Section 14: Flags212
To begin learning how to use the flags, set flag 00:
Keystrokes Display    
g SF 
  SF _    
  The HP-41C prompts you for the flag number.
  Flag 00 is now SET. Flag 00 annunciator (0) turns on in the display.
Flag decisions are made using the test flag functions ( FS? , FC? , FS?C , and FC?C ). Each of these functions asks a question about the status of the specified flag. In a program, if the answer to the test question is TRUE, the calculator executes the next line in the program (this is the ‘‘DO if TRUE’’ rule again). If the answer to the question is false, the calculator skips the next line in the program before execution continues.
For example, if you use the FS? ‘‘flag set’’ test) function to check the status of flag 01 in a program and the flag is set, the next line in the program is executed. If the flag is clear, the next line in the program is skipped.
Is flag 01 SET?    
  If YES (flag 01 is set), continue with the next line.
FS? 01
  If NO (flag 01 is not set), skip one line.
Pressed from the keyboard, these flag functions will show an answer to the test question in the display. If the answer is true, the display shows YES; if the answer is false, the display shows NO.
Two of the flag test functions perform an additional function other than asking a question. These functions, FS?C (‘‘flag set’’ test and clear) and FC?C (‘‘flag clear’’ test and clear), also clear the specified flag in addition to testing it.
If at any time you are unsure as to the status of the flags, there are two ways to tell whether a flag is set or clear. (Remember, the status of some flags is maintained by the Continuous Memory of the HP-41C.)
First, and most simply, you can check the status of flags 00 through 04 by simply looking in the display at the flag annunciator. If any of these five flags are set, the corresponding number will show in the display annunciator at the bottom of the display window.
Second, you can test the flag with FS? or FC? without changing its status. Pressed form the keyboard, these functions return a YES or NO answer in the display.