CISC

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CISC
Nathan Murata CS 147
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NOT CISC
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NOT CISC
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NOT CISC
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NOT EVEN CLOSE
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CISC
Complex Instruction Set Computer
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• CISC lt RISC

• CISC is a retroactive term to differentiate it
  from RISC (Reduced Instruction Set Computer)

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WHY CISC?

• Memory was expensive (forty years ago)

• Cycle times are high

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• Fewer calls to main memory

• CISC helps bridge the Semantic Gap between
  high-level programming languages and low-level
  instructions

• Maximize throughput, minimize cost

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What is CISC?
microprocessor instruction set architecture (ISA)
in which each instruction can execute several
low-level operations, such as a load from memory,
an arithmetic operation, and a memory store, all
in a single instruction.
TASK1 data1 TASK1 data2 TASK2 TASK3
OPERATION data1 data2
gt
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Multiplying two numbers, using CISC The
execution unit carries out computations, but only
on data in one of the registers (labeled ABCDEF).
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CISC aims to carry out this task using as few
lines of assembly code as possible. The
processor hardware is built such that certain
instructions are pre-built (such as MULT). MULT
value1, value2 would take two values loaded in
the registers and multiply the two numbers
together and store that information in another
register. This is done in one function, without
loading or storing data in intermediate
steps. This requires little translation of
high-level language into assembly code and not
much RAM to store instructions.
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The multiplication is carried out in one line of
code rather than four (as in RISC) LOAD
value1 LOAD value2 PROD value1, value2 STORE
registerSpace value1
This requires two steps to load each value, a
step to find the product, and one to store the
value.
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It is important to note 1) CISC predates
pipelining (allowing different instructions to
use different parts of the execution unit) to
keep the execution time constant. 2) CISC
allowed for compatibility and upscaling a new
processor design could incorporate the
instruction set of its predecessor as a subset of
an ever-growing language. 3) CISC made assembly
language more like high-level language, so that
the compiler does fewer functions.
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Some DISadvantages of CISC

• Incorporating more into CISC leads to increasing
  complexity
• Because all needed instructions are hard-wired,
  more obscure instructions are rarely used,
  leading to less than optimal design
• The translation of complex instructions into
  basic ones may be more time-consuming than a
  series of simple instructions.

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Timeline 1960s CISC architecture developed
and largely unquestioned as the best way to
design processors 1970's other non-CISC
architectures are developed, theorized, or
otherwise explored, although not used 1980's
computer manufacturers still build more complex
microprocessors with larger and larger sets of
instructions. Some decided to explore RISC
architecture.
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Modern era The divide between CISC/RISC has
dwindled over time. Many RISC microprocessors
support large sets of instructions associated
with CISC and CISC microprocessors incorporate
many RISC techniques of design. Intel's Pentium
chip is an amalgam of Complex/Reduced Instruction
Set Computers and Parallel Instruction Computing
has also been developed by Intel.
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EXAMPLES
Intel x86 processors
Motorola 68K series