Multi Core Processor Programming

I'm trying to find information for why multi core processor programming is harder than single core processor programming.
I know google is my friend, but it has let me down lol

There are 4 various model used in software develop ment,system development lifecyclemodel,prototyping model,rapid application development model,component Component Assembly Model.software development also depended in market reserch,analyzing the problem,implementation of software,testing software,and client requirement.

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I'm trying to find information for why multi core processor programming is harder than single core processor programming.
I know google is my friend, but it has let me down lol

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As computing requirements persist to put rising demand on the requirement for quicker and more powerful computer hardware, the dual-core processors have become approximately common place in the majority computers. Actually, a few chip makers such as AMD and Intel have already released quad core processors and further than. One group of researchers is even looking at the possibility of making a single processor containing over 100 cores.