Difference between CISC and RISC In Computer System

 CISC Vs. RISC

Hello Dear Reader,

Most Welcome in my second blog I have started this one after I got succeded in my first blog https://nishitnathwani.blogspot.com/ 

Today I will be discussing the difference between CISC and RISC two types of hardware systems inside the computer system.

CISC is the general name for CPUs that have a complex instruction set. An instruction set is said to be complex if there are some instructions that perform complex operations or the instruction formats are not uniform. The Intel x86 family, Motorola 68000 series, PDP-11, and VAX are examples of CISC. The CISC instruction set tries to enhance the code density so that a computer system can use a small amount of memory, including cache, to store as many instructions as possible for reducing the cost and improving performance. CISC adopts two measures to reduce the code size – it lets an instruction perform as many operations as possible and makes the encoding of each instruction as short as possible. The first measure results in using microcode to implement the complex instructions, and the second measure results in a variable length of the instruction formats. 

As a consequence, it becomes difficult to design and implement a pipelined CISC CPU to obtain substantial performance improvements. 

Is every instruction in a CISC complex? The answer is No. There are some very simple instructions in a CISC. The analysis of the instruction mix generated by CISC compilers shows that about 80% of executed instructions in a typical program uses only 20% of an instruction set and these instructions perform the simple operations and use only the simple addressing modes. 

RISC is the general name for CPUs that have a small number of simple instructions. In the 1980s, the team headed by David Patterson of the University of California at Berkeley investigated the existing ISAs proposed the term of RISC and made two CPU prototypes: RISC-I and RISC-II. This concept was adopted in the designs of Sun Microsystems’ SPARC microprocessors. Meanwhile, the team headed by John Hennessy of Stanford University did similar research and created MIPS CPUs. Actually, John Cocke of IBM Research originated the RISC concept in the project IBM 801, initiated in 1974. The first computer to benefit from this project was IBM PC/RT (RISC technology), the ancestor of the RS/6000 series. There are two main features in a RISC CPU. One is the fixed length of the instruction formats. This feature makes fetching an instruction in one clock cycle possible. The other feature is the so-called load/store architecture. It means that only the load and store instructions transfer data between the register file and memory, and other instructions perform operations on the register operands. This feature makes the operations of the RISC instructions simple. Both features make the design of the pipelined RISC CPUs are easier than CISC CPUs. In below Figure is given a short picture summary of this architecture.

The SUN Microsystems SPARC, AMD 29000 family, SGI MIPS, IBM PowerPC, HP PA-RISC, and ARM are examples of RISC. Was the CISC replaced by RISC? The answer is No. 

The reason is simple – the market. The Intel x86 ISA is widely used in the IBM-compatible PC, which is the most common computer system in the world. There is a huge amount of software resources that we cannot throw away. Today, most CISC CPUs use a decoder to convert CISC instructions into RISC instructions (micro-operations) and then use RISC cores to execute these instructions. Meanwhile, many RISC CPUs add more new instructions to support complex operations, multimedia operations, for instance, High-speed signal processing, etc...

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Credits:

https://www.watelectronics.com/what-is-risc-and-cisc-architecture/

Computer Principles and Design in Verilog HDL by Yamin Li

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