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The simplest way to examine the advantages and disadvantages of
RISC architecture is by contrasting it with it's predecessor: CISC (Complex
Instruction Set Computers) architecture.
Multiplying Two Numbers in Memory
On the right is a diagram representing the storage scheme for a
generic computer. The main memory is divided into locations numbered from
(row) 1: (column) 1 to (row) 6: (column) 4. The execution unit is
responsible for carrying out all computations. However, the execution unit
can only operate on data that has been loaded into one of the six registers
(A, B, C, D, E, or F). Let's say we want to find the product of two numbers
- one stored in location 2:3 and another stored in location 5:2 - and then
store the product back in the location 2:3.
The CISC Approach
The primary goal of CISC architecture is to complete a task in as
few lines of assembly as possible. This is achieved by building processor
hardware that is capable of understanding and executing a series of
operations. For this particular task, a CISC processor would come prepared
with a specific instruction (we'll call it "MULT"). When executed, this
instruction loads the two values into separate registers, multiplies the
operands in the execution unit, and then stores the product in the
appropriate register. Thus, the entire task of multiplying two numbers can
be completed with one instruction:
MULT 2:3, 5:2
MULT is what is known as a "complex instruction." It operates
directly on the computer's memory banks and does not require the programmer
to explicitly call any loading or storing functions. It closely resembles a
command in a higher level language. For instance, if we let "a" represent
the value of 2:3 and "b" represent the value of 5:2, then this command is
identical to the C statement "a = a * b."
One of the primary advantages of this system is that the compiler
has to do very little work to translate a high-level language statement
into assembly. Because the length of the code is relatively short, very
little RAM is required to store instructions. The emphasis is put on
building complex instructions directly into the hardware.
The RISC Approach
RISC processors only use simple instructions that can be
executed within one clock cycle. Thus, the "MULT" command described above
could be divided into three separate commands: "LOAD," which moves data
from the memory bank to a register, "PROD," which finds the product of two
operands located within the registers, and "STORE," which moves data from a
register to the memory banks. In order to perform the exact series of steps
described in the CISC approach, a programmer would need to code four lines
of assembly:
LOAD A, 2:3
LOAD B, 5:2
PROD A, B
STORE 2:3, A
At first, this may seem like a much less efficient way of
completing the operation. Because there are more lines of code, more RAM is
needed to store the assembly level instructions. The compiler must also
perform more work to convert a high-level language statement into code of
this form.
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CISC
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RISC
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Emphasis on hardware
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Emphasis on software
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Includes multi-clock
complex instructions
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Single-clock,
reduced instruction only
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Memory-to-memory:
"LOAD" and "STORE"
incorporated in instructions
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Register to register:
"LOAD" and "STORE"
are independent instructions
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Small code sizes,
high cycles per second
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Low cycles per second,
large code sizes
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Transistors used for storing
complex instructions
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Spends more transistors
on memory registers
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However, the RISC strategy also brings some very important
advantages. Because each instruction requires only one clock cycle to
execute, the entire program will execute in approximately the same amount
of time as the multi-cycle "MULT" command. These RISC "reduced
instructions" require less transistors of hardware space than the complex
instructions, leaving more room for general purpose registers. Because all
of the instructions execute in a uniform amount of time (i.e. one clock),
pipelining is possible.
Separating the "LOAD" and "STORE" instructions actually reduces the
amount of work that the computer must perform. After a CISC-style "MULT"
command is executed, the processor automatically erases the registers. If
one of the operands needs to be used for another computation, the processor
must re-load the data from the memory bank into a register. In RISC, the
operand will remain in the register until another value is loaded in its
place.
The Performance Equation
The following equation is commonly used for expressing a computer's
performance ability:
The CISC approach attempts to minimize the number of instructions
per program, sacrificing the number of cycles per instruction. RISC does
the opposite, reducing the cycles per instruction at the cost of the number
of instructions per program.
RISC Roadblocks
Despite the advantages of RISC based processing, RISC chips
took over a decade to gain a foothold in the commercial world. This was
largely due to a lack of software support.
Although Apple's Power Macintosh line featured RISC-based chips and Windows NT was
RISC compatible, Windows 3.1 and Windows 95 were designed with CISC
processors in mind. Many companies were unwilling to take a chance with the
emerging RISC technology. Without commercial interest, processor developers
were unable to manufacture RISC chips in large enough volumes to make their
price competitive.
Another major setback was the presence of Intel. Although their
CISC chips were becoming increasingly unwieldy and difficult to develop,
Intel had the resources to plow through development and produce powerful
processors. Although RISC chips might surpass Intel's efforts in specific
areas, the differences were not great enough to persuade buyers to change
technologies.
The Overall RISC Advantage
Today, the Intel x86 is arguable the only chip which retains CISC
architecture. This is primarily due to advancements in other areas of
computer technology. The price of RAM has decreased dramatically. In 1977,
1MB of DRAM cost about $5,000. By 1994, the same amount of memory cost only
$6 (when adjusted for inflation). Compiler technology has also become more
sophisticated, so that the RISC use of RAM and emphasis on software has
become ideal.
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