Use &SYSTEM_ID to obtain the name and release of the operating system under which your source module is being assembled. &SYSTEM_ID has a global scope.

For example, on z/OS, &SYSTEM_ID might contain one of the following:

z/OS 01.04.00
z/OS 01.05.00
... and so on

on CMS, &SYSTEM_ID might contain one of the following:

CMS 18
CMS 19
... and so on

on z/VSE, &SYSTEM_ID might contain one of the following:

VSE/AF 6.6.0
... and so on