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Personal computers have their own terminology when it comes to buses. PCs
have an internal bus (called the system bus) that connects the CPU, memory, and
all other internal components. External buses (sometimes referred to as expansion
buses) connect external devices, peripherals, expansion slots, and I/O ports to the
rest of the computer. Most PCs also have local buses, data buses that connect a
peripheral device directly to the CPU. These are very high-speed buses and can
be used to connect only a limited number of similar devices. Expansion buses are
slower but allow for more generic connectivity. Chapter 7 deals with these topics
in great detail.
Buses are physically little more than bunches of wires, but they have specific
standards for connectors, timing, and signaling specifications and exact protocols
for usage. Synchronous buses are clocked, and things happen only at the clock
ticks (a sequence of events is controlled by the clock). Every device is synchronized
by the rate at which the clock ticks, or the clock rate. The bus cycle time
mentioned earlier is the reciprocal of the bus clock rate. For example, if the bus
clock rate is 133MHz, then the length of the bus cycle is 1/133,000,000 or 7.52ns.
Because the clock controls the transactions, any clock skew (drift in the clock) has
the potential to cause problems, implying that the bus must be kept as short as
possible so the clock drift cannot get overly large. In addition, the bus cycle time
must not be shorter than the length of time it takes information to traverse the
bus. The length of the bus, therefore, imposes restrictions on both the bus clock
rate and the bus cycle time.
With asynchronous buses, control lines coordinate the operations and a complex
handshaking protocol must be used to enforce timing. To read a word of data from
memory, for example, the protocol would require steps similar to the following:
1. ReqREAD: This bus control line is activated and the data memory address is put
on the appropriate bus lines at the same time.
2. ReadyDATA: This control line is asserted when the memory system has put the
required data on the data lines for the bus.
3. ACK: This control line is used to indicate that the ReqREAD or the Ready-
DATA has been acknowledged.
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