Codes
While starting the game, hold down the keys 1, 2, and 3 (top row only, not numeric keypad), until you see a message about you being "worthy of the powers of Jaffar". This gives you access to several cheats and debugging features. Pay attention to capitalization: you have to press Shift on most of them.

d: Tension (?) R: Room Number I: Invert Screen
F: Toggle animation frame # display, use with spacebar
K: Kill 1 life B: Toggle background display
W: Weightless - you don't die from falling
+/- : Add/subtract 1 to the number of minutes remaining

If the machine seems stuck from the "F" command, try hitting F1 and/or the spacebar.

You really need an Extended keyboard or equivalent, because these cheats disable all but the numeric keypad controls for moving the Prince. Some keyboards are also known to have problems enabling the cheat, because they cannot report more than two non-control keys being held down simultaneously.