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Instruction Sets and Their Types

By Prerana Jain Last updated : April 16, 2024

What is an Instruction Set?

An instruction set can be defined as a group of instruction that a processor can execute to perform different operations.

Types of Instruction Sets

On the basis of complexity and the number of instruction used the instruction set can be classified as:

1. Complex Instruction set
2. Reduced Instruction set

1. Complex Instruction set

The complex instruction set is the set of instruction which includes very complex and large number of instructions. The number of instruction in this set varies from 100 to 250. The instruction in this set is mostly memory based instruction, which involves frequent to the memory. The complex instruction set makes use of a large number of addressing modes because of the frequent references to register as well as memory. The instruction in this instruction takes a lot of time because the instructions format which is not limited to only 32 bit.

The complex which makes use of complex instruction set is called Complex Instruction set Computer (CISC). The instruction set of CISC has a large number of instruction and for each instruction type.

Advantages of Complex Instruction Set

• There is no need to invent an instruction set for each new design. The instruction set of the predecessor is used by the new user.
• A program written in CISC require less memory space as the code is confined to less number of instruction.
• CISC makes the job of a computer easier by facilitating the implementation of high-level language constructs.

Disadvantages of Complex Instruction Set

• The inheritance of old instruction into new processor increase the complexity.
• Many CISC instructions are not frequently used.
• CISC command is translated into a large number of lines of microcode which makes the CPU processing slower.
• CISC system has a complex hardware, so they require more times for designing.

2. Reduced Instruction Set

The reduced instruction set refers to a set of instruction that contains very few instructions ranging from 0 to 100. It comprises only those instruction that is frequently used by the processor for the execution of a program. The instruction is generally very simple to execute. The memory-based instruction includes the only load and stores instructions. The instruction in this instruction set has fixed length instruction format of 32 bits. An instruction format divides the bits of instructions into small group fields. Generally, an instruction has the following fields:

• Opcode field
  It represents the operations to be performed by the instruction.
• Operand field
  It represents the data on which the operations are to be performed or the memory location or registers where the data is stored.
• Mode field
  It represents the method of fetching the operands stored at specified memory location or register.

The comparison of RISC and CISC processor indicated that the RISC processor is always preferred over the CISC processor because of their compact size and small instruction set.

Advantages of Reduced Instruction Set

The other advantages of RISC processor over the CISC processor are as follow:

• In RISC processor the instructions are executed by decoding but in CISC processors, the instructions are executed by first and then translating them into equivalent microcode instructions. The conversion of instruction into microcode consumes a lot of space in the memory thereby reducing the speed of executions.
• The RISC processor executes an instruction in a single clock cycle, while the CISC processor requires multiple clocks cycles for the execution of an instruction.

Disadvantages of Reduced Instruction Set

The only disadvantages of RISC in comparison to CISC is that the number of instructions required to perform an operation is comparatively large.