Dell OptiPlex 790 (Early 2011) - Manual (Page 60)

benefit here is that you do not have to remember the and
keystrokes (although they still work).
NOTE: The BIOS features an option to disable either or both of the keystroke
prompts under the System Security / Post Hotkeys submenu.
When you enter the or keystroke correctly, the computer beeps.
The key sequence invokes the Boot Device Menu.
Since the one-time boot menu only affects the current boot, it has the added benefit of
not requiring the technician to restore the customer's boot order after completing
troubleshooting.
Timing Key Sequences
The keyboard is not the first device initialized by Setup. As a result, if you press a
keystroke too early, you lock out the keyboard. When this happens, a keyboard error
message appears on the monitor, and you cannot restart the system with the
keys.
To avoid this scenario, wait until the keyboard is initialized before pressing the keystroke.
There are two ways to know that this has happened:
• The keyboard lights flash.
• The "F2=Setup" prompt appears in the top right-hand corner of the screen during
boot.
The second method is good if the monitor is already warmed up. If it is not, the system
often passes the window of opportunity before the video signal is visible. If this is the
case, rely on the first method—the keyboard lights—to know the keyboard is initialized.
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