CISC and RISC in Computer Architecture
CISC in Computer Architecture:
It stands for Complex Instruction Set Computer (CISC). In 1978, Intel developed the 8086 microprocessor chip. The 8086 designed to process a 16-bit data word. It had no instruction for floating-point operations. The Pentium currently processes 32-bit and 64-bit words, and it can process floating-point instructions. Intel designed the Pentium processor in such a way that it can execute programs written for earlier 80 x 86 processors.
The characteristics of 80 x 86 are called complex instruction set computers (CISC), which include instructions for earlier Intel processors. Another CISC processor is VAX 11/780, which can execute programs for the PDP-11 computer. The CISC processor contains many instructions with different addressing modes. For example, the VAX 11/780 has more than 300 instructions with 16 different address modes.
Characteristics of CISC:
1. A large number of instructions.
2. Many addressing modes.
3. Variable length of instructions.
4. Most instructions can manipulate operands in the memory.
5. Control unit is microprogrammed.
RISC in Computer Architecture:
It stands for Reduced Instruction Set Computer (RISC). Until the mid-1990s, computer manufacturers were designing complex CPUs with large sets of instructions. At that time, several computer manufacturers decided to design CPUs capable of executing only a very limited set of instructions. One advantage of a reduced-instruction set of computers is that they can execute their instructions very fast because the instructions are simple. In addition, the RISC chip requires fewer transistors than the CISC chip. Some of the RISC processors are the PowerPC, MIPS processor, IBM RISC System/6000, ARM, and SPARC.
Characteristics of RISC:
1. Require a few instructions.
2. All instructions are the same length (they can be easily decoded).
3. Most instructions are executed in one machine clock cycle.
4. Control unit is hardwired.
5. Few address modes.
6. A large number of registers.