Advantages And Disadvantages Of Risc

Reduced Instruction Set Computer (RISC)

Reduced Instruction Set Computer (RISC) pronounced as “risk” is a type of microprocessor architecture that uses relatively very less or limited number of instructions. Until mid-1980s computer manufacturers used to build computers with relatively complex Central Processing Units (CPU) and large number of instructions. At that time computer manufacturers came together to reverse the trend of using large and complex instructions by using limited set of instructions in computer architecture. “One advantage of reduced instruction set computers is that they can execute their instructions very fast because the instructions are so simple. Another, perhaps more important advantage, is that RISC chips require

Since the earliest machines were programmed in assembly language and memory was slow and expensive, the CISC viewpoint made sense, and was commonly realized in such large computers such as the PDP-11 and DEC system 10 and 20 machines. Most common microprocessor designs that uses CISC are Intel 80x86 and Motorola 68K series. But new alterations in software and hardware technology have led to re-examination of CISC and many modern CISC processors are mixtures, employing many RISC principles. CISC was developed to make compiler development simpler. It shifts most of the burden of making machine instructions to the processor. Example, in place of having to make a compiler write long machine commands to calculate a square-root, a CISC processor would have a built-in ability to do this. Simple comparison between CISC and RISC.

History of RISC.

Many system dating back to 1960s are termed as first RISC architecture because they used loading and storing of instruction concept which made computers perform faster. Although term RISC was used before it is believed that it was termed by David Patterson who was working for Berkeley RISC project. Later in 70s and 80s the RISC projects also came from IBM, Stanford and many more. Since then device manufacturers have used concept of RISC to manufacture various devices and has greatly improved till date.

Main features

The IA-32, for instance, allows register to register as well as register-to-memory operations; it does not allow memory to memory operations. The VAX 11/780 also allows memory to memory operations. RISC processors only allows special load and store operations to access memory. The remaining of the operations work on a register to register basis. This feature simplifies instruction set design as it allows execution of instructions at a one instruction/cycle rate. Limiting number of instructions to register also simplifies the control unit.

3. Uses simple addressing modes.

When simple addressing modes are used it allows fast address computation of operands. As RISC processor uses register to register instruction and most commands uses this addressing modes. In special case of load and store instructions require memort to memory addressing modes.

4. RISC employs large register