I was having problems IPLing z/OS. Once you can logon to TSO you can use SDSF to display the console. The problem is when the system fails before you can logon to TSO.

When z/OS is IPLed a few messages are displayed, then the console goes into wrap mode, and message roll past faster than I can read them.

You can change the roll mode using the command to not delete messages automatically

K S,DEL=N

You can then use

K

to clear the screen.

You can use K S,REF to display and overtype values. For example RTME=5 says roll every 5 seconds, RTME=1/4 says roll every quarter of a second.