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/               HDPMI - DPMI Server (Version 3.20)                           /
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 0. Contents

   1.    About HDPMI
   2.    Requirements
   3.    Commandline Options
   4.    Environment Settings
   5.    Returncodes and Messages
   6.    Implementation Details
    6.1  Memory Management
    6.2  Interrupts
    6.3  Exceptions
    6.4  Stacks
    6.5  Client Initialization/Termination
    6.6  DPMI API
    6.7  Vendor API
    6.8  VDS API
    6.9  DOS API Translation
    6.10 Other API Translation Support
    6.11 Debug Support
    6.12 Nested Execution of DPMI Clients
    6.13 Other Features of HDPMI
   7.    Restrictions of current Version
   8.    Known Problems/Hints
   9.    Compatibility List
  10.    License


 1. About HDPMI

   HDPMI is a DPMI server which conforms to DPMI specification v0.9, but
   supports a large subset of DPMI v1.0 as well. Although it is part of
   the HX runtime, it doesn't depend on any other runtime modules and can
   be used as a standalone host.

   HDPMI exists in two versions called HDPMI16.EXE and HDPMI32.EXE. The
   first will run 16-bit DPMI clients, the latter executes 32-Bit DPMI
   clients. The HX PE binary loader DPMILD32 is aware of HDPMI and will
   try to load it silently in the background if no DPMI server is found
   in memory yet.

   A DPMI server's purpose is to run applications in protected-mode. This
   gives the following benefits compared to real-mode DOS:

   þ breaks the DOS 640 kB (conventional) memory limitation. The 80386 
     supports 4 GB memory.
   þ extends the 16-bit segment limit to 32-bit, which allows to forget
     about segmented memory models and use the simple "flat" model.
   þ several privilege levels which allows "user" and "system" code.
   þ paging.
   þ memory and I/O protection.
   þ additional exceptions.

   The DPMI server will have to manage all resources specific to protected
   mode (GDT, IDT, LDT, TSS, page tables, extended memory, ...). To make
   DOS and BIOS accessible it also has to provide the ability to switch
   between modes. That's why a DPMI server has to run at a high privilege
   level and cannot be launched in a "DOS box".

   Extended features of a DPMI server are:

   þ real-mode interrupt translation services, thus allowing the client
     to use DOS Int 21h calls which use pointers, for example. HDPMI
     has such services implemented, see below.
   þ Swapfile to increase available memory by hard disk space (virtual
     memory). HDPMI does not implement a swapfile on its own, but since
     version 3.03 it is possible for the client to do this.
   þ "virtual machines". This is not supported, but optionally HDPMI allows
     to run clients in their own address contexts. Conventional memory
     (address space 0-10FFFFh) always is shared, though.


 2. Requirements

   HDPMI requires the following resources to run:

   þ a 80386 or better cpu. If environment switch HDPMI=1 is used, at least
     a 80486 is required (see below).

   þ a minimum of 72 kB of extended memory for host initialization.
     By (optionally) adding conventional DOS memory to the page pool,
     HDPMI may run without any extended memory at all.

   þ MS-DOS (version >= 3.3). FreeDOS or DR-DOS (version >= 5.0) should work
     as well.
   
   þ a XMS host if HDPMI cannot run in "raw" mode due to an unknown
     A20 gate switch mechanism.


 3. Commandline Options

   HDPMI accepts the following commandline options:

     -a: run clients in separate address contexts. This is similar to
         environment switch HDPMI=32. Using this option is recommended.
     -b: keep the TLB only when at least 1 client is running. This
         option is useful only in conjunction with -r and will result
         in conventional memory usage of just 5 kB when HDPMI is idle. But
         be aware that this constellation may not work with all clients!
     -d: disable the last installed instance of HDPMI. This means it won't
         allow to start new clients - clients already running in the disabled
         instance are not affected. Also, a client terminating in a disabled
         instance will automatically put it back into the enabled state.
     -e: reenable a disabled instance of HDPMI.
     -h: display help only; HDPMI will not install.
     -i: trap direct access to the IVT to ensure hardware interrupts are
         always routed to protected mode. This is similar to environment
         switch HDPMI=1.
     -k: abandoned since v3.18. ( in previous versions, this option was
         used to ensure that the screen is in text mode if a register dump
         was to be displayed ).
     -l: TLB will be allocated in low DOS memory. Useful if DOS UMBs
         are slow or not accessible by DMA.
     -m: disables support for DPMI 1.0 memory functions (AX >= 0504h).
     -n: report clients a smaller amount of free physical pages. This is
         necessary for clients which assume this amount is safe to use.
     -r: this will cause HDPMI to install permanently as a TSR. In this
         mode HDPMI will run until it is deinstalled using option -u.
         If option -r is *not* entered, HDPMI will install as a TSR as 
         well, but will terminate when the next client has terminated (*).
     -s: 'safe' mode. Prevents client from modifying system tables. Also,
         this option will HPDMI cause to set the WP bit in CR0, thus 
         preventing write access to read-only pages even for the host.
         This option is similar to environment switch HDPMI=4096.
     -u: this switch will uninstall a running instance of HDPMI.
     -v: if both XMS and VCPI hosts were detected, use VCPI memory allocation
         and ignore XMS host. 
     -x: limit reported free memory to 256 MB. The application may still
         allocate more than 256 MB, it's just DPMI function 0x500h that will
         be affected. Some old dos-extended applications simply don't expect
         GBs of free memory.
     -y: use extended memory not managed by XMS host. Usually a "modern"
         XMS host can handle all extended memory, but old ones may handle
         63 or 255 MB only. Then setting this option may be useful. For
         XMS V3 hosts the implementation of function AH=89h must return
         in ECX the true highest physical address managed by the host. 
         This value is sometimes wrong, in which case this option shouldn't
         be set (some security checks are implemented, but it isn't
         fool-proved). If in doubt run tool DPMI.EXE to see if the value
         is reasonable.

