What is CISC?

Complex Instruction Set Computing, or CISC, is a type of computer architecture that emphasizes high-level instruction sets capable of performing complex operations in a single instruction. The CISC architecture is characterized by its ability to execute instructions that manipulate memory data, perform arithmetic and logical operations, and control program flow. This is in contrast to Reduced Instruction Set Computing (RISC), which emphasizes a smaller number of simpler instructions and relies on pipelining and other techniques to achieve high performance.

CISC processors were first developed in the 1970s and were popular throughout the 1980s and early 1990s. They were favored over RISC processors because of their ability to map common programming operations into single assembly language instructions, which made it easier for programmers to write code. However, CISC processors are now less commonly used as RISC processors have become increasingly popular due to their simpler instruction sets, which are easier to optimize for performance.

CISC processors typically have larger instruction sets than RISC processors, with instructions that can perform multiple tasks. These instructions are often complex and require more hardware resources to decode and execute. CISC processors may also have specialized hardware for specific operations, such as floating-point arithmetic or graphics processing. However, these features can make CISC processors more costly to design and manufacture than RISC processors.

One of the defining features of CISC processors is their ability to execute memory-to-memory operations. This means that instructions can operate directly on data in memory instead of moving it to and from registers. For example, a CISC processor might have an instruction that adds the contents of two memory locations and stores the result in another memory location. This operation can be completed in a single instruction on a CISC processor but would require several instructions on a RISC processor.

Another feature of CISC processors is their ability to execute instructions of variable length. This allows CISC processors to have a flexible instruction set, with instructions of varying complexity. However, this feature can make CISC processors more difficult to decode and execute, as the processor must determine the length of each instruction before executing it.

CISC processors can be found in a variety of devices, from personal computers to embedded systems and mobile devices. Some popular CISC processor architectures include the Intel x86 architecture, used in most personal computers, and the Motorola 68000 architecture, used in early Apple Macintosh computers and many embedded systems.

In conclusion, CISC is a type of computer architecture that emphasizes high-level instruction sets capable of performing complex operations in a single instruction. While CISC processors have been popular in the past, they are now less commonly used as RISC processors have become more popular due to their simpler instruction sets. CISC processors can be found in a variety of devices, from personal computers to embedded systems and mobile devices, and include popular architectures such as the Intel x86 architecture and the Motorola 68000 architecture.