OUT, if call successful:
Fc = 0, keyboard purged
Registers changed after return:
The system timeout is also reset, ie. running a loop with OS_Pur and OS_Tin in a loop never switches the machine off, even though the user never touches the keyboard.
GN_Sip, system input line routine
OS_In, read (wait for) character from standard input
OS_Tin, read character from standard input, with timeout
OS_Xin, examine input