    [-t: removed since v3.19! Previously, it told HDPMI not to set the 
         CR0.NE and thus make the cpu generate an Irq 0Dh instead of a
         numeric exception on FP errors. Since v3.19, HDPMI will not touch
         CR0.NE and the default handler for Irq 0Dh will trigger an 
         exception 10h, which is supposed to be more compatible. However,
         Irq 0Dh is not called if interrupts are disabled or Irq 0Dh has
         been masked, so the behavior of HDPMI has slightly changed.]
   
   (*) Please note: Since HDPMI modifies some real mode interrupt vectors,
       it cannot terminate if any other application (it needn't be a
       protected mode app, a simple DOS TSR may suffice) has modified these
       vectors while HDPMI is running. In this case HDPMI has to remain
       resident in any case, regardless if a client is running or not.
       
   (*) If both HDPMI16 and HDPMI32 are installed permanently, the hosts
       will share the TLB, so both hosts will consume just about 13 kB
       conventional DOS memory. The order in which the host are installed
       might matter - usually it's better to install HDPMI16 first.


 4. Environment Settings

   On startup HDPMI will search for environment variable "HDPMI". By setting
   this variable one may modify HDPMI's behavior. It is interpreted as bit
   values, so one may set any combination of the following values:

   HDPMI=1:  this option makes the host trap direct IVT access
             (read/writes to linear address range 0-3FFh) in protected mode.
             Thus it can ensure that IRQs are always handled in protected mode 
             first. If the option is set, opcode INVLPG is used, which is
             implemented on 80486 or better cpus only. Same effect as
             command line option -i.
   HDPMI=2:  DOS memory is included in standard memory page pool.
             This option should be set only on machines with very
             little physical ram.
   HDPMI=4:  return 1.00 as DPMI server version. Some clients will
             refuse to work if server identifies itself as V0.90 only.
             Since HDPMI implements many DPMI V1.0 functions, such apps may
             work with this switch set.
   HDPMI=8:  allocate TLB in low DOS memory. Same effect as command line
             option -l.
   HDPMI=16: prevents HDPMI from trying to allocate a temporary 64 kB TLB
             for DOS read/write functions which exceed the static TLB size.
   HDPMI=32: run clients in separate address contexts. This option may be
             needed to allow nested execution of clients which cannot share
             an address space. The conventional memory region 0-10FFFFh
             is still shared, though. Setting this switch will increase the
             second (and further ) client's DOS memory footprint by about 
             2 kB. OTOH, the system is more stable, an abnormal termination of
             a client should leave the other clients unaffected. Same effect
             as command line option -a.
   HDPMI=64: don't use XMS v3+ functions. This should make HDPMI compatible
             with XMS hookers which aren't aware of XMS v3 (WSWAP.EXE). It
             will also restrict the amount of free memory the host reports.
   HDPMI=128: disable DOS API translation for LFN functions.
   HDPMI=256: when running as VCPI client, ensure that PTEs for HMA region
             (address space 100000h-10FFFFh) are set so that physical
             addresses match linear addresses. This may not work with all
             VCPI hosts.
   HDPMI=512: alloc IDT and LDT in client address space. Usually these 
             tables are allocated in HDPMI's reserved space above linear
             address FF800000h, but some programs seem to not expect them
             at such high addresses. Furthermore, GDT is not moved to
             extended memory if this switch is on. In 'safe' mode this
             switch is ignored!
   HDPMI=1024: disables support for DPMI 1.0 memory functions (AX >= 0504h).
             Same effect as command line option -m.
   HDPMI=2048: clear HiWord of ESI and EDI on initial switch to protected
             mode. This doesn't conform to the DPMI specs, but is required
             to make HDPMI compatible with Borlands's 32RTM.EXE when running
             on some versions of DOS.
   HDPMI=4096: 'safe' mode. Don't allow client direct read/write access to 
             GDT, IDT, LDT and page tables; also, host cannot write to pages
             marked as read-only. Same effect as command line option -s.
   HDPMI=8192: stop in debugger after client's initial switch to protected
             mode.
   HDPMI=16384: obsolete since HDPMI v3.18. Previously, it caused HDPMI not
             to respond to int 2Fh, ax=160Ah. This allowed to start Windows
             3.1 WIN.COM while HDPMI was loaded resident.
   HDPMI=32768: obsolete since HDPMI v3.19. Previously, it prevented HDPMI
             from modifying CR0 NE bit; same effect as command line option -t.


