About CISC ‘Complex Instruction Set Computing’ Architecture
The CISC Approach:
CISC stand for “Complex Instruction Set Computing”. CISC is a CPU design techniques and makes the use of complex instruction architecture for performing the processing and execution of the tasks. The primary goal of the CISC architecture was to reduce the execution of the task in as minimum assembly language programming as possible. In order to achieve this goal the main emphasis was on the computer hardware that can understand and execute a series of the operations. It was capable to execute the several low-level operations (like memory store, load from memory) and is also capable of multi- step operations within the single instructions. So for multiplying task the CISC processor would come prepared with a specific instruction in the form of “MULT”. When the MULT was executed, the instruction will load the two values into the two separate registers, multiplies the operands, and then store the product in the appropriate register. So for multiplying two numbers the instruction used was:
MULT 4:5, 7:8
MULT is known as “complex instruction” which operates directly in memory and doesn’t any loading and storing of the functions.
Examples of the CISC instruction set architectures are System/360, PDP-11, VAX, Motorola68k and x86.
The advantages include such as that compiler has to do very little work to translate the high level instruction statement into assembly and as the length of the code is small so very little RAM memory is required to store the instructions. The main emphasis of this architecture was to put on the complex instructions directly in the hardware.
But due to the design limitations and high level of complexity with the design used in CISC architecture, it saw a set back and the manufacturers move on to use the RISC which reduced the complexity to a very minimum rate, more stress was focused on programming and sequencing the code rather than implementing all of them on the hardware by the use of transistors.
Summing up the CISC architecture:
- The main emphasis was on the hardware rather than software.
- CISC architecture used the ‘multi-clock’ complex instructions.
- Commands like “LOAD” and “STORE” were incorporated in the instructions itself and were allocated from memory-to-memory rather than the registers.
- Although the code size was small but it used high cycles per second.
- Transistors used for storing the complex instructions.
- Great deal of work was on the part of the processor.
