DOS API
The MS-DOS API is an API used in MS-DOS, PC-DOS and other compatible operating systems. This API has been extended with the so-called DOS extenders. The MS-DOS API lives on in Microsoft Windows as a subsystem on cmd.exe.
Modern movements to maintain compatibility of the MS-DOS API involve FreeDOS and DOSBox. They are both an installable operating system and an emulator respectively. DOSBox has even extended the MS-DOS API to other chipsets outside of x86. The other chipsets DOSBox has extended MS-DOS's API support to include PowerPC, ARM Architecture, MIPS, SPARC, etc.