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Text File | 1993-06-18 | 17.2 KB | 325 lines

                QEMM's Stealth ROM Technology


  This QEMM 7 technote is available from the following sources:

  Quarterdeck Technical Support BBS:  STLTECH.TEC
                         CompuServe:  STTECH.ZIP
                              Q/FAX:  #168


  Subject: An overview of QEMM's Stealth ROM technology -- how it
  works, why it works so well, and what can cause complications for
  it.

  PLEASE NOTE:  Unless otherwise indicated, references to the 386
  processor include all 386 and higher processors.

  Q: What is Stealth ROM?
  Q: How does Stealth ROM work?
  Q: What is the difference between ST:M and ST:F?
  Q: Which Stealth ROM strategy is preferable?
  Q: What machines are supported by QEMM's Stealth ROM?
  Q: Does Stealth ROM slow down my system?
  Q: How can Stealth ROM fail?
  Q: If I'm having problems with Stealth ROM, what should I do?

  Q: What is Stealth ROM?
  -----------------------
  Traditionally, 386 memory managers such as QEMM have been able to
  create High RAM by associating physical extended memory with
  unused addresses between 640K and 1MB.  Quarterdeck's Stealth ROM
  technology is QEMM's method of creating more High RAM than
  previously thought possible, by mapping memory to addresses that
  are used by system, video, disk, and other ROMs.

  Q. How does Stealth ROM work?
  -----------------------------
  To understand how Stealth ROM works, it is useful to understand
  the concept of MAPPING.  MAPPING is the process by which memory
  management hardware and software can make memory appear in
  appropriate places at appropriate times; it is the process of
  associating memory with an address.  The expanded memory
  specification (EMS) uses mapping to make portions of expanded
  memory appear inside the EMS Page Frame when that memory is
  requested by a program.  When a program needs more memory than
  what is normally available to it under DOS, it can request that
  some expanded memory be allocated from either an EMS board, or
  from the EMS memory created by a 386 expanded memory manager.

  Expanded memory has no DOS addresses of its own, but can be made
  to appear at a valid DOS address -- "mapped in".  Expanded memory
  pages that are not currently needed may be "mapped out" --
  relieved of their DOS addresses and put back into the expanded
  memory pool, with code and data still intact.  When the
  application needs these pages, they are "mapped in" to the EMS
  Page Frame again.  It is therefore possible for a program that
  uses expanded memory to have access to much more memory than DOS
  itself can see of its own accord. You may know this technology as
  "bank switching", which is one of the techniques used to extend
  and add power to everything from mainframe computers to high-end
  UNIX systems... to DOS machines!

  Mapping is also useful for creating High RAM; in the same way as
  detailed above, expanded memory can be associated with unused
  addresses between 640K and 1MB.  The EMS hardware and software
  cooperate to make memory appear where there is otherwise none.

  Stealth ROM uses mapping for a new purpose.  The 386 chip can be
  made to map memory in or out of DOS' address space at any time.
  Stealth ROM uses 386 mapping to map system, disk, or video ROMs
  in and out of DOS' address space when appropriate, using one of
  two strategies -- ST:M and ST:F.

  Q. What is the difference between ST:M and ST:F?
  ------------------------------------------------
  ROMs on your system are accessed via interrupts -- which are
  conceptually similar to BASIC subroutines.  When your system
  boots up, it sets up something called an interrupt vector table.
  This is a list of addresses where specific ROM subroutines can be
  found.  When a program on your system needs a certain ROM
  function (for example, writing coloured text to the screen), it
  sets up some data in appropriate places, and then calls the
  interrupt with a processor INT instruction.  The processor then
  looks at the interrupt vector table to find out the address where
  the ROM function can be found.  The calling program jumps to that
  address, the ROM subroutine gets run, and then control is
  returned to the calling program.

  When you use Stealth ROM, as your system boots QEMM takes control
  of interrupts that are in use by the ROMs on your system and
  points those interrupts into itself.  This way, QEMM can monitor
  exactly when a ROM interrupt occurs, and can manage the interrupt
  appropriately.

  When you use ST:M ("Map Mode"), QEMM maps system, video, disk
  ROMs, and any other "Stealthable" ROMs out of the first megabyte
  to addresses elsewhere in the processor's memory map.  QEMM can
  then map memory from your "pool" of expanded memory into these
  addresses, created additional High RAM into which you can load
  device drivers and memory resident programs.  (For information on
  what is "Stealthable", see "How can Stealth ROM fail?" below.)
  When the ROM is needed by the system, QEMM maps the appropriate
  ROM code into the expanded memory Page Frame.  The ROM code now
  has a valid DOS address at which it can execute, and it does so
  normally.  When the ROM routine is finished, QEMM then remaps the
  ROM elsewhere outside of DOS' address space.

  When you use ST:F ("Frame Mode"), QEMM leaves the system, video,
  and disk ROMs where they are normally found.  QEMM then maps the
  EMS Page Frame to such that it lies on top of a ROM.  Expanded
  memory can then be mapped into the EMS Page Frame.  When the ROM
  that has been hidden by the Page Frame is needed, QEMM
  momentarily disables expanded memory usage and maps the Page
  Frame out of DOS' address space, exposing the ROM beneath.  The
  ROM code then executes normally.  When the ROM routine is
  finished, QEMM can then re-enable the EMS Page Frame, and lie it
  back down over the ROM.