 5. Returncodes and Messages

   If HDPMI is launched by a program, it will return with the following codes:

   rc   meaning
   ------------------------------------------------
   00   HDPMI is resident now in Int15 mode
   01   HDPMI is resident now in XMS mode
   02   HDPMI is resident now in VCPI mode
   03   DPMI server detected without VCPI support, HDPMI has terminated
        without taking over control. This state should be no error for
        the application since the DPMI API is available
   04   error: not enough memory for host initialization
   05   error: A20 cannot be enabled
   06   error: VCPI host reports remapped IRQs
   07   error: CPU in V86 mode, but no VCPI or DPMI host found
   08   error: DOS version isn't 3.3 or higher
   09   error: CPU not 80386 or better
   0A   error: invalid command line given

   If HDPMI is started with a nonempty command line, it may display 
   messages. These are:

   - "HDPMI already installed": may occur when starting HDPMI with 
     option "-r".
   - "HDPMI not installed or disabled or version differs": may occur if
     HDPMI was launched with option "-u" or "-d". 
   - "HDPMI is busy": may occur when starting HDPMI with option "-u". 
     Reason: The server has at least 1 client running.
   - "HDPMI *not* uninstalled because real-mode interrupts were modified"
     may occur when starting HDPMI with option "-u". Reason: Another TSR
     has modified an interrupt which HDPMI is using as well, so it cannot
     restore the old value. Most likely this is interrupt 2Fh.
   - "no disabled instance of HDPMI found": may occur when starting HDPMI
     with option "-e". 

   There are also a few runtime error messages:
   
   - "hdpmi: cannot initialize", means that the conditions required by
     hdpmi to start ( memory, DOS version,... ) were not met.
   - "hdpmi: fatal exit nnnn", where <nnnn> may be:
      þ 0005: host stack exhausted due to too many nested mode switches.
      þ 0006: no more descriptors available for translation services.
      þ 0007: locked protected-mode stack (LPMS) exhausted.
   - "hdpmi: ctrl-alt-delete detected, app terminated", if the user
     pressed the Ctrl-Alt-Del key combination while a client was active.
     Note that a DPMI-TSR cannot be removed from memory by this way,
     because it isn't regarded as "active".


 6. Implementation Details


 6.1 Memory Management

   a) Overview

     The memory usage of HDMI depends on the configuration found
     at startup. Possible configurations are:

     þ Int15 or "Raw" mode: CPU is in real-mode and no XMS host
       (HIMEM.SYS) has been found.

     þ XMS-Mode: CPU is in real-mode and a XMS host is running.

     þ VCPI-Mode: CPU is in v86-mode and a VCPI host has been detected.

     þ CPU is in v86-mode and no VCPI host has been found. HDPMI cannot
       enter ring 0 protected-mode and terminates without further message.


   b) Physical Memory Allocation

   Int15-Mode:  HDPMI allocates all of extended Memory by calling
                Int15, AX=E801h or, if this fails, by calling Int15, AH=88h. 
                When a real mode app is launched, half of currently unused
                extended memory is released. This is to allow
                nested execution of protected mode apps in this mode.
                When the launched app terminates, HDPMI grabs this memory
                again.

   XMS-Mode:    Memory is allocated dynamically from the XMS host. HDPMI 
                will detect and use XMS v3 hosts, which may allow to alloc
                up to 4 GB of physical memory. The memory blocks allocated 
                are released when HDPMI terminates. If HDPMI has been
                installed permanently with option -r, the blocks (except
                the first one) are released when the server enters idle state,
                that is, no client is running.

   VCPI-Mode:   If a XMS-Host is available in this mode, HDPMI prefers to 
                allocate memory through this host, because some VCPI host's
                memory allocation API is pretty slow. So this API will be 
                used only if no XMS host is found (which should be a very
                rare constellation) or cmdline option -v has been entered.

