CISC

Definition

CISC, or Complex Instruction Set Computing, refers to a type of computer architecture that has a rich set of instructions. This allows programmers to execute complex operations with fewer lines of code by using instructions that can perform multiple low-level tasks in a single command. CISC architectures are designed to make programming easier and reduce the number of instructions per program, which can lead to more efficient use of memory.

5 Must Know Facts For Your Next Test

1. CISC architectures typically have hundreds of instructions, which can include complex operations like string manipulation and arithmetic calculations.
2. The primary goal of CISC is to complete a task in as few instructions as possible, which can help in reducing program size and memory usage.
3. CISC processors often require more transistors than RISC processors due to their complex instruction decoding logic and additional circuitry for handling microcode.
4. While CISC allows for more powerful instructions, it can result in slower execution times because of the complexity involved in decoding and executing those instructions.
5. Common examples of CISC architectures include x86 and Intel's 8086 family of processors, which are widely used in personal computers.

Review Questions

• How does the instruction set size in CISC compare to that in RISC, and what implications does this have for programming?
  • CISC architectures feature a larger instruction set compared to RISC, allowing for more complex operations to be executed with fewer lines of code. This means that programmers can often write shorter programs using high-level commands. However, this complexity can lead to challenges in instruction decoding and execution speed, as the processor must manage more intricate operations.
• Discuss how microcode functions within CISC architectures and its impact on performance.
  • Microcode serves as an intermediary between high-level machine code instructions and the underlying hardware in CISC architectures. It breaks down complex instructions into simpler steps that the processor can execute. While this allows for versatile instruction capabilities, it may also introduce delays due to the extra processing required for decoding and executing these microcoded commands.
• Evaluate the advantages and disadvantages of CISC compared to RISC in terms of design philosophy and practical applications.
  • CISC's design philosophy focuses on reducing the number of instructions per program by allowing more complex operations with single commands, making it easier for developers to write software. However, this can lead to slower execution speeds due to the need for elaborate instruction decoding. On the other hand, RISC emphasizes simplicity and efficiency with a smaller instruction set, leading to faster execution times but potentially larger programs. The choice between CISC and RISC often depends on specific application needs, such as processing power versus memory efficiency.

Related terms

RISC:

RISC, or Reduced Instruction Set Computing, is a computer architecture that uses a small set of simple instructions, aiming for high performance through efficient instruction execution.

Instruction Set Architecture (ISA): ISA is the part of the computer architecture related to programming, defining the set of instructions that the processor can execute and how they interact with the hardware.

Microcode: Microcode is a layer of hardware-level instructions or data structures that implement the higher-level machine code instructions for CISC architectures.