Register parameters

IN:  -

OUT, if call successful:
     Fc = 0
     BHL = pointer to alarm block


OUT, if call failed:
     Fc = 1
     A = error code
          RC_ROOM ($07), no room to allocate alarm block

Registers changed after return:
     ...CDE../IXIY same
     AFB...HL/.... different

Related calls

GN_Alp, process an expired alarm
GN_Fab, free alarm block
GN_Lab, link an alarm block into the alarm chain
GN_Uab, unlink an alarm block from the alarm chain
OS_Alm, alarm manipulation (internal OZ usage)