C preprocessor

I saw in a document ,which says that (it was on C preprocessor) the OS will set a macro names SYSTEM which will be equal to the OS currently running. For example if it is a MS-DOS system , SYSTEM will qual to MSDOS, and the example given was,
#if SYSTEM==MSDOS
.
.
.

How true is the fact that the OS sets a macro named SYSTEM. Pls explain.

> How true is the fact that the OS sets a macro named SYSTEM.
Not true at all generally speaking. Any such macro name would be set by the compiler, and as such would be compiler specific.