CISC

Abstract

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• Stands for Complex Instruction Set Computer
• Instruction in CSIC is like pieces built with unique lego pieces that can be used to get CPU to carry out multiple operation in one single instruction to complete a particular task. Unlike RISC which only provides unique lego pieces that can be used to get CPU to carry out one operation at a time. Thus, RISC has fewer possible Instruction

Takes less resources to build applications

First, the programmers can code faster with complex instructions, because each instruction is able to do multiple things, you can think of it as a small function. Second, since one instruction is able to do multiple things, less instructions are needed, thus less usage on the Main Memory.

However, thanks to Compiler, programmers seldom code in complex instructions anymore. And with modern Main Memory, the space saved with CISC isn’t significant anymore.

More complex decoder

[[ISA Instruction Format#]]. For x86, the length of instructions range from 1-15 bytes. So the decoder needs extra logic to determine if it is processing a new instruction. However, we can make use of micro-op to handle this.

Requires more transistors

The design of CPU needs to be complex to achieve complex computation with fewer Instruction, so less Transistors (晶体管) can be used improve overall computing performance. Thus, more power-hungry, and more wasted power when performing simple instruction.

Common CISC

X86

• Developed by Intel

x86-64

• The 64bits version of X86

AMD64

• 64-bit extension of the x86, created by AMD

i486

• A 32-bit x86 processor released by Intel in 1989, very outdated and mostly found in retro computing or embedded systems with minimal requirements.

i686

• Intel’s 6th generation x86 processor introduced in 1995. It designates a baseline of features present in most modern x86 processors (both Intel and AMD)