Intel 8080 manual Typical Computer System, Architecture of a CPU, Accumulator

This chapter introduces certain basic computer con- cepts. It provides background information and definitions which will be useful in later chapters of this manual. Those already familiar with computers may skip this material, at their option.

A TYPICAL COMPUTER SYSTEM

A typical digital computer consists of:

a)A central processor unit (CPU)

b)A memory

c)Input/output (I/O) ports

The memory serves as a place to store Instructions, the coded pieces of information that direct the activities of the CPU, and Data, the coded pieces of information that are processed by the CPU. A group of logically related instruc- tions stored in memory is referred to as a Program. The CPU "reads" each instruction from memory in a logically deter- mined sequence, and uses it to initiate processing actions. If the program sequence is coherent and logical, processing the program will produce intelligible and useful results.

The memory is also used to store the data to be manip- ulated, as well as the instructions that direct that manipu- lation. The program must be organized such that the CPU does not read a non-instruction word when it expects to see an instruction. The CPU can rapidly access any data stored in memory; but often the memory is not large enough to store the entire data bank required for a particular appli- cation. The problem can be resolved by providing the com- puter with one or more Input Ports. The CPU can address these ports and input the data contained there. The addition of input ports enables the computer to receive information from external equipment (such as a paper tape reader or floppy disk) at high rates of speed and in large volumes.

A computer also requires one or more Output Ports that permit the CPU to communicate the result of its pro- cessing to the outside world. The output may go to a dis- play, for use by a human operator, to a peripheral device that produces "hard-copy," such as a line-pri nter, to a

peripheral storage device, such as a floppy disk unit, or the output may constitute process control signals that direct the operations of another system, such as an automated assembly line. Like input ports, output ports are addressable. The input and output ports together permit the processor to communicate with the outside world.

The CPU unifies the system. It controls the functions performed by the other components. The CPU must be able to fetch instructions from memory, decode their binary contents and execute them. It must also be able to reference memory and I/O ports as necessary in the execution of in- structions. In addition, the CPU should be able to recognize and respond to certain external control signals, such as INTERRUPT and WAIT requests. The functional units with in a CPU that enable it to perform these functions are described below.

THE ARCHITECTURE OF A CPU

A typical central processor unit (CPU) consists of the following interconnected functional units:

•	Registers	\
•	Arithmetic/Logic Unit (ALU)
•	Control Circuitry

Registers are temporary storage units within the CPU. Some registers, such as the program counter and instruction register, have dedicated uses. Other registers, such as the ac- cumulator, are for more general purpose use.

Accumulator:

The accumulator usually stores one of the operands to be manipulated by the ALU. A typical instruction might direct the ALU to add the contents of some other register to the contents of the accumulator and store the result in the accumulator itself. In general, the accumulator is both a source (operand) and a destination (result) register.

Often a CPU will include a number of additional general purpose registers that can be used to store operands or intermediate data. The availability of general purpose

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