   If all extended memory has been allocated, HDPMI will try to use
   conventional memory if environment variable HDPMI=2 is set. So in tight
   memory conditions DOS memory may be a full part of HDPMI's memory pool.
   This is optional because there is a chance that this behavior may cause
   problems (some clients simply allocate the largest available memory block
   as reported by the host, which will then make HDPMI to alloc all free DOS
   memory).

   c) Address Space Management

   Paging will always be enabled with HDPMI. It reserves the last 8 MB of
   the address space for its usage (GDT, IDT, LDT, page tables, host code).
   The rest is left for the client, so 4088 MB address space is available 
   in the range 0 - FF7FFFFFh. Optionally HDPMI can be instructed to locate
   IDT and LDT in the client address space (see environment variable HDPMI).

   d) Virtual Memory

   HDPMI itself doesn't create a swap file. But support for "exception
   restartability" has been implemented in version 3.03. This allows a
   client to catch page faults occuring inside the host, which makes it
   possible to support swapfiles (or memory-mapped files) on the client
   level.

   e) Conventional Memory

   Conventional memory (address space 0-10FFFFh) is under control of DOS.
   In XMS and raw mode HDPMI will initialize the page table for this region
   so that physical memory addresses and linear memory addresses are identical.
   DOS memory usage of HDPMI is just about 10 kB, of which 8 kB are used
   as a "translation buffer" (TLB) - hence using cmdline option -b will reduce
   the conventional memory footprint of HDPMI while no client is running to 
   a little more than 2 kB!


 6.2 Interrupts

   According to DPMI specs a host must ensure that hardware interrupts (IRQs)
   are routed to protected mode if they have occured in real/v86-mode.
   HDPMI fullfills this requirement; for this it intercepts the real-mode
   IVT vectors of all IRQs. Routing the IRQs to protected mode, however,
   is done only if a client has changed the protected-mode interrupt vector.

   There exist some DPMI clients which get in the way, however. Most 
   likely this is because some hosts had difficulties with the IRQ
   routing to protected-mode. So these clients try to do this routing
   on their own - which may cause problems with HDPMI. For this reason,
   HDPMI has an option to catch all direct accesses to the IVT in protected
   mode (this feature is activated by setting environment variable HDPMI=1
   or command line option -i). This will slow down any access to page 0,
   however, so it should be set only if it becomes obvious that the client
   will need it to run.

   Besides the hardware interrupts there are 3 software interrupts which
   will be routed to protected-mode as well: Int 1Ch, Int 23h and Int 24h.

   Since version 3.19, the HDPMI default handler for IRQ 13 (FPU exception)
   will trigger an exception 10h (numeric exception), as if the NE bit in 
   register CR0 has been set. Previously, HDPMI had set CR0.NE to 1 and thus
   no IRQ 13 had occured at all.


 6.3 Exceptions

   If an exception occurs and the client hasn't installed an exception
   handler (or if it decides to not handle the exception), HDPMI's default
   exception handler will get the exception and will do:

   Exc    Action
   ----------------------------------------------------------------
   00-05  exception is routed to protected-mode INT 00-05.
   06     client is terminated.
   07     exception is routed to protected-mode INT 07.
   08-0E  client is terminated.
   10-11  client is terminated.

   The default handler for protected-mode interrupts 00-07 will do:

   Int    Action
   ----------------------------------------------------------------
   00     client is terminated.
   01-04  interrupt is routed to real-mode.
   05     interrupt is routed to real-mode if it is a programmed INT, else
          client is terminated.
   06     interrupt is routed to real-mode.
   07     client is terminated.

   In other words, exceptions 00 and 05-07 will not arrive in real-mode.
   This is not what DPMI docs are telling, but it wouldn't make sense,
   since these exceptions are faults and the exception cause must be cured
   to continue execution. This can't be done by a real-mode interrupt handler.

   A special case is interrupt 75h (coprocessor numeric error). Since v3.19,
   the default handler for this protected-mode interrupt will trigger an 
   exception 10h. OTOH, HDPMI does not longer set the CR0.NE bit when 
   starting.

   If a client is terminated, a register dump is displayed. If HDPMI is
   installed residently, it will first try to terminate the client with
   an int 21h, ax=4CFFh in ring 3 protected mode. If another exception
   occurs, HDPMI will terminate itself, which may leave the system in an
   unstable state.


 6.4 Stacks

   When a client is running the host will switch among 4 stacks:

   þ the protected mode stack (PMS), which is the stack the DPMI
     client uses. It can switch these stacks as it likes and there
     are no special requirements about it.

   þ the locked protected mode stack (LPMS). The DPMI host switches
     the client application to this stack when
     - a hardware interrupt (IRQ) has occured
     - an interrupt is reflected from real mode (mostly IRQs, but
       also software interrupts 1Ch, 23h and 24h.
     - the DPMI client executes a real mode callback
     - the server notifies the client about an exception
     There exists 1 LPMS only and its size must be at least 4 KB.
     Once it is "in use" the host will no longer switch stacks until
     the LPMS is free again.

