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Basics and Architectures
RISC and CISC
Processor
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Introduction
 There are two fundamentally different ways of
designing CPUs
 The CPU can be designed to have an instruction set
with:
 very basic instructions OR
 a wide range of complex instructions
 Exercise – List typical instructions for each case
 add, move-data etc
 multiply, dsp orientated instructions etc
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CISC Processor
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Complex Instruction Set Computer (CISC).
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 data1, data2
•
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
“data1” and "b" represent the value of “data2”, then this command is
identical to the C statement "a = a * b."
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• 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.
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RISC Processor
• Reduce Instruction Set Computer (RISC).
• 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, data1
LOAD B, data2
PROD A, B
STORE data1, A
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CISC v/s RISC
CISC
RISC
• Emphasis on hardware.
• Emphasis on software.
• Includes multi-clock complex
instructions
• Single-clock, reduced instruction only
• Memory-to-memory:
"LOAD" and "STORE"
incorporated in instructions
• Register to register:
"LOAD" and "STORE"
are independent instructions
• Small code sizes,
high cycles per second
• Low cycles per second,
large code sizes
• Transistors used for storing
complex instructions
• Spends more transistors
on memory registers
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Conclusion CISC v/s RISC
• 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.
• 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.
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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.
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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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PIPELINING CONCEPT
Pipelining Concept
Fetch
• The instruction is
fetched
from
memory
Decode
Execute
• The
instruction
is
decoded and the data
path
• control signals prepared
for the next cycle
• The operands are read
from the register bank,
shifted, combined in the
ALU and
• The result written back
3 stage Pipelining
• That is basically 3 stage pipelining,
– Fetch
– Decode
– Execute
How it works????
3 Basic Architecture
1. Super Scalar Architecture/Von-Neuman
Architecture
2. Very Long Instruction word Architecture
(VLIW)/ HARWARD Architecture
3. Single Instruction Multiple data (SIMD)
Superscalar Architecture
• Superscalar processing is the ability to initiate multiple
instructions during the same clock cycle.
• A superscalar architecture consists of a number of pipelines
that are working in parallel.
• Depending on the number and kind of parallel units available,
a certain number of instructions can be executed in parallel.
• It is known as Instruction level parallelism (ILP).
Degree of Superscalar Architecture
Superscalar Execution
Advantage of Superscalar Architecture
• Maximum Performance
• Propagation delay is less.
Disadvantage of Superscalar Architecture
• Problem in Scheduling.
Example of Superscalar Architecture
• Athlon(AMD) processor has these type of Architecture
Very Long Instruction Word (VLIW)
Architecture/ HARWARD Architecture
• Problem in Superscalar Architecture :
– Problem of Scheduling which operation and which one have to wait
and has done sequentially after others.
• A typical VLIW (very long instruction word) machine has
instruction words hundreds of bits in length.
• Every VLIW contains enough bits to specify not just one
machine operation but several to be performed
simultaneously.
Operand 3C
Operand 3B
Operand 3A
Operation
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Operand 2C
Operand 2B
Operand 2A
Operation
2
Operand 1C
Operand 1B
Operand 1A
Operation
1
How it can be achieved ?
The Word format of VLIW Architecture
• It’s name implies: a machine language instruction format that is fixed in
length(as in RISC Arch.) but much longer than 32 bit to 64 bits.
• The format of VLIW word has fixed length and each instructions are
scheduled to execute. Now compiler has to more work than a control unit.
• It is compiler’s job to analyze the program for data and resources and
pack the slots of each VLIW with as many concurrently executable
operations as possible
Advantages of VLIW
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Reducing delay by Scheduling.
Allow to work the system at higher frequency.
It can execute more operation per cycle.
Increases CPU Clock speed.
Disadvantages of VLIW
• Poor code density ( Number of bits required to
store each instruction).
• Wasted bit fields.
• Take up more space.
SIMD Architecture
• SIMD stands for single instruction, multiple data.
• It is the ability to do the same instruction on multiple pieces of
data. This can lead to significantly better performance, since it is
using less cycles with the same amount of data.
• This is done by packing the numbers into a vector. For instance, to
add {1,2,3,4} to {5,6,7,8} you could add 1 to 5, 2 to 6, and so on. Or
you could use SIMD and add 1,2,3,4 to 5,6,7,8, and the result gets
stored, when you can then unpack it and store it in a register. This is
why it is sometimes called vector math.
• SIMD has far-reaching applications; although the bulk and focus has
been on multimedia. Why? Because it is an area of computing that
needs as much computing power as possible, is popular, and in
most cases, it is necessary to compute a lot of data at once.
• SIMD works by performing an instruction on multiple pieces of data
at the same time by packing a vector with data and sending each
data parallel to one another.
Application of SIMD
• Two such real world examples of usage of SIMD are FastFourier Transform (FFT) and three-dimensional transform.
• Fast Fourier Transform is used primarily with applications
dealing with waveforms, such as Digital Signal Processors
(DSPs), radar, sonar, and more popularly, audio/mpeg
encoding (MP3s).