Unformatted text preview: the instructions in the instruction set of its predecessor CPU, plus some new
ones. This manufacturing strategy is known as upward compatibility, and the new CPU is
said to be upward compatible with its predecessor. This feature allows software written
for a computer with a particular CPU to work on computers with newer processors of the
same family. In turn, it allows the users of these computer systems to easily upgrade their
system without worrying about converting all their existing software.
As the instructions are interpreted and executed by the CPU, there is a movement of
information between the various units of the computer system. In order to handle this
process satisfactorily and to speed up the rate of information transfer, the computer uses a
number of special memory units called registers. These registers are used to hold
information on a temporary basis and are part of the CPU (not main memory). The length of a register equals the number of bits it can store. Thus a register that can
store 8 bits is normally referred to as an 8-bit register. Most CPUs sold today have 32-bit
or 64-bit registers. The size of the registers, which is sometimes called the word size,
indicates the amount of data, which the computer can process in a given period. The
bigger the word size, the faster the computer can process a set of data. With all other
parameters being same, a CPU with 32-bit registers can process data twice as fast as one
with 16-bit registers.
The number of registers varies among computers as does the data-flow pattern. Most
computers use several types of registers, each designed to perform a specific function.
Each of these registers possesses the ability to receive information, to hold it temporarily,
and to pass it on as directed by the control unit. Although the number of registers varies
from computer to computer, there are some registers that are common to all computers.
The function of these registers is described below.
Memory Address Register (MAR)....
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