   þ the real mode stack (RMS). Usually this stack is located in the
     memory block that the DPMI client has to deliver to the server on its
     initial switch to protected mode. This stack's size will be at
     least 200h bytes. It is used when there is a reflection from protected
     mode interrupts to real mode or when one of the "call real-mode"
     functions ( int 31h, ax=300h/301h/302h ) is called with registers SS:SP
     set to 0000:0000. If protected-mode is reentered (by a real-mode
     callback, a raw jump or a hardware IRQ), the current RMS is assumed to
     be in use and HDPMI will then use the real mode SS:SP of the last entry
     to protected mode as current real-mode stack; this is the strategy also
     used by Windows 3x/9x.

   þ the ring 0 stack. This stack is invisible to the client, it is
     used by the dpmi host. Since HDPMI v3.18, this stack is 4 kB in size and
     is located in a protected system region; previously, this stack was 2 kB
     in size and was located in conventional memory. The host uses this stack
     for its normal processing and to save the client's segment register
     values.


 6.5 Client Initialization/Termination

   Usually clients running on the same instance of HDPMI will share address
   space, IDT and LDT. On client initialization HDPMI will do:

   - save the current IDT
   - save the current LDT
   - save other client resources, among which are
     + real-mode callbacks
     + ring 3 interrupt vectors
     + ring 3 exception vectors

   These resources are saved, but not initialized, that is, a new client
   will inherit the state of the previous client.

   On client termination, HDPMI will do:

   - free memory handles the client has allocated
   - restore the IDT to the previous state
   - restore the LDT to the previous state
   - restore other resources to the previous state

   So there is no need for a client to free memory or LDT selectors before
   terminating. Conventional DOS memory blocks and file handles will be 
   released by DOS, but any real-mode interrupt vectors that were modified
   must be restored by the client.

   If HDPMI has been told to give each client its own address context, things
   are different of course. Address space, IDT and LDT are no longer shared,
   and the client will get "clean" copies of IDT and LDT. On termination, the
   client's resources are just "thrown away".


 6.6 DPMI API

   HDPMI fully supports DPMI version 0.9.

   Additionally, the following DPMI v1.0 features are supported:

   Int AX    comment
   -----------------------------------------------------------------
   2F  168A  get vendor-specific API entry point
   31  000E  get multiple descriptors
   31  000F  set multiple descriptors
   31  0210  get protected-mode extended exception handler
   31  0212  set protected-mode extended exception handler
   31  0401  get DPMI capabilities, Vendor is "HDPMI". This is the
             recommended way to detect that HDPMI is present.
  *31  0504  allocate linear memory block
  *31  0505  resize linear memory block
  *31  0506  get page attributes
  *31  0507  modify page attributes
  *31  0508  map device in memory block
  *31  0509  map conventional memory in memory block
  *31  050A  get memory block size and base
  *31  050B  get memory information
   31  0801  unmap physical region
   31  0E00  get coprocessor status
   31  0E01  set coprocessor emulation

  * support for these functions can be disabled by commandline parameter
    or setting environment variable HDPMI.

   So most DPMI V1.0 features are implemented, what's missing are:

   - 0211h + 0213h, get/set handler for real-mode exceptions
   - 0C00h + 0C01h, DPMI TSRs
   - 0D00h - 0D03h, shared memory

   Int 31h, AX=508h (map device in memory block) will set paging flags
   "write through" and "cache disabled". OTOH, function int 31h, AX=800h
   will NOT set those flags. Int 31h, AX=509h (map conventional memory) 
   will copy PTEs, so the destination region will become a clone of the
   source region, with UMBs mapped.

   Some privileged opcodes are emulated (this feature is not active if
   cmdline option -s ['safe' mode] is given):

   - HLT (F4) 
   - MOV reg,CRx (0F 20 xx) - using ESP for <reg> will not work!
   - MOV CRx,reg (0F 22 xx) - using ESP for <reg> will not work!


 6.7 Vendor API

   HDPMI supports the so-called "vendor" API in protected-mode. Those APIs
   usually offer capabilities not covered by the standard DPMI API. To get
   the API entry point, int 2Fh has to be called in protected-mode, with
   register AX=168Ah and DS:E/SI pointing to the "vendor" name; on return,
   register ES:E/DI should contain the entry point and register AL is zero.
   The following "vendors" are recognized:
   
   - "MS-DOS": supports function ax=100h (get LDT selector)
   - "HDPMI": see HDPMIAPI.TXT for details.


 6.8 VDS API

   The following VDS functions (Int 4Bh, AH=81h) are supported in
   protected-mode:

   AL  Function               Comment
   -----------------------------------------------------------------------
   03  lock region            routed to v86-mode with translation of ES:E/DI
   04  unlock region          routed to v86-mode with translation of ES:E/DI
   05  scatter/gather lock    handled by HDPMI. ES:E/DI must point to a EDDS.
   06  scatter/gather unlock  handled by HDPMI. Indeed it is mostly a No-Op,
                              just the parameters are checked for validity.
   07  request DMA buffer     routed to v86-mode with translation of ES:E/DI
   08  release DMA buffer     routed to v86-mode with translation of ES:E/DI
   09  copy into DMA buffer   routed to v86-mode with translation of ES:E/DI
   0A  copy out of DMA buffer routed to v86-mode with translation of ES:E/DI

   VDS functions 02 and 0Bh-0Ch are routed to real/v86-mode without 
   translation.
   Implementation of function 05 will allow linear-to-physical address
   translation for any valid linear address. If an address space contains
   uncommitted pages, bit 6 of DX must be set to return PTEs, else the call
   will fail. 
   Functions which are routed to v86-mode with ES:E/DI translation will not
   be able to handle linear addresses managed by HDPMI, that is, they might
   work for address space 0-110000h only.
   
