If you are a computer enthusiast, then you have probably heard the names CISC (Complex Command Computer) and RISC (Low Command Computer). In fact, the two terms refer to differences in the way a processor is designed. In the following, we intend to pay more attention to the difference between CISC and RISC.
For example, the ARM processor in your phone uses the RISC architecture. At the same time, the x86 processor in your computer has a CISC design.
What is the meaning of Instruction Set?
Whenever discussing differences between CPUs, then one of the things to discuss is the set of instructions. A set of instructions for a CPU is actually a set of operations that a CPU can perform naturally. These are operations that are encrypted on the CPU and at the hardware level. This set can contain several to thousands of instructions, depending on the design of your CPU.
In other words, if an operation is outside the scope of a set of instructions, then the CPU will not be able to execute it because it does not have the necessary hardware for it. Let’s take a look at a similar example to make it easier to understand. Consider a light bulb. The lamp manufacturer has designed it to convert electrical energy into light. The reason for this is that the lamp can do so in terms of hardware.
In principle, it can be said that only one lamp can convert electricity into light and nothing else is made of it. Similarly, a CPU set of instructions is actually a set of operations that the CPU is able to perform. For example, there is a “move” command within almost all instructions for all CPUs. This command takes some data from the source storage memory and then transfers it to the destination storage memory.
Whenever a CPU wants to transfer data, it knows exactly how to do it, because its hardware is built around such issues. In short, the Instruction Set includes all the operations that a CPU supports at the hardware level.
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How does a CPU work?
A CPU can be considered a spiral of electrical circuits. These electrical circuits are designed to provide the CPU with a set of natural instructions. So, since the required circuits are embedded inside the CPU, the hardware knows how to execute the operations inside a set of instructions.
The circuits associated with an operation are activated by an electrical signal, so a CPU can perform a specific operation. When a circuit is activated, the CPU will then execute the routine associated with that circuit.
If a processor wants to perform a complex operation such as sending a tweet, then software programs must send millions of electrical signals every second. Each of these signals also targets one of the commands in the CPU instruction set. This is where the concept of RIS and CIS comes into play.
What is RISC?
We begin the difference between RISC and CISC by discussing the former. As its full name suggests, a RISC-based CPU has a simple set of operations. These simplified commands can achieve simple goals and take just one cycle to complete.
Since RISC has simple instructions, such processors do not need complex circuits to perform this simple operation. That’s why RISC designs cost less.
To better understand the details of such processors, let’s take a look at their design concepts. First, RISC-based CPUs complete each instruction in a single cycle. Second, these processors only operate on data stored in registers. This is because one of the main bottlenecks associated with CPUs in terms of task performance is the mismatch between CPU speed and main memory (RAM) speed. The core is much slower than a CPU.
Thus, if a processor is forced to work with data stored in RAM, then this process causes a bottleneck and as a result, the data processing speed is reduced. In RISC design, data is stored and loaded on the registers in the CPU. Naturally, the reason for this is due to the higher speed of the registers compared to RAM.
Third, RISC structures are so simple that there is no layer of interpretive microcode to translate instructions into simpler forms. Finally, RISC designs support pipelining. In this way, parts of several commands can be executed simultaneously. Since these types of processors have higher clock speeds, it is natural for them to be faster as well. Information routing is one way to use this higher speed and execute multi-command broadcasts, thus increasing efficiency.
Finally, RISC processors have simple instructions, have a higher clock speed, have a more efficient data routing structure, load and store operations on registers, and can execute commands in a single cycle.
We continue the difference between RISC and CISC processors by discussing the latter. The CISC opposes the RISC in almost all key cases. Almost all desktop chips have this design. First of all, the CISC design is complex and therefore requires a layer of microcode to turn complex instructions into simpler ones.
Second, CISC instructions can take several CPU cycles to execute. Third, the routing of information on such processors is not as efficient as on RISC processors, and it is also more difficult to do so due to the complex nature of CISC instructions.
In short, such CPUs can perform multiple operations on a single complex instruction, but the same complex instruction takes several cycles to complete. Information routing is more difficult for these architectures, and there are also many circuits on the processor itself.
Key differences between RISC and CISC
The main difference between CISC and RISC is the type of commands executed by the two. RISC instructions are simple, perform only one operation, and the processor can execute them in one cycle. CISC guidelines, on the other hand, perform several operations. Therefore, the CPU cannot run them in a single cycle.
The reason why RISC processors use information routing more efficiently and easily compared to CISC processors goes back to the instructions. In the first, the commands are so simple that they can be executed in sections. This is now more difficult to do in the latter, due to the more complex nature of the commands in CISC processors.
Unlike RISC, in CISC you can work directly with commands through RAM. So there is no need to load and save operations separately. Finally, the hardware requirements of the CISC design are greater than those of the RISC design, as the former must execute complex instructions. Basically, what CISC design does with hardware is done by software in RISC architecture.
This is why programs that target CISC design have fewer lines of code than RISC because the instructions themselves do a large part of the operation.
Modern processors do not use the CISC or RISC design at all. In fact, both of these architectures are used today to achieve the best results. For example, AMD’s x86 architecture essentially uses CISC but also has a microcode that converts complex instructions into simpler instructions similar to RISC. Thus, unlike older processors, modern CPUs have evolved beyond a simple RISC or CISC classification.