INT 2A - NETWORK - PCIBM PC LAN PROG v1.31+ - GET LOGGED ON USER NAME
AX = 7802h
ES:DI -> 8-byte buffer to be filled
Return: AL = 00h if no user logged on to Extended Services
AL <> 00h if user logged on to Extended Services
buffer at ES:DI filled with name, padded to 8 chars with blanks.