OUT, if call successful:
Fc = 0, input MAY BE available : a following OS_In will return immediately possibly with error
Fc = 1, A=RC_EOF ($09), no input, OS_In will wait
Registers changed after return:
This call return if a key is pressed or not. It avoids OS_In usage which has more overhead. For example, Pipedream uses OS_Xin during calculations, system menu and help looks for a key press with OS_Xin during menu and help rendering.
The function of this call is not so much to tell you when input is available, but rather to indicate when it definitely is not.
The call might be useful to examine if OS_In is going to return a pre-emption error like RC_SUSP or RC_QUIT. Possible reasons are:
exception code (e.g. RC_SUSP due to machine reviving)
GN_Sip, system input line routine
OS_In, read (wait for) character from standard input
OS_Pur, purge keyboard buffer
OS_Tin, read character from standard input, with timeout