4.0 The Language of the CPU: CISC vs. RISC
The “instruction set” is the vocabulary of commands that a processor understands. Just as humans use different languages, processors are designed around different instruction set philosophies. The two dominant philosophies are CISC and RISC.
4.1 CISC: The Complex Instruction Set Computer
CISC stands for Complex Instruction Set Computer. As the name implies, a CISC processor is designed to understand and execute a large number of complex instructions.
4.2 RISC: The Reduced Instruction Set Computer
RISC stands for Reduced Instruction Set Computer. The RISC philosophy is to use a smaller, highly optimized set of instructions. Each instruction is simple and can be executed much faster within the CPU, without needing to access memory as often.
4.3 Core Distinctions
The core distinctions between CISC (Complex Instruction Set Computer) and RISC (Reduced Instruction Set Computer) are summarized below.
| CISC | RISC |
| Instruction Set | Larger set of instructions, making it easier to program directly. |
| Compiler Design | Simpler compiler design. |
| Addressing & Formatting | Many addressing modes and variable-length instruction formats. |
| Execution Speed | Higher clock cycles per instruction, resulting in slower execution. |
| Design Emphasis | Emphasis is on the hardware to handle complexity. |
| Pipelining | Pipelining is difficult to implement due to variable instruction length and complexity. |
4.4 The ‘So What?’: Hardware vs. Software Complexity
The fundamental difference between CISC and RISC is where the complexity lies. CISC architecture places the burden on the hardware. The goal is to make the programmer’s job easier by providing a rich set of powerful instructions that can accomplish complex tasks in a single step. In contrast, RISC architecture simplifies the hardware and shifts the burden to the software. It relies on a sophisticated compiler to break down high-level commands into many simple, fast instructions that the streamlined hardware can execute very quickly.
With these core architectural comparisons in mind, we can now see the bigger picture of computer design.