RISC

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RISC and The Modern Computer
By Craig Long
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INTRODUCTION How many of you have taken as
assembly language class?
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Brief history of microprocessors
1978 Intel releases Intel 8086, considered
first modern processor 1981 IBM-PC released
with Intel 8088 processor 1982 First RISC
processor developed at UC Berkeley 1983
Original Macintosh released with Motorola 68000
(CISC) processor 1992 First PowerPC (RISC)
processor released 1993 First Pentium processor
released 1994 Power Macintosh released with
PowerPC-based processor
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What is CISC?
Complex Instruction Set Computer - large
instruction set - few instructions per program -
instructions tend to be complex - instructions
take longer to decode Dominant in early
computing before 1980s.
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What is RISC?
Reduced Instruction Set Computer - small
instruction set - more instructions per program -
complex operations composed of simpler
instructions - instructions take much less time
to decode First major RISC processor developed
in early 80s, first commercial RISC success was
PowerPC in 1992.
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A BRIEF DIVERSION...
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How is code generated inside the computer?
Steps in compilation - lexical analysis
reducing text to symbols of the language -
parsing reducing symbols to program
structure - semantic analysis associating names
with correct variables, memory locations, and
types - code generation creating assembly code
from statements
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An example...
Consider the Java statement int x y 2 -
lexical analysis int x y 2
TYPE IDENTIFIER DENOTATION
(etc)? - parsing int x y 2 TYPE
IDENTIFIER EXPRESSION EXPRESSION
IDENTIFIER CONSTANT - semantic analysis int x
y 2 x, y, 2 int associated with
memory address - code generation (in IA-32
assembly)? assume x and y have been declared as
integer-size memory locations mov eax, y add
eax, 2 mov x, eax
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So...what's the problem here?
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The semantic gap
The semantic gap is the gap between the meaning
of statements in a high-level language and the
meaning of statements at the machine
level. Researchers found that many complex
instructions were not being used in CISC machines.
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CISC vs RISC
So (you might ask) if an advantage of CISC is
complex instructions that aren't being used, is
it time for a new approach? Some people thought
so, and RISC was developed.
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So why is RISC so great?
...well I'll tell you!
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There are many reasons, including - More
available registers, so that memory is accessed
less - Load-store idiom means less frequent
data movement from external memory to program
space - Simple instructions which can be used to
compose complex operations
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Pipelining A Story Imagine you own a toy
factory. Material comes in, goes through 23
stations, and leaves fully processed. You start
to wonder why your competitors can make 23 times
more toys than you can. Then, one day, you watch
your assembly line and gasp in astonishment! As
each worker does their part, they step back and
wait until the entire toy is assembled before
starting on the new one (and take a nice nap
while waiting)!
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Why is pipelining only possible with
RISC? (well, it really isn't)?
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While RISC has not caught on in the personal
computer market, it remains dominant in other
areas, especially the embedded market and the
high-end server market. For example, almost all
chips in Apple iPods are RISC, as are the chips
in the next-gen video game consoles and IBM-built
Roadrunner, the fastest supercomputer in the
world. Many RISC ideas, such as pipelining and
standard-size instructions, have also been taken
by CISC-type processors, such as the x86
architecture that forms the current generation of
desktop chips.
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SOURCES
Murdocca, Miles and Vincent Heuring, Computer
Architecture and Organization, 2007, John
Wiley Sons, Inc Irvine, Kip R., Assembly
Language for Intel-Based Computers, 2007,
Pearson Prentice Hall Wikipedia The Free
Encyclopedia. 17 Nov. 2008. Wikimedia
Foundation. lthttp//en.wikipedia.orggt