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.