Instruction set design includes opcode and operand specifications. Two basic categories of Instruction Set or Instruction Set Architecture are:
Reduced Instruction Set Computers(RISC)
For faster execution of instructions, instructions should be fewer in number so that they make less use of memory. The type of computers using fewer instructions and executing them at faster speed are called Reduced Instruction Set Computers.
The basic concept behind designing of RISC is to reduce execution time. It has some peculiar characteristics:
- Hardwired rather than microprogrammed control.
- Fixed length instruction format.
- Memory is accessed only for storing instructions and fetching them on demand.
- Few number of instructions.
- Few addressing modes.
- Single cycle instruction execution.
- Relatively large number of registers in processor.
- Instruction pipeline is efficient.
Complex Instruction Set Computers(CISC)
A computer with a large number of instructions is classified as a Computer Complex Instruction Set Computer.
- A large number of instructions(typically from 100-250 instructions) are used with such computers.
- Variable length instruction formats.
- Instructions which manipulate operands in memory.
- A large number of addressing modes.
- Some special infrequently used instructions are provided to for performing some special tasks.