INT 68 - APPC/PC
        AH = 03h
        DS:DX -> control block (see below)

Format of control block:
Offset  Size    Description
 00h 12 BYTEs   reserved
 0Ch    WORD    verb (action)
 0Eh  6 BYTEs   0
 14h    DWORD   (high byte first) return code (see AH=01h)
 18h    WORD    0
 1Ah  8 BYTEs   (high byte first) logical unit ID
---if verb=2400h (TP Started), control block continues---
 22h  8 BYTEs   (high byte first) TP ID
---if verb=2800h (Get ALLOCATE), control block continues---
 22h    BYTE    type
                00h dequeue
                01h test
 23h    DWORD   pointer to CREATE_TP record
---if verb=2A00h (Change Logical Unit). control block continues---
 22h    DWORD   pointer to CREATE_TP_EXIT routine
                FFFFFFFFh reject incoming ALLOCATEs
                00000000h queue ALLOCATEs
 26h    DWORD   00000000h
 2Ah    DWORD   pointer to SYSTEM_LOG_EXIT routine, FFFFFFFFh= don't log errors
 2Eh    DWORD   00000000h
 32h    BYTE    maximum TPs
 33h    BYTE    00h stop QUEUE_ALLOCATEs
                01h resume QUEUE_ALLOCATEs
 34h    DWORD   pointer to LU_LU_PASSWORD_EXIT routine, FFFFFFFFh = no exit
 38h    DWORD   00000000h