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Because of this sharing, the bus protocol (set of usage rules) is very important.
Figure 4.2 shows a typical bus consisting of data lines, address lines, control
lines, and power lines. Often the lines of a bus dedicated to moving data are
called the data bus. These data lines contain the actual information that must be
moved from one location to another. Control lines indicate which device has permission
to use the bus and for what purpose (reading or writing from memory or
from an I/O device, for example). Control lines also transfer acknowledgments
for bus requests, interrupts, and clock synchronization signals. Address lines indicate
the location (in memory, for example) that the data should be either read
from or written to. The power lines provide the electrical power necessary. Typical
bus transactions include sending an address (for a read or write), transferring
data from memory to a register (a memory read), and transferring data to the
memory from a register (a memory write). In addition, buses are used for I/O
reads and writes from peripheral devices. Each type of transfer occurs within a
bus cycle, the time between two ticks of the bus clock.
Due to the different types of information buses transport and the various
devices that use the buses, buses themselves have been divided into different
types. Processor-memory buses are short, high-speed buses that are closely
matched to the memory system on the machine to maximize the bandwidth
(transfer of data) and are usually very design specific. I/O buses are typically
longer than processor-memory buses and allow for many types of devices with
varying bandwidths. These buses are compatible with many different architectures.
A backplane bus (Figure 4.3) is actually built into the chassis of the
machine and connects the processor, the I/O devices, and the memory (so all
devices share one bus). Many computers have a hierarchy of buses, so it is not
uncommon to have two buses (for example a processor-memory bus and an I/O
bus) or more in the same system. High-performance systems often use all three
types of buses.
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