CISC and RISC Computers
Computers can be divided into two classes depending on their method of using instructions:
• Complex instruction set computers (CISCs)
• Reduced instruction set computers (RISCs)
The reasons for designing CISCs are to simplify compilers and to improve performance.
Underlying both of these reasons was the shift to high-level languages (HLLs) in computer programming. Computer architects attempted to design machines that provided better support for HLLs. The CISC was expected to yield smaller programs that would execute instructions more quickly.
The most basic command a computer CPU can understand and accept is an instruction set. Following any command is a two-step process for a computer. First, the computer must find the currently running program in order to figure out what step it is supposed to do next.
Then it simply follows the instructions. Based on this, computers were originally designed to run as few instructions as possible in order to maintain higher operational speeds. The premise behind this idea was that the CPU would save time by having to retrieve instructions less frequently. As an example, imagine a parent telling a child to ‘‘clean your room’’ as opposed to saying, ‘‘pick up your clothes,’’ waiting for the clothes to be picked up, then saying, ‘‘now put them in the hamper,’’ and so forth.
Computer designers soon learned that giving a complex command like ‘‘clean your
room’’ could often take longer to retrieve than several simple commands. This led to the
development of RISCs.
Today, the actual architecture and styles of the chips determine the classification of
chips because of the significant increase in chip speeds. Differentiation in instruction sets
used within the chip are no longer the primary discerning factor because both CISC and
RISC chips perform at a negligible speed difference. As an amusing point, many CISC chips
actually have a smaller instructional set than many RISC chips.
In general, modern computer users will never notice a difference in usage from either
the RISC or CISC chips. Intel’s Pentium chip is a CISC chip, while Macintosh uses a RISC
design. Some computers even use combinations of both RISC and CISC, further eliminating
the differentiation between the two classes.
RISC technology is very new by comparison. However, RISC computers use fewer and
simpler instructions than conventional CISC computers. Simpler instructions reduce the complexity
of the circuits required to implement an instruction, thereby allowing individual
instructions to execute quickly. RISC machines generally show a higher level of performance
in comparison to a comparably complex CISC system, despite the fact that a RISC processor
executes more instructions to accomplish a given task than does a CISC processor. The
following characteristics are common on all RISC architecture computers: one instruction
per machine cycle, unique register-to-register operations, and an instruction pipeline. RISC
architecture often includes more general-purpose registers to maximize the number of operations
that take place on the CPU. CISC computers, in contrast, employ more memory
referencing. Studies have shown that the compilers on CISC machines tend to favor simpler
instructions, such that the conciseness of the complex instruction sets seldom comes into
play. The expectation that a CISC computer would produce smaller programs may not be
realized because the more complex the instruction set, the more processor time is required
to decode and execute each instruction. Longer opcodes required in CISC architecture produce
longer instructions. In computationally intensive applications such as FEA, where calculation
times are often measured in hours, RISC processors are generally more efficient at
performing floating-point operations than CISC processors.
Most RISC architecture can be found in workstations which run on the UNIX operating
system, such as Sparc MIPS, DEC Alpha, PA-RISC, and PowerPC. The Pentium architecture,
however, is an excellent example of CISC design. It represents the result of decades of CISC
architecture evolution. Pentium architecture incorporates the sophisticated design principles
once found only on mainframes, supercomputers, and servers.
The RISC versus CISC debate continues to drive computer technology in new directions.
There is a growing realization in the industry that RISC and CISC may benefit from each
other. Notably, more recent designs, like the PowerPC line of the Apple Macintosh, are no
longer ‘‘pure’’ RISC, and the more recent CISC designs, like the Pentium P4, have incorporated
some RISC-like features.
Emory W. Zimmers, Jr. and Technical Staff
Enterprise Systems Center
Lehigh University
Bethlehem, Pennsylvania