  Q. Which Stealth ROM strategy is preferable?
  -------------------------------------------
  Since ST:M is capable of mapping almost all ROMs out of DOS'
  address space, and thus leaves you with much more High RAM, it is
  the better of the two options.  ST:F is needed on a very small
  number of systems; its object is to ensure compatibility with
  machines that have ROMs that jump to each other without using an
  interrupt to do so.

  Q. I have to have a special version of QEMM so that Stealth ROM
  will work on my system, right?  My system has to be one that
  Stealth ROM knows about, right?
  ------------------------------------------------------------------
  No to both questions!  Stealth ROM is designed to work on ANY
  system, regardless of brand, model, or ROM BIOS revision.  You do
  not need a special version of QEMM or Stealth ROM that has been
  customized for your machine, because Stealth ROM's strategy
  merely remaps you ROMs instead of replacing them.  Stealth ROM
  does not modify, compress or replace your ROM BIOS, and it does
  not depend on being aware of the brand or revision of your ROMs.
  Additionally, Stealth ROM will typically create more High RAM on
  your system than any other memory management technique.  You do
  not have to disable any of QEMM's features -- EMS, XMS, DPMI, or
  VCPI memory management -- as other memory managers force you to
  do in order to squeeze every last byte of High RAM from your
  system.

  Q. Does Stealth ROM slow my system down?
  ----------------------------------------
  Stealth ROM does add some tiny amount of overhead to ROM BIOS
  interrupts. Since most application programs spend very little
  time calling ROM code, the slowdowns are usually imperceptible or
  insignificant to the user.  Ironically, since benchmark programs
  often call ROM interrupts repeatedly (some do almost nothing but
  this), the greatest slowdown will be seen in benchmark results;
  these results rarely have much to do with the speed of useful
  programs, however.  Since your application programs typically
  have much more conventional memory to deal with when Stealth ROM
  is invoked, you are much more  likely to observe faster
  performance.  It should be noted that QEMM's ROM parameter can
  provide significant performance increases.  Using Stealth ROM
  along with the ROM parameter is typically significantly faster
  than not using QEMM at all.

  Q. How can Stealth ROM fail?
  ----------------------------
  Stealth ROM is a robust and proven technology.  However, some
  programs and some system ROM implementations can interfere with
  Stealth ROM's strategies. Note that the problems described here
  are infrequent and/or system-specific. Most users will experience
  no difficulty at all with Stealth ROM.

  Note that in the above description of how Stealth ROM works, each
  strategy depends on a processor interrupt being asserted.  This
  is the normal way of calling a piece of ROM code; processor
  registers are loaded with data and with information which denotes
  exactly which ROM service is being requested, and then a
  processor INT instruction is called.  BASIC programmers will
  recognize that this is similar to the process of loading a few
  variables with data, and then calling a subroutine with a GOSUB
  instruction; most good texts favour this method of programming.
  However, it is possible (though relatively uncommon) for a piece
  of ROM code to JUMP to a specific address without asserting an
  interrupt.  This is analogous to a BASIC GOTO, rather than a
  GOSUB; GOTOs are frowned upon by expert programmers, since a GOTO
  presumes that the address to which the code is jumping will
  remain constant and unchanging.  This is less of a problem if one
  person writes all the code, since it is easier for one person to
  keep track of the proper destination addresses; when more than
  one person is involved, it's more difficult to determine why and
  where the code should branch.

  Prudent programmers do not depend on ROM code being at a specific
  address, since the code's address could easily vary when the ROM
  code is revised (and, like any software, ROM code is constantly
  under revision). Programs that jump to an invalid ROM address
  (and therefore invalid code) will quickly crash in ANY
  circumstance.  In the case of a direct jump to ROM code when
  Stealth ROM is invoked, QEMM will not be able to intercept an
  interrupt, and thus may not have a chance to make sure that the
  appropriate portion of the ROM code is mapped into the Page Frame
  (ST:M).  In such circumstances, ST:F may be helpful, since the
  Page Frame will be made to disappear, exposing the ROM beneath,
  and will not reappear until after the ROM BIOS call is completely
  done.

  * Many ROM-BIOS-based system setup programs exhibit this same
  sort of behaviour, as do installation routines for some video
  cards. If you wish to run such programs, it is far better to
  disable QEMM temporarily than to sacrifice the large amounts of
  extra High RAM that ST:M can provide.  Setup programs should need
  to be run infrequently, and typically require a reboot before the
  modified settings take effect. High RAM is generally much more
  useful.  It is worth weighing the benefits of instant access to
  your setup program against the extra High RAM that Stealth ROM
  can provide; the decision should not be a difficult one.

