The I/O Bus
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The I/O Bus A personal computer may transfer data from disk to CPU, from CPU to memory, or from memory to the display adapter. A PC cannot afford to have separate circuits between every pair of devices. A mechanical switch, like the old phone systems used, would be too slow. The solution is a Bus. The Bus is simply a common set of wires that connect all the computer devices and chips together. Some of these wires are used to transmit data. Some send housekeeping signals, like the clock pulse. Some transmit a number (the "address") that identifies a particular device or memory location. The computer chips watch the address wires and respond when their identifying number is transmitted. They then transfer data on the other wires. From the first IBM PC up through the first PS/2 computers (introduced in 1987) a computer had one bus and all of its devices and chips ran at the same speed. On those systems, additional computer memory was often added by plugging an adapter card into the same slots that held I/O adapters. Starting with machines that used the 386 CPU, the memory and CPU of the system ran faster than the I/O devices. The solution was to separate the CPU and memory from all the I/O. Today, memory is only added by plugging it into special sockets on the main computer board. Whether there is more than one Bus, or one Bus with different speeds, is a matter of perspective. A car drives down the local streets at 25 miles per hour. Then it turns onto a highway ramp and accelerates to 55. Is there one road system, or two? The important thing is that there is a connection that allows a flow of traffic between the two speed zones. Within the PC, data can flow from any chip to any other chip, but different parts of the path run at different speeds. Analogy Alert: Electricity flows through wire at the same speed everywhere (at about the speed of light). So a "faster" bus is not one where the electrons move faster, but rather one in which the time between meaningful events (the "clock speed") is faster. On a faster bus the chips have to react more quickly, and the engineering has to be more rigorous so that voltage signals can be clean and tight. In a modern PC, there may be a half dozen different Bus areas. There is certainly a "CPU area" that still contains the CPU, memory, and basic control logic. There is a "High Speed I/O Device" area that is either a VESA Local Bus (VLB) or an PCI Bus. An very low cost home computer may have no high speed devices. A more typical desktop system connects the high speed bus on the mainboard to the display adapter and IDE disk interface chip. Then one or two extra I/O slots may allow adapter cards to connect to the high speed bus. The remaining I/O device slots support standard "ISA" bus cards. Some computers will also provide sockets for a number of PCMCIA "credit card" adapters commonly found in laptop computers. |
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