In the field of processor design, the CISC (Complex Instruction Set Computing) and RISC (Reduced Instruction Set Computing) architectures have represented two different approaches to addressing how instructions are processed by a CPU. Although both have evolved since the 1970s, their design and differences significantly influence the performance, energy efficiency and complexity of the systems and applications in which they are used.
In today’s article, we take a detailed look at the characteristics of each architecture, how they work and the environments in which they excel, providing a clear view of how they have made an impact on today’s technology.
CISC architecture
As its name suggests, the CISC architecture is characterized by a complex instruction set, where a single instruction can perform multiple operations at the hardware level. This approach was designed in the context of the 1970s, a time when memories were expensive and compilers were not advanced enough to efficiently optimize code. Therefore, the main goal of this architecture was to minimize the number of instructions needed in the program, reducing the size of the source code and maximizing the use of available resources. Examples of CISC architectures include Intel’s popular x86 version and AMD’s x86-64.
In the following, we will take an in-depth look at the key features of CISC, how it works, as well as its advantages, disadvantages and applications.
Main CISC features
Large and complex instruction set: Instructions in CISC are usually large and varied, allowing complex operations such as multiplications, function calls, loop handling and data manipulation directly in memory. These instructions are usually of variable length, depending on the task to be performed.
- Direct support for in-memory operations: CISC processors allow arithmetic operations to work directly with values stored in memory, instead of requiring data to be loaded into registers beforehand.
- More sophisticated hardware: The complexity of the instruction set requires advanced hardware to decode and execute the instructions. This includes microprogramming control units that translate complex instructions into simpler steps.
- Emphasis on code size reduction: One of the original motivations of the CISC design is to minimize the number of instructions in the code, thus optimizing memory usage.
Hardware operation in CISC
The core of the CISC design lies in its control unit, which uses microprogramming to decompose complex instructions into multiple micro-operations. This approach allows the processor to execute a wide variety of tasks without requiring each instruction to be directly encoded in hardware.
- Instruction decoding: A complex instruction is parsed and decomposed into simpler operations, so that, for example, a single instruction could combine loading data from memory, performing a calculation and storing the result.
- Execution in multiple clock cycles: Because instructions are complex, their execution often requires multiple clock cycles (the basic unit that measures the rate at which a processor can complete operations, determined by the CPU clock frequency). This implies that CISC instructions can be slower to process compared to simple instructions that execute in a single cycle.
- Flexibility and compatibility: The use of microprogramming facilitates the upgrade and extension of the instruction set, allowing backward compatibility with older software.
RISC architecture
Unlike the CISC model, RISC focuses on a reduced and highly optimized instruction set, so this design promotes a more efficient and faster approach by simplifying the processor hardware. Therefore, RISC architecture works with a processor design approach that prioritizes simplicity, speed and efficiency, and emerged as an alternative to the complexity of CISC architectures, aiming to execute fast and uniform instructions, optimizing performance through pipelining and minimizing hardware logic.
In addition, this design model is widely used in embedded systems, mobile devices and high performance applications where energy efficiency is very important. An example of this would be ARM architectures, which are often used in mobile devices and PowerPC.
RISC main features
- Reduced instruction set: RISC architectures limit the number of instructions available, simplifying them so that each one performs an elementary operation, such as a calculation or data transfer. Instructions are usually of fixed length, making them easy to decode.
- Fast, uniform execution: Each instruction is designed to complete in a single clock cycle, maximizing processing speed and facilitating performance predictability.
- Register-intensive: Operations in RISC are generally performed between registers rather than interacting directly with memory, minimizing slower memory accesses.
- Simplified hardware: Due to the simplicity of the instruction set, the hardware can be optimized for faster, parallel operations.
- Optimized segmentation: The RISC architecture is designed to take advantage of pipeline segmentation, dividing instruction execution into stages that can be used as a basis for faster, parallel operations.
Hardware operation in RISC
RISC design seeks to maximize performance through simplicity and regularity in instruction execution.
- Simple decoding: Since instructions are of fixed length and uniform, the processor can decode them quickly without the need for complex hardware.
- Execution in one clock cycle: Each instruction is decomposed into basic operations (such as register reading, calculation and result writing) that are performed in a single cycle.
- Extensive use of pipelining: Segmentation divides processing into several stages (such as fetch, decode, execute), allowing multiple instructions to be processed simultaneously.
- Compiler optimization: Compilers are fundamental in RISC, since they must translate high-level code into instructions that efficiently use registers and minimize memory accesses.
Comparing architectures: CISC vs RISC
| CISC | RISC | |
| Instruction set | Complex and diverse (variable length) | Reduced and uniform (fixed length) |
| Cycles per instruction | Varied (often multiple cycles) | Usually one per instruction |
| Hardware complexity | High (advanced decoding required) | Low (simplified design) |
| Code size | Reduced, due to more complex instructions | Larger, as it requires more instructions |
| Power usage | Larger, due to complex hardware | Minor, ideal for mobile devices |
| Standard applications | Personal computers or servers | Mobile devices, embedded systems, IoT, etc. |
Future of CISC and RISC
As technology has evolved, the distinction between the two architectures has blurred, since today, CISC processors such as Intel’s incorporate many typical RISC features, such as segmentation and out-of-order execution. In turn, RISC processors have expanded their instruction sets, moving closer in functionality to CISC.
The battle between CISC and RISC has no clear winner, as both architectures are designed to solve specific problems in different contexts. Still, what is clear is that the balance between complexity, efficiency and compatibility will continue to drive innovation in processor design.
On the other hand, ARM and RISC-V architectures are rapidly growing in popularity due to their energy efficiency, while CISC remains the standard choice for desktops and servers.
Conclusion
As we have seen along the article, CISC and RISC are different paradigms that address similar problems but in a different way.
The CISC architecture has been instrumental in the evolution of processors, providing an effective solution for reducing code size and optimizing memory usage in traditional systems. In addition, its ability to handle complex instructions directly in hardware makes it an ideal choice for environments where backward compatibility and legacy software execution are important, such as in computers and servers.
However, this complexity comes at a cost: higher power consumption, more expensive hardware and potentially slower execution speeds compared to more modern architectures such as RISC, which, with its focus on simplicity and efficiency, has become the favored choice in industries where performance per watt is key. Therefore, although it requires more instructions to perform complex tasks, its optimized design for segmentation and low power consumption makes it highly effective in modern contexts such as mobile devices, IoT and supercomputers.
Resources:
[1] Wikipedia – RISC
[2] Wikipedia – CISC
[3] Universidad Autónoma Metropolitana – Arquitectura RISC vs CISC
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