Int 21 Fn FFBF  - Topware Network OS V5.10+ - Get Logon User Information   [N]

   AX = FFBFh
   DX:BX -> buffer for logon information (see below)

Return: AL = status (00h successful, else failed)
   AH = number of logged-in stations

Format of logon information:
Offset Size    Description
 00h   BYTE    station address
 01h 15 BYTEs  username