Features of 32-bit and 64-bit Microprocessors

In this section we describe the basic aspects of typical 32- and 64-bit microprocessors.
Topics include on-chip features such as pipelining, memory management, floating-
point, and cache memory implemented in typical 32- and 64-bit microprocessors. The
first 32-bit microprocessor, Intel’s problematic iAPX432, was introduced in 1980. Soon
afterward, the concept of mainji-ame on a chip or micromainframe was used to indicate the
capabilities of these microprocessors and to distinguish them from previous 8- and 16-bit
microprocessors.

The introduction of several 32-bit microprocessors revolutionized the
microprocessor world. The performance of these 32-bit microprocessors is actually more
comparable to that of superminicomputers such as Digital Equipment Corporation’s
VAXl1/750 and VAX11/780. Designers of 32-bit microprocessors have implemented
many powerful features of these mainframe computers to increase the capabilities of
microprocessor chip sets: pipelining, on-chip cache memory, memory management, and
floating-point arithmetic.In pipelining, instruction fetch and execute cycles overlap. This method allows simultaneous preparation for execution of one or more instructions while another instruction is being executed. Pipelining was used for many years in mainframe and minicomputer CPUs to speed up the instruction execution time of these machines. The 32-bit microprocessors implement the pipelining concept and operate simultaneously on several 32-bit words, which may represent different instructions or part of a single instruction.

Although pipelining greatly increases the rate of execution of nonbranching code,
pipelines must be emptied and refilled each time a branch or jump instruction appears in
the code. This may slow down the processing rate for code with many branches or jumps.
Thus, there is an optimum pipeline depth, which is strongly related to the instruction set,
architecture, and gate density attainable on the processor chip.With memory management, virtual memory techniques, traditionally a feature of mainframes, are also implemented as on-chip hardware on typical 32-bit microprocessors.
This allows programmers to write programs much larger than those that could fit in the
main memory space available to microprocessors; the programs are simply stored on a
secondary device such as a hard disk, and portions of the program are swapped into main
memory as needed.Segmentation circuitry has been included in many 32-bit microprocessor chips.With this technique, blocks of code called segments, which correspond to modules of the
program and have varying sizes set by the programmer or compiler, are swapped. For many
applications, however, an alternative method borrowed from mainframes and superminis
calledpaging is used. Basically, paging differs from segmentation in that pages are of equal
size. Demandpaging, in which the operating system swaps pages automatically as needed,
can be used with all 32-bit microprocessors।

The 64-bit microprocessors such as Power PC 750 include all the features of 32-bit
microprocessors. In addition, they contain multiple on-chip integer and floating-point units
and a larger address and data buses. The 64-bit microprocessors can typically execute four
instructions per clock cycle and can run at a clock speed of over 2 GHz. The original Pentium
microprocessor is a CISC microprocessor. Pentium Pro and other succeeding members of
the Pentium family are designed using a combination of mostly microprogramming (CISC)
and some hardwired control (RISC) whereas the PowerPC is designed using hardwired
control with almost no microcode. The PowerPC is a RISC microprocessorand therefore
includes a simple instruction set. This instruction set includes register-to-register, load, and
store instructions. All instructions involving arithmetic operations use registers; load and
store instructions are utilized to access memory. Almost all computations can be obtained
from these simple instructions. Finally, 64-bit microprocessors are ideal candidates for
data-crunching machines and high-performance desktop systems and workstations.