$ZST[EP] contains a string value specifying the default action for the ZSTEP command. $ZSTEP provides the ZSTEP action only when the ZSTEP command does not specify an action.
$ZSTEP initially contains the string "B" to enter direct mode. $ZSTEP is a read-write Intrinsic Special Variable, (i.e., it can appear on the left side of the equal sign (=) in the argument to the SET command).
GTM>WRITE $ZSTEP B GTM>
This example displays the current value of $ZSTEP, which is the default.
GTM>SET $ZSTEP="ZP @$ZPOS B"
This example sets $ZSTEP to code that displays the contents of the next line to execute, and then enters Direct Mode.