The controller connects
to its device by a cable attached to a connector on the back of the box.
The job of a controller is to control its I/O device and handle bus access for it.
When a program wants data from the disk, for example, it gives a command to the
disk controller, which then issues seeks and other commands to the drive. When
the proper track and sector have been located, the drive begins outputting the data
as a serial bit stream to the controller. It is the controller's job to break the bit
stream up into units and write each unit into memory, as it is assembled. A unit is
typically one or more words. A controller that reads or writes data to or from
memory without CPU intervention is said to be performing Direct Memory Ac-
cess , better known by its acronym DMA . When the transfer is completed, the con-
troller normally causes an interrupt , forcing the CPU to immediately suspend run-
ning its current program and start running a special procedure, called an interrupt
handler , to check for errors, take any special action needed, and inform the operat-
ing system that the I/O is now finished. When the interrupt handler is finished, the
CPU continues with the program that was suspended when the interrupt occurred.