INT 68 - APPC/PC - TRANSACTION PROCESSING
        AH = 04h
        DS:DX -> control block (see below)
Return: control block updated

Format of control block:
Offset  Size    Description
 00h 12 BYTEs   reserved
 0Ch    WORD    verb (action)
                2500h TP_ENDED
                2900h TP_VALID
 0Eh  6 BYTEs   00h
 14h    DWORD   (big-endian) return code (see AH=01h/SF=1B00h)
 18h    WORD    00h
 1Ah  8 BYTEs   (big-endian) TP_ID
 22h    DWORD   -> CREATE_TP record (only if verb = 2900h)

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