GN_Aab, Allocate alarm block
Register parameters
RST 20H, DEFW $6809
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