RISC vs CISC still relevant?

There has been long time discussion on if RISC or CISC processor architecture is more efficient. CISC (Complex instruction set computing) was the most popular architecture before RISC became popular. According to Wikipedia before the RISC philosophy became prominent, many computer architects tried to bridge the so-called semantic gap, i.e. to design instruction sets that directly supported high-level programming constructs such as procedure calls, loop control, and complex addressing modes, allowing data structure and array accesses to be combined into single instructions. CISC designs involve very complex architectures including a large number of instructions and addressing modes, whereas RISC designs involve simplified instruction that the compilers needed to combine to do those complex tasks. To enable efficient compilation of high level language programs, CISC and RISC designs are the two options. Both of them work and work well today.

Why did the RISC idea take over the microprocessor market in the 1980′s? Because these machines were manifestly faster. But over years also CISC processors became faster with new technologies and borrowing some ideas from RISC: modern x86 processors also (typically) decode and split instructions into dynamic sequences of internally buffered micro-operations. CISC became a catch-all term meaning anything that’s not a load-store (RISC) architecture.

In today’s computing environment Intel’s x86 based design are the most popular CISC processors and different ARM processors are the most popular RISC design. Today, the Intel x86 is arguable the only chip which retains CISC architecture. For information on the differences of those most popular processors read through ARM vs X86 – Key differences explained! article.

The terms CISC and RISC have become less meaningful with the continued evolution of both CISC and RISC designs and implementations. The X86 was originally a pure CISC design, but over the years has taken on a much more RISC-like structure while ARM’s approximately RISC architecture has taken on more CISC features. It means that the majority of today’s processors can’t rightfully be called completely RISC or completely CISC. Chips that implement the x86 CISC ISA have come to look a lot like chips that implement various RISC ISA’s.

We are now in in Post PC era where a market trend involving a decline in the sales of personal computers in favor of post-PC devices; which include mobile devices such as smartphones and tablet computers. Most smartphones and tablets utilize low-power ARM processors manufactured by companies such as Nvidia and Qualcomm instead of the x86 processors sold by companies such as Intel and AMD. ARM-based chips have dominated the designs for mobile processors in much the same way that Intel’s x86 family of processors locked up the Windows PC market a few decades ago. Over the years ARM (RISC) Vs Intel (x86/CISC) war has been been strong on mobile front because Intel is hungry to grab a piece of the mobile market – a market they were not not interested  years ago because they did not see the potential in it then. Nowadays they have the engineers, fabs, and other resources to make this happen. Android supports 3 different processor architectures: ARM, Intel and MIPSIn short, ARM is the current winner and Intel is its big brand rival.

At the same time Linux ARM servers are on their way to the data center as big cloud companies start to deployed servers based on 64-bit ARM processors to their data centers where they make sense. There is also interest in using ARM processors to power network switches and storage devices in data centers.

A big part of ARM’s success, however, has little to do with better technology. It’s the company’s business model – ARM only designs chips, leaving the physical fabrication to its customers/licensees. Once ARM gained a toehold in the mobile market, others followed. As with PCs, servers, and most processor-based systems, later products were based on the same processor family as the earlier products. After all, why switch? If the previous generation worked, make the next generation similar but faster. Contrary to popular belief, RISC processors are neither faster nor slower than their doddering CISC forebears. But they are generally more power-efficient, which turns out to be a key differentiator when you’re making battery-powered gizmos. Maybe ARM architecture is inherently slightly more power efficient than x86 CISC, but  Intel has the most advanced chip manufacturer in the world which will help to get power efficiency benefits of it’s own. Which one is really the winner technology?

According to RISC vs CISC: What’s the Difference? article if you are one of the few hardware or software developers out there who still think that instruction set architectures, reduced (RISC) or complex (CISC), have any significant effect on the power, energy or performance of your processor-based designs, forget it. Ain’t true. What is more important is the processor microarchitecture. A new study “ISA Wars: Understanding the Relevance of ISA being CISC or RISC” comparing the Intel X86, the ARM and MIPS CPUs finds that microarchitecture is more important that instruction set architecture, RISC or CISC.

A new study comparing the Intel X86, the ARM and MIPS CPUs finds that microarchitecture is more important that instruction set architecture, RISC or CISC:  “While there may have been differences in the past between RISC and CISC ISAs in current architectures, there certainly aren’t now in terms of the parameters we focused on: performance, power, and energy,” Sankaralingam told EE Times. “Based on this study, developers can safely consider ARM, MIPS, and x86 processors simply as engineering design points optimized for different levels of performance,” he said. “It appears that decades of hardware and compiler research has enabled efficient handling of both RISC and CISC ISAs, and both are equally positioned for the coming years of energy-constrained innovation.”There’s nothing fundamentally more energy-efficient in one ISA class versus another.” To me it seems that there is place in market for both ARM and x86.

If you want to do your own analysis, the raw data for the study can be downloaded from the University of Wisconsin VTG web page.

Thus the “RISC vs. CISC” debate really exists only in the minds of marketing departments and platform advocates whose purpose in creating and perpetuating this fictitious conflict is to promote their pet product by means of name-calling and sloganeering.


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