Some of the functionality of DST is duplicated in the System Service Tools (STRSST) command that can be called from an OS/400 green screen, but because there are several items in DST that IBM doesn’t want activated by the casual user, you have to explicitly put your system into manual mode to enter DST.
DST is available when the Licensed Internal Code has been started, even if OS/400 has not been loaded. SST is available from OS/400. DST physical access through console during a manual IPL or by selecting option 21 on the control panel. SST access through interactive job. There is a way to force Dedicated Service Tools via SST but won’t discuss here and personally haven’t done so since v5r2.