CISC (COMPLEX INSTRUCTION SET COMPUTER) ARCHITECTURE MICROCONTROLLERS
Almost all of today’s microcontrollers are based on the CISC (Complex Instruction Set Computer) concept. When an microcontroller has an instruction set that supports many addressing modes for the arithmetic and logical instructions, data transfer and memory accesses instructions, the microcontroller is said to be of CISC architecture.
The typical CISC microcontroller has well over 80 instructions, many of them very powerful and very specialized for specific control tasks. It is quite common for the instructions to all behave quite differently. Some might only operate on certain address spaces or registers, and others might only recognize certain addressing modes.
The advantages of the CISC architecture are that many of the instructions are macrolike, allowing the programmer to use one instruction in place of many simpler instructions. An example of CISC architecture microcontroller is Intel 8096 family.
RISC (REDUCED INSTRUCTION SET COMPUTER) ARCHITECTURE MICROCONTROLLERS
The industry trend for microprocessor design is for Reduced Instruction Set Computers (RISC) designs. When a microcontroller has an instruction set that supports fewer addressing modes for the arithmetic and logical instructions and for data transfer instructions, the microcontroller is said to be of RISC architecture.
The benefits of RISC design simplicity are a smaller chip, smaller pin count, and very low power consumption. Some of the typical features of a RISC processor- Harvard architecture are:
1. Allows simultaneous access of program and data.
2. Overlapping of some operations for increased processing performance.
3. Instruction pipelining increases execution speed.
4. Orthogonal (symmetrical) instruction set for programming simplicity.
5. Allows each instruction to operate on any register or use any addressing mode.
SISC (SPECIFIC INSTRUCTION SET COMPUTER)
Actually, a microcontroller is by definition a Reduced Instruction Set Computer. It could really be called a Specific Instruction Set Computer (SISC). The basic idea behind the microcontroller was to limit the capabilities of the CPU itself, allowing a complete computer (memory, I/O, interrupts, etc) to fit on the single chip. At the expense of the more general purpose instructions that make the standard microprocessors (8088, 68000, 32032) so easy to use, the instruction set was designed for the specific purpose of control (powerful bit manipulation, easy and efficient I/O, and so on).
For more info refer Types of Microcontroller