   Please be aware that if DOS is running in real-mode (no EMM is loaded),
   then calling functions other than 05-06 most likely will fail. Therefore
   bit 5 at 40h:007Bh should be checked if VDS is available.


 6.9 DOS API Translation

   HDPMI supports DOS-API translation, both in 16-bit mode and in
   32-bit mode. For translation purposes, HDPMI allocates a static
   translation buffer of 8 KB. For the important read/write functions
   HDPMI will try to allocate a temporary 64k buffer, if the size of
   the i/o-operation exceeds the size of the static translation buffer.

   Supported DOS extended INT 21h functions in detail:

    AH  Comment  
    -----------------------------------------------------------
    00  close current PSP without terminating client
    09  Write String DS:E/DX to Standard Output
    0A  Buffered Input into DS:E/DX
    0C  AL=0A, Flush Buffer and read Standard Input into DS:E/DX
    11  Find First File using FCB in DS:E/DX
    12  Find Next File using FCB in DS:E/DX
    13  Delete File using FCB in DS:E/DX
    1A  Set Disk Transfer Area to DS:E/DX
    1B  Get Allocation Information for Default Drive in DS:E/BX
    1C  Get Allocation Information for Specific Drive in DS:E/BX
    1F  Get Drive Parameter Block for Default Drive in DS:E/BX
    25  Set Interrupt Vector in DS:E/DX
    29  Parse Filename in DS:E/SI into FCB in ES:E/DI
    2F  Get Disk Transfer Area in ES:E/BX
    32  Get Drive Parameter Block for Specific Drive in DS:E/BX
    34  Get Address of InDOS Flag in ES:E/BX
    35  Get Interrupt Vector in ES:E/BX
    38  DX!=FFFF, Get Country-Specific Information in DS:E/DX
    39  Create Subdirectory DS:E/DX
    3A  Remove Subdirectory DS:E/DX
    3B  Set Directory DS:E/DX
    3C  Create File DS:E/DX 
    3D  Open File DS:E/DX
    3F  Read E/CX bytes from File to DS:E/DX
    40  Write E/CX bytes from DS:E/DX to File 
    41  Delete File DS:E/DX
    43  Get/Set File DS:E/DX Attributes
    44  AL=02 Read from Char Device Control Channel into buffer DS:E/DX
    44  AL=03 Write to Char Device Control Channel from buffer DS:E/DX
    44  AL=04 Read from Block Device Control Channel into buffer DS:E/DX
    44  AL=05 Write to Block Device Control Channel from buffer DS:E/DX
    44  AL=0D Generic Block Device Request, parameter block in DS:E/DX
    47  Get Directory Path into DS:E/SI
    48  Allocate Memory Block E/BX paragraphs
    49  Free Memory Block in ES
    4A  Resize Memory Block ES, new size in E/BX
    4B  AL=00 - Load and Execute Program DS:E/DX, exec parm ES:E/BX
    4C  terminate client
    4E  Search for First Filename Match, DS:E/DX=file spec
    50  Set current PSP to BX (selector)
    51  Get current PSP in BX (selector)
    52  Get List of Lists in ES:E/BX
    53  Translate BPB in DS:E/SI to DPB in ES:E/BP
    55  Create Child PSP from DX (selector)
    56  Rename File DS:E/DX to ES:E/DI
    5A  Create temporary File DS:E/DX 
    5B  Create new File DS:E/DX 
    5D  AL=06 Get Address of DOS Swappable Data Area in DS:E/SI
    5D  AL=0A Set extended error information from DS:E/DX
    5E  AL=00 Get Machine Name into DS:E/DX
    60  Canonicalize Filename or Path in DS:E/SI to ES:E/DI
    62  Get current PSP in BX (selector)
    63  AL=00 Get DBCS table in DS:E/SI
    65  AL=00 Set extended Country Information from ES:E/DI
    65  AL=01-07 Get extended Country Information into ES:E/DI
    65  AL=21|22|A1|A2 capitalize string in DS:E/DX
    69  Get/Set disk serial number in DS:E/DX 
    6C  Extended Open/Create, file name in DS:E/SI 
   *71  AL=39|3A|3B LFN subdirectory functions DS:E/DX
   *71  AL=41 LFN delete file DS:E/DX
   *71  AL=43 LFN get/set file attributes DS:E/DX
   *71  AL=47 LFN get current directory DS:E/SI
   *71  AL=4E LFN get first file DS:E/DX into ES:E/DI
   *71  AL=4F LFN get next file into ES:E/DI
   *71  AL=56 LFN rename file DS:E/DX to ES:E/DI
   *71  AL=60 LFN get canonical/short/long path DS:E/SI to ES:E/DI
   *71  AL=6C LFN open file DS:E/SI
   *71  AL=A0 LFN get volume info DS:E/DX, return file sys into ES:E/DI
   *71  AL=A6 LFN get file info by handle into DS:E/DX
   *71  AL=A7 LFN filetime DS:E/SI to dostime - dostime to filetime ES:E/DI
   *71  AL=A8 LFN generate short filename from DS:E/SI to ES:E/DI 
   *71  AL=AA LFN create SUBST DS:E/DX, query SUBST DS:E/DX
    73  AL=02 Get extended DPB into ES:E/DI
    73  AL=03 Get extended free space into ES:E/DI
    73  AL=04 Set DPB for formatting in ES:E/DI
    73  AL=05 Extended absolute disk read/write with params in DS:E/BX

