INT 5B - ISOLAN Multi Protocol Software
        ES:BX -> Transfer Control Block
Return: AL - status
Note:   this software interface allows multiple protocols/software packages
          to access a BICC 411x network card

Format of Transfer Control Block:
Offset  Type    Description
 00h    BYTE    command code
                B3h Status
                F2h Activate
                F3h Deactivate
                F4h Send Data
 01h    BYTE    command identity
 02h    BYTE    virtual circuit ID
 03h    WORD    buffer length
 05h    DWORD   buffer pointer
 09h    BYTE    expedited data flag
 0Ah    BYTE    cancelable flag
 0Bh 16 BYTEs   local network address
 1Bh 16 BYTEs   remote network address
 2Bh    DWORD   asynchronous notification routine
 30h    DWORD   local network number
 34h    DWORD   remote network number
 38h    BYTE    call timeout
 39h    BYTE    not used
 3Ah  8 BYTEs   reserved
 42h    BYTE    command code extension
 43h    WORD    Blue Book MAC type