| The central processing unit, CPU, or processor, is the nerve center of the computer system. It performs the central control functions. All the computational, logical, and operational decisions are made here. It contains the logic circuitry for performing the various computational activities. It controls the operation of all the functional units. It fetches machine instructions from memory, decodes these instructions and insures that the operations called for by the instructions are executed correctly. In order to do all this, it communicates or interfaces with the input and output units and the memory. |
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| Digitally coded information travels among the CPU, memory, and Input/Output units on electrical connections called busses. Each I/O unit of the computer system is given an address and each information location in memory is given an address. To locate specific information in memory, a digital code - the address of the information - is sent on the address bus to memory by the CPU. At the same time, other digital codes representing control signals are sent on the control bus to tell the memory what to do - either to read or to write information from the memory location designated by the address on the address bus. |
| The CPU controls when and at what time each operation is to be accomplished. It does this by following instructions stored in memory in a step-by-step sequence called the program. Inside the computer, the information is not recognizable as written letters or numbers because it is all converted to electrical signals of digital codes so that it may be handled easily inside the computer system. The digital codes carrying the information inside the computer system must be coupled from one unit to another. The multiple wires which do this are called busses. The three principal busses in a computer system - the address bus, the control bus, and the data bus - are illustrated to the right. |
| Sometimes an additional special processor is used to provide more efficient system operations by relieving some of the control requirements of the CPU. When used as such, the I/O processor is referred to as a data channel, direct memory access channel, or peripheral processor. For more efficiency, some computer systems use a supplementary processor to control the interaction between the CPU and external devices. |
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| When punched cards (containing the instructions that make up the computer program and the data to be used in the program) were still used, they were read into the system by a card reader and stored in memory.
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Each instruction was then read from memory (fetched) by the CPU in the order directed by the program. This is still the way the computer operates today. The CPU interprets (decodes) the instruction and performs (executes) the operation called for in the instruction. The final results are normally outputted to the printer. Intermediate or temporary results are stored (stores) in memory for later use. This typical sequence of events is known as one machine cycle: the program and data are encoded and read into computer memory (the first two steps together are called instruction time or I-time). The CPU executes the program, and the final results are outputted to a printer (the last two steps together are called exectution time or E-time). |
| Logical functions like AND, OR, and NOT form the basis of many different types of digital devices. They are called logic circuits because they make logic decisions electronically. The logic circuit is called such because electronically it performs the logical AND function. Decisions such as "A" AND "B", "A" OR "B" and NOT "A" or NOT "B" being a particular value are the types used over and over again in computer systems hardware. These circuits can be represented symbolically. Individual inputs and outputs in digital circuits are always in either of only two possible states: "1" or "0" (high/on or low/off) (the binary number system). The signal level can only have one of two states; therefore, it is called a binary circuit and it can only represent one bit of information which can either have a value of "1" or "0". A "truth table," illustrated below, is used to demonstrate the combination of logical or binary input signals which yield a binary output for a specific gate type.The output goes high when both inputs are high. The output drops when either input drops. Output "C" is a "1" only when "A" AND "B" are both a "1" at the same time. The OR gate is different from the AND gate in that it requires only one of the two input signals to be a one (or 2.4 volts) at any given instant of time in order for the output "C" to be a logical one. Only when both of the input signals "A" and "B" are zero ("0") will the output signal "C" be zero. The NOT gate simply performs an inversion on a signal. When the input signal is a one, the output is a zero and visa versa. |
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| Integrated circuits contributed significantly to the advancement of the use of computers. Large numbers of logic circuits are contained on a single integrated circuit. The density of modern IC's is possible in part because of the similarity of the circuitry in them. The computer is made up of thousands and sometimes millions of such circuits. These circuits handle the digital information in digital codes, detect the codes, and execute the operations called for in the codes. Because all of this is being done within the machine in digital code, the codes are called machine language or machine instructions. |
| 0011 0111 0110 0011 1001 0101 1101 0011 0110 1111 1111 1010 1111 0011 1001 1011 1011 0001 1100 1010 1110 1001 1011 1010 1011 1101 0010 1001 1100 1101 1101 1000 1110 0101 1111 0100 1101 1001 0101 1111 1001 0100 1110 0110 1110 1100 0111 0010 1011 1010 0110 1111 1111 1111 1111 0100 1010 0111 1011 1111 0110 0011 1001 0101 1101 0011 1111 0101 0111 1111 0101 0011 1001 1011 1011 0001 1100 1110 1110 1001 1011 0010 1011 1101 0110 1111 1101 1101 1101 1000 1111 0101 0111 0100 1101 1001 0101 1110 1111 0100 1110 0101 0100 1001 1100 1101 1000 1110 0101 1111 0100 1111 |