   * LFN (AH=71h) API translation only works if LFN is installed in 
     real-mode DOS. Usually this requires a driver to be installed (DOSLFN).


 6.10 Other API Translation Support

   Int Function Comment
   ---------------------------------------------------------------------
   10  ax=1002h set all palette registers ES:E/DX
   10  ax=1009h get all palette registers ES:E/DX
   10  ax=1012h set DAC registers ES:E/DX
   10  ax=1017h get DAC registers ES:E/DX
   13  ah=02h   disk read into buffer ES:E/BX
   13  ah=03h   disk write from buffer ES:E/BX
   13  ah=08h   for FD return drive parameter table in ES:E/DI
   15  ah=C0h   read configuration into ES:E/BX
   15  ax=C207h set pointing device event proc ES:E/BX
   25           absolute disk read buffer in DS:E/BX
   26           absolute disk write buffer in DS:E/BX
   33  ax=0009h define graphics cursor ES:E/DX
   33  ax=000Ch define interrupt subroutine ES:E/DX
   33  ax=0012h define large graphics cursor ES:E/DX
   33  ax=0014h exchange interrupt subroutine ES:E/DX
   33  ax=0016h save driver state ES:E/DX
   33  ax=0017h restore driver state ES:E/DX
   33  ax=0018h set alternate mouse user handler ES:E/DX


 6.11 Debug Support

    Since v3.20, debug support is enabled (again) in the standard variant.
   For other variants, debug support may be enabled in the makefiles by
   setting ?WDEB386=1.

    HDPMI's default support for kernel debugging is based on the API used
   by MS WDeb386. Optionally, the API may be changed to the 386SWAT style,
   by setting ?386SWAT=1 in the makefile. However, this variant is mostly
   untested by now.

   The kernel debugger support consists of 

   - a real-mode API ( interrupt 68h ). This interrupt is used to allow
     the kernel debugger to modify GDT entries and to get a protected-mode
     address to be called as soon as protected-mode has been activated.

   - protected-mode init ( FAR32 call ), to allow the debugger to setup
     its vectors in the IDT.
   
   - interrupt 41h in protected-mode, to notify the kernel debugger of
     certain events.

   - interrupt 22h, supposed to be called by the kernel debugger to get
     information from the host ( i.e. linear addresses of paging tables ).

    If an exception occurs, the default exception handler will execute a
   breakpoint, so the kernel debugger has a "last" chance to examine the
   status of host and client.

    Since WDeb386 requires a serial connection and a second machine, there's
   a simple replacement available, Deb386, that doesn't need this setup.
   

 6.12 Nested Execution of several DPMI Clients

   As already mentioned, usually if two or more clients share one instance
   of HDPMI, they also have to share address space, IDT and LDT. For most
   clients this is no problem at all. But on certain conditions it may be
   necessary to not share these resources. Therefore HDPMI optionally allows
   to run each client in its own address context. Thus the clients will only 
   share DOS specific resources (address space 0-10FFFFh, files, IVT vectors)
   and (physical) extended memory. 


 6.13 Other Features of HDPMI

  þ  clients run with CPL 3; IOPL is 3 in the standard version. This means,
     instructions like STI, CLI, IN or OUT are executed in real time. Since
     v3.20, there are additionally supplied 2 binaries, named HDPMI16i.exe and
     HDPMI32i.exe, that run clients with IOPL 0.
  þ  during ring 0 processing NMIs are ignored.
  þ  HDPMI's ring 0 ESP will always have its hiword cleared. This is because
     of a design weakness in Intel CPUs, which don't touch the Hiword of ESP
     on a switch to a lower privilege level if the D bit of the destination
     ring SS descriptor is not set (16-bit stack). Some 32-bit DPMI clients 
     which (temporarily) use a 16-bit stack aren't aware of this potential
     problem, among these are apps written with the DOS/4G extender.
  þ  A client can allocate up to 16 real-mode callbacks. 


