CISC Characteristics :
The design of an instruction set for a computer might take into consideration not only machine language constraints, but also the requirements imposed on the use of high-level programming languages. The translation from high-level to machine language programs is done by means of a compiler program. One cause for the trend to provide a complex instruction set is the desire to simplify the compilation and improve the overall computer performance. The task of a compiler is to produce a sequence of machine instructions for each high-level language statement. The job is simplified if there are machine instructions that implement the statements directly. The important aim of CISC architecture is to attempt to provide a single machine instruction for each statement that is written in a high-level language .An Examples of CISC architectures are the Digital Equipment Corporation VAX computer and the IBM 370 computer.
The main characteristics of CISC architecture are:
1. A large number of instructions-usually from 100 to 250 instructions.
2. Some instructions that perform specialized tasks and are used infrequently.
3. A vast variety of addressing modes-typically from 5 to 20 different modes.
4. Variable-length instruction formats.
5. Instructions that manipulate operands in memory.