RISC versus CISC
by Lloyd Borrett
Australian Personal Computer, June 1991
microprocessors used in personal computers become faster and
more powerful they are increasingly being compared with the
processors used in the low-end workstation and minicomputer
market. Essentially the debate becomes a comparison of
Reduced Instruction Set Computing (RISC) and Complex
Instruction Set Computing (CISC).
History of RISC and CISC
was developed at IBM in the late 1960s before
microprocessors were even invented. Yet despite that, until
recent times, all of the major manufacturers of
microprocessors used CISC based designs.
The reason for
this is that microprocessors were introduced in the early
1970s to be used in simple electronic products such as
calculators, stereos and video games, not personal
computers. CISC technology was more suitable for this
CISC based microprocessors found their way into personal
computers. The PCs evolved with generation after generation
of CISC based microprocessors being introduced to meet the
ever increasing demands of PC users. The microprocessor
manufacturers began to refocus their CISC microprocessor
design efforts away from general purpose designs to a
high-performance computing orientation.
RISC designs made
a comeback in the mid 1980s via workstations. The first
workstations were based on CISC designs, but the workstation
manufacturers were keen to find a technology that broke the
market control of Intel and Motorola.
designs were able to be produced and implemented faster than
CISC based designs. More importantly the workstation
manufacturers could be seen to be offering a very different
product than that offered by personal computer
An instruction is
the lowest level command that can be executed by the
microprocessor. The instruction set is simply the list of
instructions that a microprocessor will understand. CISC
microprocessors typically have 100-150 instructions, while
RISC microprocessors tend to have 70-80 instructions.
Many of the extra
instructions on CISC microprocessors are really supersets of
the simpler instructions in a RISC instruction set. Thus
CISC microprocessors are complex in two senses: they have
more instructions and some of these instructions are
The speed at
which a computer can accomplish any task is a function of
the number of instructions that the task has to be broken in
to, the number of cycles it takes to complete one
instruction, and the number of cycles that can be
accomplished in one second.
Speed = (Instructions/Task) *
(Cycle/Instruction) * (Time/Cycle)
microprocessors aim to achieve speed by having such a rich
instruction set that it takes fewer instructions to complete
a task. RISC microprocessors require more instructions to
complete a given task, but they focus on reducing the number
of cycles it takes to complete an instruction. Clock cycles
per second is largely independent of the microprocessor
The case for
higher performance of RISC based microprocessors is based on
the premise that superiority in instructions per clock cycle
(the MIPS rating) more than offsets the handicap of
requiring more instructions to complete the task. This is
highly situation dependent and the factors involved include
system hardware configuration, memory speed and capacity, as
well as the application software.
improvements offered in current RISC based workstations has
very little to do with inherent differences in RISC/CISC
architectures. The Intel 80486 and Motorola 68040 are the
first generation of CISC microprocessors that specifically
set speed as a performance objective. There performance
compares very well with RISC microprocessors.
The factors that
make the difference are the non RISC/CISC parts of the
systems, that is, register capacity, cache memory,
pipelining and floating point processing. Thus it would be
easy to see CISC processors evolve and match the RISC
performance, in part by adopting some of the features
currently associated with RISC but that are in fact
independent of it.
We believe that
the future will see a convergence of microprocessor
architectures as the CISC designers adopt some of the
features of RISC microprocessors, and RISC microprocessors
enlarge their instruction set so as to improve performance.
In fact we're
already seeing this. The IBM RS/6000 microprocessor that
powers IBM's RISC System/6000 systems is actually more
complex than the CISC based design of the Intel 80386. The
RS/6000 has 184 instructions and the 80386 has 150
- RISC and CISC
are merely different design methodologies. Neither is
inherently superior to the other.
architectures are equally capable of evolving to the
levels of performance being claimed by RISC.
of microprocessor based computers is a function of more
factors than just the microprocessor.
Saturday, 15 October 2011