The CPU or processor is the brain of the computer and responds to all the commands you give the computer. It is one of the primary factors in determining the power of the system. Measured in Gigahertz, the higher the number, the more powerful the processor. Modern processors have multiple cores. You might see a dual core or quad core processor. A core is an independent processing unit, meaning the processor can execute more than one instruction at a time, so the more cores your processor has, theoretically the faster it is. The operation of most CPUs, is to execute a sequence of stored instructions called a program. The instructions are kept in the computer’s memory (or RAM). There are four steps that nearly all CPUs use in their operation: fetch, decode, execute, write back. Processors running at 2Ghz or more, do this billions of times a second.
Which CPU you use with your current motherboard depends mostly on the motherboard itself. While nearly all motherboards can be upgraded with a new processor and chipset, exactly which CPU and chipset is totally dependent on the configuration of the motherboard. If you have a Pentium 75 MHz processor and wish to move up to a Pentium III Xeon, you can count on replacing the motherboard and CPU and perhaps the power supply and more. However, if you merely want to step up to the next level of processor, as long as the processor you wish to move to is within the specification of the motherboard, the move should be fairly effortless. There are some processors, such as the Pentium Pro and Pentium II processors, which have unique motherboard configurations and aren’t typically compatible with other Pentium-based motherboards.
CPU’s Bus System
The bus, as it relates to the pathways on the computer and in the processor, carries the various signals, addresses, and data (remember, data is plural) that are transferred around the computer between its components. Although quite dissimilar, it can be related to the routes of your local transit company in many ways. On the computer, a bus structure is a group of electronic transmission lines that connect the various components of the CPU, motherboard, and expansion cards to each other. Bus structures have different sizes, ranging from 16 to 64 bits on modern microprocessors, and their size determines the amount of data that can be transmitted. Obviously, a 64-bit bus carries more data than a 16-bit bus. Within the computer, there are several bus structures, as illustrated in Figure 3-4. The
most important of these are as follows:
Data bus Carries information to and from the CPU.
Address bus Carries the address from where data is to be read to where data is to be written.
Control bus Carries the signals used by the CPU and the other components of the computer to communicate with each other, including when data is ready to be read, when another device wishes to use the bus, and the type of operation to be performed (read, write, interrupt).
CPU Wait States
It should also be noted that RAM, which operates in nanoseconds, is faster than most CPUs. This suggests a problem, but the CPU works through wait states, which are intervals of a set number of cycles between CPU actions, such as data requests, reads, writes, moves, etc., to allow the requests to be carried out. To read data from memory, the CPU may use three wait states, as illustrated in pic 1. The CPU issues the request for data along with an address. Receiving the address and transferring it to the memory controller uses about one wait state. Finding the data in memory also takes about one wait state. Transferring the data to the CPU’s storage areas (called registers) uses a third wait state.
Even if each wait state only took about 1/400 millionth of a second (based on a 400MHz CPU), RAM only requires perhaps 50 to 60ns to do its part. The significance here is that the closer the RAM’s speed is matched to that of the data bus and CPU clock, more data will be transferred fromRAMto the CPU and other components of the PC on each cycle. Another speed in the PC that must be considered is the speed of the data and address buses. Like the CPU, the bus transfer speed is in megahertz, which represents the speed used to move data and instructions between structures, such as the CPU and memory. Most RAM manufacturers include online guides on their Web sites to helpmatch RAM and RAM speeds to bus and CPU speeds. pic 2 contains a sampling of RAM speeds and matching bus speeds.
Having more RAM in the PC does not improve the overall speed of the processor, but it does improve how much data the processor can access without the need to go to the slower hard disk drive. You may have heard that adding RAM to a slow PC will speed it up. Yes, but only because the processor was able to perform faster input/output (I/O) operations.
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