INT 68 - APPC/PC - CHANGE NUMBER OF SESSIONS
        AH = 06h
        DS:DX -> control block (see below)

Format of control block:
Offset  Size    Description
 00h 12 BYTEs   reserved
 0Ch    WORD    1500h
 0Eh  6 BYTEs   0
 14h    WORD    (high byte first) primary return code (see AH=02h)
 16h    DWORD   (high byte first) secondary return code (see below, AH=01h)
 1Ah  8 BYTEs   (high byte first) logical unit ID
 22h  8 BYTEs   blanks
 2Ah  8 BYTEs   (high byte first) partner logical unit name
 32h  8 BYTEs   (high byte first) mode name
 3Ah    BYTE    bit 7: use MODE_NAME_SELECT_ALL rather than MODE_NAME
                bit 6: set negotiable values
 3Bh    BYTE    partner logical unit mode session limit
 3Ch    BYTE    minimum CONWINNERS_SOURCE
 3Dh    BYTE    maximum CONWINNERS_TARGET
 3Eh    BYTE    automatic activation
 3Fh    BYTE    0
 40h    BYTE    bit 7: drain target
                bit 6: drain source
                bit 5: target responsible, not source

Values for secondary return code (see also AH=01h):
  0000h accepted
  0001h negotiated
  0003h bad logical unit ID
  0004h allocation failure, no retry
  0005h allocation failure, retry
  0151h can't raise limits
  0153h all modes must reset
  0154h bad SNASVCMG limits
  0155h minimum greater than total
  0156h mode closed (prim return code = 1)
        CNOS mode closed (prim return code = 18h)
  0157h bad mode name (prim return code = 1)
        CNOS bad mode name (prim return code = 18h)
  0159h reset SNA drains
  015Ah single not SRC response
  015Bh bad partner logical unit
  015Ch exceeds maximum allowed
  015Dh change SRC drains
  015Eh logical unit detached
  015Fh CNOS command race reject