What is the interface that is the boundary between the hardware and programming software ISA or API?

The interface that serves as the boundary between hardware and programming software is the Instruction Set Architecture (ISA). The ISA defines the set of instructions that a processor can execute, along with the hardware capabilities it provides, allowing software to effectively communicate with and utilize the underlying hardware. In contrast, an Application Programming Interface (API) is a set of protocols and tools for building software applications, operating at a higher level than the ISA.