Hardware Reference
In-Depth Information
Monitor
Hard
disk drive
CD-ROM
drive
Keyboard
Hard
disk
controller
CD-ROM
controller
Video
controller
Keyboard
controller
CPU
Memory
Bus
Figure 2-30.
Logical structure of a simple personal computer.
The bus is used not only by the I/O controllers but also by the CPU for fetch-
ing instructions and data. What happens if the CPU and an I/O controller want to
use the bus at the same time? The answer is that a chip called a
bus arbiter
decides who goes next. In general, I/O devices are given preference over the CPU,
because disks and other moving devices cannot be stopped, and forcing them to
wait would result in lost data. When no I/O is in progress, the CPU can have all
the bus cycles for itself to reference memory. However, when some I/O device is
also running, that device will request and be granted the bus when it needs it. This
process is called
cycle stealing
and it slows down the computer.
This design worked fine for the first personal computers, since all the compo-
nents were roughly in balance. However, as the CPUs, memories, and I/O devices
got faster, a problem arose: the bus could no longer handle the load presented. On
a closed system, such as an engineering workstation, the solution was to design a
new and faster bus for the next model. Because nobody ever moved I/O devices
from an old model to a new one, this approached worked fine.
However, in the PC world, people often upgraded their CPU but wanted to
move their printer, scanner, and modem to the new system. Also, a huge industry
had grown up around providing a vast range of I/O devices for the IBM PC bus,
and this industry had exceedingly little interest in throwing out its entire invest-
ment and starting over. IBM learned this the hard way when it brought out the suc-
cessor to the IBM PC, the PS/2 range. The PS/2 had a new, faster bus, but most
clone makers continued to use the old PC bus, now called the
ISA
(
Industry Stan-
dard Architecture
) bus. Most disk and I/O device makers also continued to make
controllers for it, so IBM found itself in the peculiar situation of being the only PC
maker that was no longer IBM compatible. Eventually, it was forced back to sup-
porting the ISA bus. Today the ISA bus has been relegated to ancient systems and
computer museums, since it has been replaced by newer and faster standard bus
