HP 15c Scientific manual Flags,  n will clear flag number n

92 Section 8: Program Branching and Controls

Following a conditional test, program execution follows the "Do if True" Rule: it proceeds sequentially if the condition is true, and it skips one instruction if the condition is false. A t instruction is often placed right after a conditional test, making it a conditional branch; that is, the t branch is executed only if the test condition is met.

Flags

Another conditional test for programming is a flag test. A flag is a status indicator that is either set (= true) or clear (= false). Again, execution follows the "Do if True" Rule: it proceeds sequentially if the flag is set, and skips one line if the flag is clear.

The HP-15C has eight user flags, numbered 0 to 7, and two system flags, numbered 8 (Complex mode) and 9 (overflow condition). The system flags are discussed later in this section. All flags can be set, cleared, and tested as follows:

Fn will set flag number n (0 to 9).

"n will clear flag number n.

?n will check if flag n is set.

A flag n that has been set remains set until it is cleared either by a " n instruction or by clearing (resetting) Continuous Memory.