Information Technology Reference
In-Depth Information
is needed the next time, it may be retrieved from the relatively fast
cache rather than the fairly slow main memory. As this scheme may
suggest, cache can work only when coupled with an effective book
keeping system, so the location of a piece of data (in cache or main
memory) can be identified quickly. Without good bookkeeping, iden
tifying the location of data could take as long as getting the infor
mation directly from main memory—thus undermining any advan
tage cache might have. In practice, such matters can be handled well
most of the time, and a CPU will usually find most of the data it
needs in cache memory. Thus, one section of a Pentium IV chip
includes a cache memory containing a few thousand data items.
Although data still must move between cache and a Pentium's full
capability registers, the availability of the highspeed cache is very
effective in allowing the Pentium to process information efficiently.
Busses: Conceivably, increasing the number of busses in a com
puter would improve the computer's performance by allowing sev
eral pieces of data to move between components at the same time.
Unfortunately, connecting all components together with multiple
busses is quite difficult technically, because activity along one bus
might need to be coordinated with other activity along another bus.
Thus, few computers use multiple busses to connect large numbers
of components.
However, hardware designers may use specialized busses to con
nect a few (two or three) isolated components. This works particular
ly well if those elements are close to each other—perhaps parts of the
same chip. For example, busses within the Pentium connect the full
capability registers with highspeed cache. This enables data to move
between the cache and registers without interfering with data transfer
between other components over a primary bus, such as might be need
ed between an I/O device and main memory.
Moving Work Outside the CPU: Although a CPU serves as the coor
dinator of all work in a computer, common tasks sometimes are dele
gated to specialized components. The idea is that when the CPU
decides that a task is required, a separate component could be instruct
ed to perform that work rather than require the CPU do it. For coor
dination, a component usually signals when the task is done by send
ing a signal, called an
interrupt
, back to the CPU. This entire approach
is similar to the way a site manager may assign skilled workers at a
construction site. At the start of a day, one electrician may be asked to