  The easiest way to deal with this is to disable QEMM, run your
  Setup program, and reboot with QEMM active again.  To disable
  QEMM temporarily, hold down the <Alt> key immediately after you
  hear a beep on bootup.  QEMM will post a message telling you to
  press <Escape> to unload QEMM, or any other key to continue with
  QEMM.  Press <Escape>, and run your Setup program.  (If you are
  using the DOS-UP feature from QEMM version 7.0 you will first see
  a message asking if you want to unload DOSDATA.  Press <Escape> to
  unload DOS-UP, then hold down the <Alt> key again until you see
  the message telling you to press <Escape> to unload QEMM.  After
  unloading QEMM, run your Setup program, then reboot the machine
  normally (without holding down <Alt>; your revised Setup will be
  in effect, and so will QEMM.

  * Some programs find the address of a given piece of ROM at
  startup, and then jump directly to that address later on, at a
  time when the ROM may not be mapped into memory.  Programs like
  these will often require that a portion of the ROM be EXCLUDEd on
  the QEMM386.SYS line in CONFIG.SYS.

  * Some ROMs do not have any interrupts pointing to them at
  startup.  If this is the case, QEMM will not be able to detect
  where a given interrupt should point, and thus may not invoke
  Stealth ROM for that ROM.  Some device drivers refuse to load
  unless they see an interrupt pointing to its normal location;
  these programs can be loaded before QEMM if necessary.

  * Some programs make invalid assumptions about the EMS Page
  Frame.  In some cases, programs assume that the state of the EMS
  Page Frame will remain unchanged even after they abandon an EMS
  handle; this is akin to assuming that you can get your property
  back after leaving it at the end of the driveway on garbage pick-
  up day.  This fails with Stealth ROM because, by default, the
  Page Frame is immediately unmapped after a handle has been
  abandoned -- as if, in the above example, the city picks up the
  garbage pretty much immediately --as soon as you get back into
  your house.  The UFP:N parameter suppresses this feature, perhaps
  at the expense of some speed.

  * Some applications assume that the contents of the Page Frame
  will be preserved across a hardware interrupt -- like assuming
  that your coat will never get moved from the place in which you
  saw the cloakroom attendant put it.  This is an invalid
  assumption, and can cause problems not only with Stealth ROM, but
  with EMS-using TSRs as well.

  * Other programs outright violate the Expanded Memory
  Specification by placing their interrupt stacks in the Page
  Frame.  This is not simply a problem for Stealth ROM or for QEMM;
  this can cause a conflict with programs using expanded memory and
  ANY expanded memory manager.

  Fortunately, programs that exhibit the above problems are rare.
  If you experience difficulty that is found to be Stealth-related,
  you might wish to encourage the developer of the faulting program
  to make the program more compatible with Stealth ROM.
  Quarterdeck is very happy to assist the developer of any
  commercial hardware or software who wishes added compatibility
  with our products.

  ROM code is normally read 8 or 16 bits at a time, and 32-bit RAM
  is therefore much faster.  (You can see this in action by looking
  at Manifest First Meg / Timings, first without the ROM parameter
  on the QEMM386.SYS line in CONFIG.SYS, and then with ROM added to
  the end of that line.) Some video ROM speed-up drivers
  (TVGABIO.SYS, RAMBIOS.SYS, FASTBIOS.SYS, SPEED_UP.SYS... these
  typically come on the utilities diskettes provided with your
  video card) work by copying the contents of video ROM to
  conventional RAM.  These programs will often fail if Stealth ROM
  is active; again, they assume that the video ROM is in its normal
  place, and has not been moved by Stealth ROM.  QEMM's ROM
  parameter provides the same feature as these drivers do, with
  three important advantages:  first, QEMM copies the video ROM
  into 32-bit RAM, and then write-protects the RAM so that some
  errant program does not overwrite the ROM code.  Second, QEMM's
  ROM parameter costs neither conventional memory nor High RAM to
  provide this feature -- the video drivers mentioned above will
  typically take 32K of one or the other.  Finally, the ROM
  parameter is fully compatible with Stealth ROM.

  Q. If I experience a problem when using Stealth ROM, what should
  I do?
  ------------------------------------------------------------------
  Stealth ROM problems can be resolved by consulting Quarterdeck
  Technical Note #205, "Troubleshooting Stealth ROM" (STEALTH.TEC).
  This technote was installed in your QEMM\TECHNOTE directory when
  you installed QEMM 7.

  SUMMARY
  -------
  Stealth ROM is a robust and proven technology.  It is an easy-to-
  use, safe, and efficient way of creating more High RAM on your
  system, providing more memory for your TSRs, your device drivers,
  DESQview 386, MS Windows, and your application programs.  It is
  likely to speed up your system rather than slowing it down.  It
  is designed to be effective on any 386 or higher processor,
  regardless of the ROM's manufacturer or version.  Programs that
  cause conflicts with Stealth ROM are apt to cause problems with
  other programs and memory managers.  Stealth conflicts are rare,
  and troubleshooting is straightforward.  Stealth ROM is the
  easiest way to provide the optimal amount of High RAM on your
  system.

  ***************************************************************
            Trademarks are property of their respective owners.
   This technical note may be copied and distributed freely as long
  as it is distributed in its entirety and it is not distributed
  for profit.  Copyright (C) 1993 by Quarterdeck Office Systems
  ****************** E N D   O F   F I L E ***********************