Microprocessors
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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.
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See Also:
What is a
Semiconductor?; DSP's; SRAMs; DRAMs
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