Owner's handbook and programming guide, HP-41C

Now, to compute the factorial of 6, key in the letters of the function name:

Keystrokes	Display

ALPHA	XEQ _	Places the HP-41C into ALPHA mode.
FACT	XEQ FACT _	Tells the HP-41C that you want to execute the FACT (factorial) function.
ALPHA	720.0000	When you take the HP-41C out of ALPHA mode, the function in the display is executed. The answer is in the displayed X-register, just as with any other function.

Let’s try another function. When you execute a function that requires some input, such as FIX (which requires a number from 0 through 9), the HP-41C will prompt for the input. (Note that FIX may also be executed directly from the keyboard.)

For example, set the calculator to FIX 6.

Keystrokes	Display

XEQ	XEQ _ _	The HP-41C prompts: Execute what?
ALPHA	XEQ _	Place the HP-41C into ALPHA mode...
FIX	XEQ FIX _	...and spell the function name, FIX.
ALPHA	FIX _	The HP-41C now prompts you for the input required by FIX.
6	720.000000	The FIX function is executed when you enter the required digit.

Any function requiring input, such as the FIX function shown above, is executed when you enter the last required digit. FIX requires one digit, so it executes when that digit is entered. Some other functions require two or three digits, and they are executed when the final required digit is entered

Note that the contents of the ALPHA registers are not disturbed when you execute a function from the display.