Which acronym refers to a processor architecture with a large set of instructions and emphasis on backward compatibility?

• A. Complex Instruction Set Computing (CISC)
• B. Reduced Instruction Set Computing (RISC)
• C. Instruction Set Architecture (ISA)
• D. Operating System

Answer: (A)

Complex Instruction Set Computing is known for a large instruction set and an emphasis on backward compatibility. In CISC designs, there are many instructions, including some that perform complex tasks in a single operation, and there are multiple ways to address memory. This richness helps ensure older software continues to run on newer processors because the instruction formats and behaviors remain compatible, often supported by microcode inside the CPU. In practice, you’ll see architectures like x86 described as CISC for this reason. This contrasts with designs that use a small, fixed set of simple instructions, which aim for straightforward decoding and execution but may require more instructions to accomplish the same task, and less emphasis on maintaining compatibility with very old code.