Microprocessors (Page 2 of 2)

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A microprocessor uses the instruction register to store the instruction code last fetched from memory.  The first byte of an instruction code is fed by the instruction register to the instruction decoder, which 'decodes' it to determine which operation must be carried out, how many bytes of data will be processed, and where to get these data. After instruction decoding, the execution of the instruction proceeds.

Registers are elements composed of a set of flip-flops where data are stored temporarily for subsequent processing or transfer, as the microprocessor goes about its task of executing its instructions one at a time. The accumulator is a special register used by the microprocessor for holding operands, or data to be manipulated by the ALU.  Aside from the accumulator, several general-purpose registers are also available to the microprocessor for holding data that need to be operated on.

Microprocessors also have Status Flags, which are really just special registers for storing the state of a condition that results from a previous operation.  Examples of status flags include: 1) the Carry Status Flag, which indicates if there's a need to do a 'carry' after addition or a 'borrow' after subtraction; 2) the Zero Status Flag, which indicates if a given operation in the ALU results in a 'zero'; 3) the Sign Status Flag, which indicates whether the result of an ALU operation is negative or positive; 4) the Overflow Status Flag, which indicates if an operation produces a result that can't fit into the specified word length; and 5) the Parity Status Flag, a flag (used in error detection) that is set if the result of an operation contains an even number of 1's.    

The microprocessor has been around for more than two decades already.  It now comes in many forms, sizes and levels of sophistication, powering all kinds of applications that rely on 'computer control'.  Although it is the central processing unit of a computer system, it also needs to interact with other semiconductor devices in order to perform its functions.  These 'other' devices include the memory and input/output devices that constitute the rest of the computer system.