Information Technology Reference
In-Depth Information
install wiring in a room, a plumber might run a pipe to a drain, and a
wall finisher might install sheetrock in a room. At various times in the
day, each worker may report that the required task is done, and the
manager then assigns the next job for that specialty.
For example, to print, the CPU may tell a processing chip locat
ed in an I/O device that data in a specified area of main memory
should be printed on a designated printer. This separate I/O chip
then can handle the details of moving the data to the printer, and the
printer can follow the steps required to shape the relevant characters
or graphical elements on the paper. When the printing job is com
plete, the separate I/O chip generates an interrupt, indicating the
work is done. Thus, instead of taking time to interact with the print
er and move data, the CPU simply issues one command to the I/O
chip and then moves on to other tasks. The CPU decides what doc
uments are printed when, but the mechanics are offloaded to other
components.
The CPU also delegates tasks to processing chips located in I/O
devices when reading from the keyboard, moving data between
main memory and a disk, and interacting with outside computer
networks. In this way, the CPU saves its own time for processing of
other data.
The “brain” of a computer is the CPU. As we have already
noted, in real computers, the modern CPU chip is a combination of
several smaller components; typically registers, circuitry for various
processing tasks (e.g., addition of numbers), and cache are packaged
together within a single chip.
To see just how this works, consider an early PowerPC chip, the
603, built through a combined effort of Motorola and IBM. The
actual chip is built on a wafer of silicon and measures just 7.4 by
11.5 millimeters. Circuits etched within the chip involve some 1.6
million transistors. Although such circuits are much too small for an
unaided eye to distinguish, Figure 1.4 shows a greatly enlarged pic
ture of the basic PowerPC 603 circuitry.
Although many details of this CPU chip are well beyond the
scope of this topic, we can relate many of the main sections of the
chip to the basic components already discussed.
