System Debug Reference Manual (32650-90888)

48 Chapter2

User Interface

Control-Y

System Debug allows the user to prematurely terminate command execution by entering a

Control-Y (press and hold the

CONTROL key and press Y). Command loops, display loops

and modiﬁcation loops can be interrupted with this mechanism.

When Control-Y is entered during window updates, interrupted output lines may disturb

portions of the windows. When this occurs, redraw the windows with the RED (redraw)

command.

NOTE

There is only one Control-Y handler per session. When Debug is entered, it

takes ownership of the Control-Y handler. When Debug is exited, it returns

the Control-Y handler to the process that owned it when Debug was entered.

If other processes are active in a session while Debug is being used, it is possible for one of

the other process to steal Control-Y ownership from Debug. In this situation, when Debug

exits it will, in effect, steal Control-Y back from the current owner and give it to the process

that owned it when Debug was entered. If Control-Y is stolen from Debug, it is also

possible to create inﬁnite loops in Debug from which there is no way out (for example,

"while TRUE do {}").

Both DAT and Debug rearm the Control-Y trap after every CI command (for

example, the ":" command).

Command History, REDO

System Debug maintains a very short history of command lines in the form of a stack.

Commands in the stack can be displayed with the HIST (or LISTREDO) command, and may

be reexecuted with the DO command or edited prior to reexecution with the REDO command.

Commands read from outer level or interactive input are pushed onto the history stack.

Currently, commands read from USE ﬁles are also pushed onto the stack. Commands

executed as part of macro commands are not pushed.

Debug Input/Output: The System Console

Under normal circumstances, Debug Input/Output is typically directed to the user's

terminal. However, during the following occasions, Debug I/O is redirected to the MPE XL

system console:

• During the bootstrap process (until the system is up), all Debug I/O is directed to the

system console.