Overview
An instruction set architecture (ISA) is the programmer-visible contract between software and hardware.
If two processors implement the same ISA correctly, the same machine code should run on both even if their internal circuits are very different.
What belongs to the ISA
- the instruction set and opcodes
- the register set and other programmer-visible state
- the supported data types
- the available addressing modes
- the memory organization visible to software
- the meaning of control-flow instructions
This is why LC-3 and MIPS are both examples of an ISA, not examples of a particular hardware implementation.
The instruction set as the core of the ISA
The instruction set defines three especially important things:
- opcodes: which operations exist
- data types: which kinds of values can be manipulated
- addressing modes: how operands are found in registers or memory
These choices strongly affect compiler design, code density, and hardware complexity.
ISA trade-offs
Instructions / opcodes
- more complex instructions can do more work per instruction and may reduce code size
- simpler instructions are easier to decode, control, and optimize in hardware
This is one of the classic trade-offs behind RISC vs. more complex instruction sets.
Data types
- more data types map more directly to high-level languages
- fewer data types simplify hardware, but software may need extra instructions or emulation
Addressing modes
- more addressing modes can make memory access more expressive and compact
- fewer addressing modes simplify decode logic and usually fit a cleaner load/store design
For example, LC-3 uses several addressing modes, while MIPS keeps a simpler load/store style.
Semantic gap
The semantic gap is the distance between high-level programming languages and the ISA.
A smaller semantic gap usually means the ISA provides more complex instructions or richer data types, so software maps more directly onto the machine. A larger semantic gap usually means a simpler ISA, so compilers do more work while hardware can stay simpler.