What is RISC and CISC architecture? The former stands for Reduced Instruction Set Computing, while the latter means Complex Instruction Set Computing – two approaches to processor architecture. In this article, we focus on thoroughly explaining the details between these two approaches – if it’s a topic of your interest, then we encourage you to read on.
What Is RISC and CISC Architecture
To begin with, let’s focus on defining what RISC and CISC architectures are.
A reduced instruction set computer (RISC) is a type of microprocessor architecture that embraces simplicity and speed. The core philosophy of RISC is to optimize the execution of instructions by using a smaller, more straightforward set of commands. This approach enables faster instruction execution and facilitates pipelining, a technique where multiple instructions are overlapped in the execution process.
Complex Instruction Set Computer (CISC) is a microprocessor architecture that attempts to improve performance by using complex instructions. It carries more complex operations within one instruction, reducing the number of instructions required per task and thus the amount of code needed, but hence using more than one clock cycle to execute an instruction.
What Are the Advantages and Disadvantages of RISC and CISC Architectures?
If you know the definition of a microprocessor and each of these two architectures, you’re probably interested in more details: their main advantages and disadvantages. In the end, this is the core of any decision. So, what is good in RISC and CISC architectures, and what could be better? Let’s take a look:
- Simple instructions – Easy to decode and execute, the simplicity of instructions in RISC architecture makes it easy to process them quickly.
- Lower power consumption – When compared to CISC-architecture-based microprocessors, RISC ones are significantly less power-consuming. This is especially important for smaller embedded systems with limited access to power.
- Pipelining – In RISC the execution of a task is often broken down into single instructions. As a result, multiple instructions may be executed at the same time, which makes the overall execution time faster.
- Smaller chip size – While this means cost-effectiveness in general, this is an extreme advantage in smaller devices where each inch of space is precious.
- Too many instructions – Performing more complex tasks in RISC architecture requires operating with numerous simple, usually one-word instructions – you need significantly more of them than in CISC.
- Increased memory usage – As the number of instructions grows, the amount of memory needed to store them – RISC is extremely memory-consuming when used for complex tasks.
- Expensive – Usually, developing RISC microprocessors is visibly more expensive and difficult than building CISC ones.
- Limited capabilities – The fact that RISC is based on simple instructions renders some complex instructions impossible to complete.
- Reduced code size – With more complex instructions, less code is needed to complete tasks, even those more complicated ones.
- Memory-efficiency – CISC consumes less memory than RISC due to the lower number of instructions, so it can perform better even on slightly lower resources.
- Spread – CISC has preceded RISC, so there are simply more resources, larger communities, and more software that might help you design processors or systems within this architecture.
- Slower executions – Since they are more precise, the execution of instructions in CISC takes more time than in RISC architectures, especially when considering basic tasks.
- Power consumption – Microprocessors in CISC architecture require more energy than those in RISC architecture. This can be the make or break for smaller devices with limited power sources.
- Larger chip size – Contrary to RISC, CISC processors need to be larger, so they might not always be the most suitable choice for smaller devices.
RISC vs. CISC – Usage
Finally, knowing what RISC and CISC are, we can proceed with their usage in practice and what interests us the most – understanding which one is better for embedded systems.
The answer to that question is simple: RISC processors. Not only are they faster when it comes to simple instructions (and those are most likely in embedded systems), but they are also physically smaller and require less energy which is a requirement for small embedded systems. But it doesn’t mean that CISC processors are utterly useless.
In fact, the CISC architecture is still the basis of many devices dealing with graphics and videos. Plus, they are utilized in computers used for scientific purposes or for complex data analysis. Simply, you don’t have a good or a bad choice between these two. But you can have a better-suited or worse-suited one.
Both older CISC architecture and newer RISC architecture have their advantages and disadvantages. The former use complex instructions that cannot be executed in one clock cycle – they consume more energy but need less RAM; the latter are based on simple, one-word instructions, use up less energy, and are smaller but require more memory and have limited capabilities. There is no better or worse choice – whether a CISC or RISC processor will be finer depends purely on the type of device that you want to install it in. And what about embedded systems? In their case, usually RISC processors are the superior choice.
Was this article helpful? In that case, we recommend reading our other post: RISC-V vs ARM: What're the Differences?