ISA

An instruction set (ISA) is a group of commands for a CPU in machine language. It refers to all possible instructions for a CPU including  :-

•  native data types,
•  instructions,
•  registers,
•  addressing modes,
•  memory architecture,
•  interrupt
•  exception handling,
•  external I/O. 

An ISA includes a specification of the set of opcodes , and the native commands implemented by a particular processor. All CPUs have instruction sets that enable commands to the processor directing the CPU to switch the relevant transistors. Some instructions are simple read, write and move commands that direct data to different hardware.

In CISC (Complex Instruction Set Computer) processors there is also a microcode layer, which involves programmable instructions stored in fast memory that may be updated. A complex instruction set computer (CISC) has many specialized instructions, some of which may only be rarely used in practical programs.

The RISC (Reduced Instruction Set Computer) architecture, on the other hand, has hard-wired control; it does not require microcode but has a greater base instruction set. A reduced instruction set computer (RISC) simplifies the processor by efficiently implementing only the instructions that are frequently used in programs, while the less common operations are implemented as subroutines, having their resulting additional processor execution time offset by infrequent use.

Instruction Types :- 

• Logical
• Arithmetic
• Control
• Data Transfer

Instruction Format

 

 k