$STACK contains an integer value indicating the "level of nesting" caused by DO commands, XECUTE commands, and extrinsic functions in the M virtual stack.

$STACK has an initial value of zero (0), and increments by one with each DO, XECUTE, or extrinsic function. Any QUIT that does not terminate a FOR loop or any ZGOTO command decrements $STACK. In accordance with the M standard, a FOR command does not increase $STACK. M routines cannot modify $STACK with the SET or KILL commands.

Example:

        GTM>WRITE $STACK
        2
        GTM>WRITE $ZLEVEL
        3
        GTM>
          

This example shows the current values for $STACK and $ZLEVEL. $ZLEVEL is like $STACK except that uses one (1) as the starting level for the M stack, which $STACK uses zero (0), which means that $ZLEVEL is always one more than $STACK. Using $ZLEVEL with "Z" commands and functions, and $STACK with standard functions avoids the need to calculate the adjustment.