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Personal computers have their own terminology when it comes to buses. PCshave an internal bus (called the system bus) that connects the CPU, memory, andall other internal components. External buses (sometimes referred to as expansionbuses) connect external devices, peripherals, expansion slots, and I/O ports to therest of the computer. Most PCs also have local buses, data buses that connect aperipheral device directly to the CPU. These are very high-speed buses and canbe used to connect only a limited number of similar devices. Expansion buses areslower but allow for more generic connectivity. Chapter 7 deals with these topicsin great detail.Buses are physically little more than bunches of wires, but they have specificstandards for connectors, timing, and signaling specifications and exact protocolsfor usage. Synchronous buses are clocked, and things happen only at the clockticks (a sequence of events is controlled by the clock). Every device is synchronizedby the rate at which the clock ticks, or the clock rate. The bus cycle timementioned earlier is the reciprocal of the bus clock rate. For example, if the busclock 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) hasthe potential to cause problems, implying that the bus must be kept as short aspossible so the clock drift cannot get overly large. In addition, the bus cycle timemust not be shorter than the length of time it takes information to traverse thebus. The length of the bus, therefore, imposes restrictions on both the bus clockrate and the bus cycle time.With asynchronous buses, control lines coordinate the operations and a complexhandshaking protocol must be used to enforce timing. To read a word of data frommemory, 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 puton the appropriate bus lines at the same time.2. ReadyDATA: This control line is asserted when the memory system has put therequired 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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