DOS Idle/Power control in mTCP Applications
2013-04-26 Version
Michael Brutman (mbbrutman@gmail.com)


Introduction

  Part of the mTCP design requires the application programmer to
  periodically check for new packets that have arrived so that they
  may be processed.  That check is supposed to be done when the
  application is not actively working on something else; it is a
  core part the idle loop in an event driven program.
  
  On old classic hardware running DOS such as the IBM PC or PCjr
  this polling behavior does no harm.  But it does keep the CPU busy
  and that can show up when using mTCP applications in a virtual
  environment such as VirtualBox or VMWare.  A side effect is higher
  power usage which is an important consideration for laptop users.


How mTCP minimizes CPU usage
  
  Starting with the 2012-04-29 version mTCP applications will try
  to minimize unnecessary CPU usage.  When in the idle loop 
  each application will check for packets and work to do and if
  nothing is found they will try to "sleep" for a short period.
  The mechanisms to do this are: 
  
    - Calling the DOS "idle" interrupt 0x28.  Traditionally this
      allows TSR programs to come active while DOS is waiting for
      a keystroke but it is also a general purpose way for DOS
      to signal that the machine is generally idle.
      
    - Calling int 0x2F function 0x1680.  This is the "VM yield
      time slice" signal.


  The first method (int 0x28) is controversial; many people will
  claim that it is not needed.  It has existed since DOS 2.x
  and it is documented as being used by DOS to signal to TSRs
  that there is idle time.  After doing a lot of research and
  testing there is no reason why an application should not
  use it too, especially if the application does not use DOS for
  keyboard input (thus depriving DOS the ability to call int 0x28
  itself.)

  If power saving is enabled in mTCP (which is the default) the
  calls to int 0x28 will always be made during idle loops.

  The second method (int 0x2f 0x1680) is well documented and should
  work in other environments such as DOS boxes in Windows, OS/2,
  and other virtual environments.  I've tested it in Windows XP
  when using SwsVpkt.

  When mTCP first starts up it will use int 0x2f function 0x1680
  to see if the function is supported.  If it is supported mTCP
  will use it during idle loops.


Notes on POWER, FDADPM, and DOSIDLE

  POWER

    The standard power management utility for MSDOS and PCDOS is
    POWER.EXE which is installed as a device driver and controlled
    from the command line.

    While POWER seems to work to reduce CPU consumption at the DOS
    command line, it does not help with the mTCP idle loop.  So it
    does not harm anything, but it doesn't help either.

    POWER works correctly on real hardware and in VMWare.  But the
    current version of VirtualBox seems to have a bug that causes
    the FTP client to hang up and freeze when POWER is loaded.
    If you see weird behavior like this under VirtualBox get rid
    of POWER and use a different utility.


  FDAPM

    FDAPM is an open source replacment for POWER that ships with
    FreeDOS and can be downloaded separately.

    The FDAPM power control utility detects the calls to int 0x28 to
    see when DOS is idle.  This has a very noticeable effect when
    running mTCP applications with FDAPM loaded; instead of making
    a processor fully busy the processor will idle at a low CPU
    utilization.  Here is the command line I use to enable FDAPM:

      fdapm apmdos

    If you run that FDAPM will detect calls to int 0x28 and try
    to adjust your hardware to save power.  For more information
    on FDAPM go to: http://help.fdos.org/en/hhstndrd/base/fdapm.htm


  DOSIDLE

    DOSIDLE is an older power management utility that can replace
    POWER.  I've not experimented a lot with it, but it works so well
    that it forced mTCP to save power even when you have told mTCP
    not to try to save power!

    DOSIDLE can be found at:

    http://maribu.home.xs4all.nl/zeurkous/download/mirror/dosidle.html


Disabling power control

  In the unlikely event that you need to disable the power control
  calls in the mTCP idle loop you can set an environment variable.
  The name of the variable is "MTCPSLEEP" and an example of how to
  set it to disable power control is shown:

    set MTCPSLEEP=0

  If you do this mTCP will not attempt to call int 0x28 or
  int 0x2f function 0x1680 while it is in an idle loop.



More information: http://www.brutman.com/mTCP


Created April 29th, 2012, Last updated April 26th, 2013
(C)opyright Michael B. Brutman, mbbrutman@gmail.com

