Hardware Reference
In-Depth Information
Early personal computers had a single external bus or
system bus
. It consisted
of 50 to 100 parallel copper wires etched onto the motherboard, with connectors
spaced at regular intervals for plugging in memory and I/O boards. Modern per-
sonal computers generally have a special-purpose bus between the CPU and mem-
ory and (at least) one other bus for the I/O devices. A minimal system, with one
memory bus and one I/O bus, is illustrated in Fig. 3-35.
CPU chip
Buses
Registers
Memory bus
Bus
controller
Memory
I/O bus
ALU
Disk
Network
Printer
On-chip bus
Figure 3-35.
A computer system with multiple buses.
In the literature, buses are sometimes drawn as ''fat'' arrows, as in this figure.
The distinction between a fat arrow and a single line with a diagonal line through it
and a bit count next to it is subtle. When all the bits are of the same type, say, all
address bits or all data bits, then the short-diagonal-line approach is commonly
used. When address, data, and control lines are involved, a fat arrow is more com-
mon.
While the designers of the CPU are free to use any kind of bus they want in-
side the chip, in order to make it possible for boards designed by third parties to
attach to the system bus, there must be well-defined rules about how the external
bus works, which all devices attached to it must obey. These rules are called the
bus protocol
. In addition, there must be mechanical and electrical specifications,
so that third-party boards will fit in the card cage and have connectors that match
those on the motherboard mechanically and in terms of voltages, timing, etc. Still
other buses do not have mechanical specifications because they are designed to be
used only within an integrated circuit, for example, to connect components toget-
her within a system-on-a-chip (SoC).
A number of buses are in widespread use in the computer world. A few of the
better-known ones, historical and current (with examples), are the Omnibus
(PDP-8), Unibus (PDP-11), Multibus (8086), VME bus (physics lab equipment),
IBM PC bus (PC/XT), ISA bus (PC/AT), EISA bus (80386), Microchannel (PS/2),
Nubus (Macintosh), PCI bus (many PCs), SCSI bus (many PCs and workstations),
