Compiler directive that instructs the compiler to look for assembler routines in the specified LIB file.
The $LIB directive is used internally by the compiler in order to enable the customizing of the assembler routines by the user.
You can use it to specify your own libraries. You can for example copy the mcs.lib file to a new file named mylib.lib and delete the content of the mcs.lib file. This way the compiler will use your routines. The mcs.lib file must exist in the \LIB subdirectory and that is why you may not delete it.
Always make a backup of the mcs.lib file before you change it.
It is not encouraged to change the mcs.lib file itself other than making a dummy because updates will contain more asm routines and you have to change everything for each update.