Throughout this series, we have looked at instruction formats, addressing modes, and pipelining, always working with fairly simple, uniform instructions like ADD R1, R2. It turns out that the very philosophy behind how a CPU's entire instruction set is designed can differ significantly between processors, and this difference is best captured by comparing two major approaches: RISC and CISC.
RISC, short for Reduced Instruction Set Computer, and CISC, short for Complex Instruction Set Computer, represent two fundamentally different design philosophies for building a CPU's instruction set. Rather than being a strict technical rule, this comparison is really about a set of design trade-offs, where each approach prioritizes different qualities, such as instruction simplicity, code compactness, and how much work is expected from hardware versus software.
In this tutorial, you will learn about the core design philosophy behind RISC and CISC, compare their instructions using worked examples, and explore their respective advantages, limitations, and typical real-world applications.
CISC processors are built around the idea of providing a rich, extensive set of instructions, where each individual instruction can perform a relatively complex task, sometimes even combining several operations that might otherwise require multiple simpler instructions. The underlying philosophy is that moving complexity into the hardware itself, through complex instructions, can simplify the work required from software and compilers.
CISC-style instruction: MULT R1, memory_address_2000, memory_address_3000 This single instruction might internally handle loading values from two separate memory addresses, multiplying them together, and storing the result into R1, all as one combined instruction
In this CISC example, a single instruction directly expresses an entire operation, including memory access and a calculation, without the program needing to break this down into multiple separate steps.
RISC processors take the opposite approach, deliberately keeping the instruction set small and each individual instruction simple, typically capable of performing only one basic operation at a time. The idea behind RISC is that simpler instructions can be executed more quickly and predictably, and that more complex operations can still be accomplished by combining several of these simple instructions together in sequence.
The same overall task using a RISC-style instruction sequence LOAD R2, memory_address_2000 (load first value into R2) LOAD R3, memory_address_3000 (load second value into R3) MULT R1, R2, R3 (multiply the two loaded values and store the result in R1)
Notice that this RISC example accomplishes the exact same overall task as the earlier CISC example, but it requires three separate, simpler instructions instead of one single complex instruction. Each individual RISC instruction, however, is simpler and more predictable for the hardware to execute quickly.
| Characteristic | CISC | RISC |
|---|---|---|
| Instruction Complexity | Complex, often multi-step instructions | Simple, single-step instructions |
| Number of Instructions per Task | Fewer instructions per task | More instructions per task |
| Instruction Set Size | Large, with many specialized instructions | Small, with fewer, general-purpose instructions |
In the previous chapter, we studied pipelining in detail, seeing how overlapping the fetch, decode, execute, and store stages of multiple instructions dramatically improves CPU performance. RISC's philosophy of simple, uniform instructions turns out to be an especially good match for pipelining, since instructions of consistent size and complexity are much easier for the pipeline's stages to process predictably and efficiently.
RISC pipeline example (each simple instruction takes a consistent amount of time at each stage) LOAD, LOAD, and MULT instructions from the earlier RISC example can move smoothly through the pipeline's fetch, decode, execute, and store stages, since each instruction follows a similarly simple, predictable structure A complex CISC-style instruction, in contrast, might take a variable and unpredictable amount of time to execute, since it could involve multiple internal memory accesses and calculations bundled into a single instruction, making it considerably harder to pipeline smoothly
This connection between RISC's simplicity and pipelining efficiency is one of the major reasons RISC-based designs became especially popular as pipelining techniques matured in modern CPU architecture.
Another important distinction between RISC and CISC lies in how each approach handles memory access. CISC instructions often allow direct memory operands within a single instruction, similar to the earlier MULT example, which accessed memory addresses directly. RISC instructions, on the other hand, typically follow a load-store philosophy, requiring data to be explicitly loaded into registers before any calculation can be performed on it.
RISC load-store philosophy Before performing a calculation, data must first be loaded into a register: LOAD R1, memory_address_5000 Only once the value is safely inside a register can it be used in an arithmetic operation: ADD R2, R1, R3 Directly performing arithmetic on a memory address without first loading it is generally not permitted in a pure RISC design
This load-store restriction keeps RISC instructions simple and predictable, since every arithmetic instruction only ever needs to deal with registers, never needing to also manage a potentially slower memory access at the exact same time.
