Instruction Set Architecture (ISA)

Instruction Set Architecture (ISA) is the part of a computer architecture that defines the instructions that a processor can execute. The instruction set provides the interface between the hardware and the software that runs on a computer. An ISA defines the data types, registers, memory organization, and instruction formats that a computer can execute. The ISA also defines the number of instructions that a processor can execute and the number of cycles required to execute each instruction.

CISC and RISC

The instruction set can be classified as CISC (Complex Instruction Set Computing) and RISC (Reduced Instruction Set Computing). CISC architectures use instructions that are capable of performing complex operations in a single instruction, while RISC architectures use a large number of simple instructions to perform complex operations. RISC architectures are faster and more efficient than CISC architectures because they require fewer clock cycles per instruction.

Addressing Modes

The instruction set can also be classified based on the addressing modes used by the processor. The addressing mode determines how the processor accesses memory to read or write data. Some common addressing modes are:

• Immediate: used when the operand is specified in the instruction itself.
• Direct: used when the operand is located in a memory location.
• Indirect: used when the memory location of the operand is specified in a register.
• Indexed: used when the memory location of the operand is specified by adding an offset to a register value.
• Relative: used when the memory location of the operand is specified relative to the program counter.

Data Types

The instruction set can also define the size and format of the data types that the processor can handle. The most common data types are byte, word, double word, and quad word. The data format can be little-endian or big-endian. In little-endian format, the least significant byte of a word is stored first, while in big-endian format, the most significant byte of a word is stored first.

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