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Cisc v risc. To begin this assignment , I will outline the definition of cisc and risc . Cisc stands for complex instruction set computing and Risc stands for reduced instruction set computing . these two concepts are both types of instruction sets used

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Cisc v Risc


To begin this assignment , I will outline the definition of cisc and risc . Cisc stands for complex instruction set computing and Risc stands for reduced instruction set computing . these two concepts are both types of instruction sets used in computer architecture for processors and microprocessors . computer architecture is a way of examining the structure of a computer, we mainly focus on the how the cpu performs internally and how it accesses addresses in computer memory. The objective of the microprocessor is to advance the performance of the machine. Performance can be defined as the amount of work a processor can do in a set time .


In order to improve the computers performance we would have to increase the speed of how we process these instructions or else make these instructions more complex so they will perform more work, but we can’t have both. Speeding up the instructions (cisc) means changing the clock speed of the computer , while making the instructions more complex (risc) means we will require more power to process them. An real life analogy of this instruction set concept would be to take a bike for example , if we are in a low gear we can pedal really fast to get where we are going but in a high gear we can go the same distance but we require more power to push the pedals. In the following sections I will try and explain the two concepts of Cisc and Risc  and where they originated from and the advantages and disadvantages .



The cpu ( central processing unit ) can be sometimes referred to as the brain of the computer as it does all the work for the programmes and applications .

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Decode : decodes the instruction and looks for operands.Execute : performs the instruction e.g( addition , substraction)Memory : accesses the memory and writes data or retrieves data from it.Write back : records the calculated value in a register.

As mentioned above the goal of pipelining is to perform each step paraellel , so as one instruction is being retrived ( fetch ) the previous one is being read ( decode ) while the previous one to that is being performed ( executed ) and so on .. there can be many steps in pipelining but in general the main ones are outline above , to understand this pipeling process more see diagram below,


From the 1960scisc microprocessors became prevalent , and each successive processor having more complicated hardware. Some examples of cisc architecture in processers are the ibm360, intel 80386 , and the Motorola 68030. Most of intels chips from the 8008 microprocessor to the Pentium 4 are all based around the cisc approach.

Ibm 360 :

Back in 1960 most computers would have been designed independently or machines were custom modified for a customer . in 1964 ibm changed all this when they brought out the ibm 360 computers. The computers could range from the model 20 ( 24 kb of memory ) to the model 91 super computer which was bult for missile defence in north America . the computers had different operating systemsbut all had the same instruction set. The low end machines didn’t or could support operating systems  and a program was loaded from binary punched cards at start up. The higher end machines called 0s/360 established 32 bits as the standard for computers. See below ibm 360 computer.


The advantages of cisc:

When cisc instruction sets were first developed , cisc machines used technologies available to optimize computer performance . micro programming was a assembly language that was reatively easy to introduce and was at less cost than hardwiring a control unit.

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Summary :

in todays computer world the vast majority of risc cpus are 32 –bit . the risc design offers  power in even small sizes.Today, the Intel x86 is arguable the only chip which retains CISC architecture. In some cases we can see that both architectures are becoming more and more alike . most of todays risc chips support as many instructions as previous cisc chips and also todays cisc chips can be seen using the same techniques which would have been associated with the risc chip . even intel today which is the main player for the cisc architecture can be seen using some of the risc technology approaches .


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Here's what a teacher thought of this essay

A lot of copied and misunderstood material and far too detailed at this level. Also lots of spelling, grammar and some factual errors. However the second half of the piece is generally accurate and informative

Marked by teacher Ivor Borkin 30/11/2013

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