Hardware abstractions are sets of routines in software that provide programs with access to hardware resources through programming interfaces. The programming interface allows all devices in a particular class C of hardware devices to be accessed through identical interfaces even though C may contain different subclasses of devices that each provide a different hardware interface. Hardware abstractions often allow programmers to write device-independent, high performance applications by providing standard operating system (OS) calls to hardware. The process of abstracting pieces of hardware is often done from the perspective of a CPU. Each type of CPU has a specific instruction set architecture or ISA. The ISA represents the primitive operations of the machine that are available for use by assembly programmers and compiler writers. One of the main functions of a compiler is to allow a programmer to write an algorithm in a high-level language without having to care about CPU-specific instructions. Then it is the job of the compiler to generate a CPU-specific executable. The same type of abstraction is made in operating systems, but OS APIs now represent the primitive operations of the machine, rather than an ISA. This allows a programmer to use OS-level operations (e.g. task creation/deletion) in their programs while retaining portability over a variety of different platforms. Many early computer systems did not have any form of hardware abstraction. This meant that anyone writing a program for such a system would have to know how each hardware device communicated with the rest of the system. This was a significant challenge to software developers since they then had to know how every hardware device in a system worked to ensure the software's compatibility. With hardware abstraction, rather than the program communicating directly with the hardware device, it communicates to the operating system what the device should do, which then generates a hardware-dependent instruction to the device.