@DEBUG
Syntax: Debug
Usage : Interactive Debugger

Debug command summary:

[SPACE]expression     Evaluate expression; display result in
                      hex and decimal form.
.                     Display Dot address and contents.
..                    Restore last Dot address; display address
                      and contents.
.expression           Set Dot to result of expression; display
                      address and contents.
=expression           Set memory at Dot to result of expression.
-                     Decrement Dot; display address and
                      contents.
[ENTER]               Increment Dot; display address and
                      contents.

:                     Display all registers' contents.
:register             Display specified register's contents.
:register expression  Set specified register to result of
                      expression.

E module-name         Prepare for execution.
G                     Go to the program.
G expression          Go to the program at the address specified
                      by the result of expression.
L module-name         Link to module-name; display address.

B                     Display all breakpoints.
B expression          Set a breakpoint at the address specified
                      by expression.
K                     Kill (remove) all breakpoints.
K expression          Kill breakpoint at the address specified
                      by expression.

M expr1 expr2         Display memory from expr1 to expr2.
C expr1 expr2         Clear and test memory from expr1 to expr2.
S expr2 expr2         Search memory from Dot to expr1 for
                      value expr2.

$text                 Call OS-9 shell.

Q                     Quit (exit) debug.

The full manual is here:
https://sourceforge.net/p/nitros9/wiki/Interactive_Debugger