Because CISC instructions can accomplish more work per individual instruction, CISC programs often require fewer total instructions to express the same overall task, which can result in more compact program code. RISC programs, needing more individual instructions to accomplish the same task, often result in a somewhat larger overall program size, measured in terms of the number of instructions and the memory space they occupy.
Comparing instruction counts for the same overall task CISC version: 1 instruction (MULT with direct memory operands) RISC version: 3 instructions (LOAD, LOAD, MULT) While the RISC version requires more individual instructions, each one is simpler and generally faster for the hardware to execute reliably
| Aspect | RISC | CISC |
|---|---|---|
| Instruction Set Size | Small and simple | Large and complex |
| Memory Access | Load-store philosophy, registers only for calculations | Direct memory operands often permitted |
| Pipelining Suitability | Very well suited, due to instruction uniformity | More challenging, due to variable instruction complexity |
| Code Size | Generally larger, due to more instructions per task | Generally smaller, due to fewer instructions per task |
| Hardware Complexity | Simpler control unit design | More complex control unit design |
Modern real-world CPU designs are rarely purely RISC or purely CISC in the strictest sense, but the underlying philosophies remain highly relevant when discussing different processor families. ARM processors, widely used in smartphones and many modern laptops, are generally associated with RISC design principles, prioritizing simplicity, power efficiency, and pipelining performance. Traditional x86 processors, commonly used in desktop computers, historically originated from CISC design principles, though modern x86 processors internally translate their complex instructions into simpler, RISC-like micro-operations before actually executing them.
CS Engineering Gyan's simulated comparison A smartphone using an ARM-based processor benefits from RISC's power efficiency and pipelining strengths, which is especially valuable for battery-powered devices A desktop computer using an x86-based processor benefits from decades of CISC-style software compatibility, while modern hardware internally applies many RISC-inspired techniques to maintain competitive performance
This blending of philosophies in modern hardware shows that RISC and CISC are best understood as two ends of a design spectrum, rather than as two completely separate, mutually exclusive categories.
| Advantages | Limitations |
|---|---|
| RISC's simple instructions pipeline efficiently and often improve raw execution speed. | RISC programs typically require more total instructions to accomplish the same task. |
| CISC's complex instructions can result in smaller, more compact program code. | CISC's variable instruction complexity makes efficient pipelining considerably more challenging. |
| Both philosophies offer valuable lessons that continue to influence modern hybrid CPU designs. | Choosing between the two involves genuine trade-offs, rather than one approach being universally superior. |
| Mistake | Correct Practice |
|---|---|
| Assuming RISC always results in faster overall program execution in every situation. | Understand that RISC pipelines efficiently, but often requires more total instructions to complete the same task. |
| Believing CISC instructions can never be pipelined at all. | Understand that CISC pipelining is more challenging, not impossible, and modern CISC processors use various techniques to improve it. |
| Assuming modern processors are purely RISC or purely CISC. | Remember that many modern CPUs blend ideas from both philosophies, such as x86 processors using internal RISC-like micro-operations. |
| Forgetting the load-store restriction that defines RISC's approach to memory access. | Remember that RISC generally requires data to be loaded into registers before any calculation can be performed on it. |
RISC and CISC represent two different philosophies for designing a CPU's instruction set, with RISC favoring a small set of simple, uniform instructions, and CISC favoring a larger set of complex, multi-step instructions. We compared these two approaches directly using worked examples, seeing how the same overall task can be expressed as a single complex CISC instruction or as a short sequence of simpler RISC instructions.
We also explored how RISC's simplicity connects directly to the pipelining efficiency covered in the previous chapter, how RISC's load-store philosophy differs from CISC's direct memory operand support, and how real-world processors like ARM and x86 relate to these underlying design philosophies, even as modern hardware increasingly blends ideas from both approaches.
With RISC versus CISC covered, you are now ready to explore parallel processing, where we will look at how modern computer systems go even further than pipelining, using multiple processing cores and other techniques to execute genuinely multiple instructions at the exact same time.