 7. Restrictions of current Version

  þ If no XMS host is found, HDPMI must switch the A20 gate on its own. Only
    standard AT (keyboard controller) and PS/2 (bit 1 of port 92h) methods
    are supported. The gate is enabled once on server initialization.
  þ Int 25h/26h (and Int 21h, ax=7305h for FAT32) translation services:
    the amount of bytes to read/write per call is limited to the size of
    the static translation buffer, which is 8 kB. 
  þ Exceptions >= 12h aren't supported/detected.


 8. Known Problems / Hints

  þ The DOS/4G(W) DOS-extender will crash in XMS mode if it isn't loaded
    in an EMB that starts below the 16 MB barrier. That's why it usually
    isn't very compatible with other extenders, including HDPMI ( even if
    the HDPMI instance is disabled ). To overcome this issue one can use
    a compatible extender like DOS32A or run DOS/4G(W) as VCPI client. 
    OTOH, DOS/4G(W) programs may run under HDPMI. They may require option -i
    or environment variable HDPMI=1 to be set. This most often is the case
    if the DOS-extended application uses sound. Generally, setting this option
    should be the first thing to do if a client doesn't run with HDPMI.

  þ Some DOS extenders will prefer to use a VCPI host over DPMI and thus
    ignore an installed copy of HDPMI. Usually there is a way to make them
    change their mind:

    - CauseWay: set environment variable "CAUSEWAY=DPMI"
    - PMODE/W: use tool PMWSETUP to set switch "prefer DPMI over VCPI"
    - DOS/32A: set environment variable "DOS32A=/DPMITST:1" or run SS.EXE

  þ Some programs don't expect system tables (GDT, IDT, LDT) being located
    at very high linear addresses, as it is the case with HDPMI by default.
    To make HDPMI compatible with such programs, environment variable
    HDPMI=512 has to be set before HDPMI is loaded. The SoundBlaster 16
    emulation for SB-PCI or SB-Live sound cards (SBINIT.COM/SBEINIT.COM)
    may belong to these kind of programs. 

  þ Some clients may require to disable HDPMI's support for DPMI V1.0
    memory functions (examples: 1. apps built with DJGPP v2 because of
    the NULL pointer protection problem. 2. the game "Inner Worlds"). 
    This can be achieved by starting HDPMI with parameter -m or by setting 
    environment switch HDPMI=1024.

  þ Some combinations of DOS, HDPMI32 and Borland's 32RTM.EXE will only
    work with environment switch HDPMI=2048. This is due to a bug in
    32RTM. Additionally, it may be necessary to make HDPMI declare itself
    as a v1.0 host, so probably the best option to run 32RTM on top of
    HDPMI32 is to set HDPMI=2052. 

  þ Some clients are incompatible with HDPMI if it hasn't been installed
    with option -r. Usually this is because they try to restore real-mode
    interrupt vector 2Fh too late (after HDPMI has done its cleanup).
    PMODE/W and DOS/32A applications may belong to these type of programs.

  þ Obsolete since v3.18: [If HDPMI is loaded and a client refuses to run 
    and tells that it doesn't like to run in a Win9x DOS box, try to set 
    HDPMI=16384 before HDPMI is loaded.]

  þ if HDPMI is loaded residently and the Watcom debugger WD is launched
    to debug a protected mode application it may display a message that
    it cannot do that on a DPMI 0.9 host. Then set environment variable
    HDPMI=36. This will report a DPMI 1.0 host to WD and separate the
    debugger's and debuggee's address space.

  þ Some clients refuse to run if the amount of free memory is too *large*
    (for example the game 'System Shock'). Simplest workaround is to use
    cmdline switch -x ( restricts reported free memory ). If this doesn't
    help: the HX runtime contains a DPMI test program called DPMI.EXE, which
    can be used to grab some extended memory and then start a DOS shell;
    it should work with any host. Start DPMI.EXE with option -? to see more
    details.

  þ Some clients regard the amount of free physical pages as safe to
    allocate. This may work ok for hosts which use a swapfile, but HDPMI
    doesn't and therefore the request is likely to fail. Option -n might 
    make such clients work (example: "Blood" game).

  þ If the machine reboots on certain actions it's not unlikely that this
    is due to a real-mode stack overflow. What sometimes may help then is:
    - set STACKS=9,512 in CONFIG.SYS (may introduce new problems though)
    - try to load another mouse/network/sound driver

  þ in Int15h or "raw" mode HDPMI expects Int 15h, ax=e801h to "work".
    However, there exist some versions of the GRUB bootloader which intercept
    this interrupt and return "wrong" values in AX, causing HDPMI to fall back
    to Int 15h, ah=88h. Extended memory is then limited to 63 MB.


 9. Compatibility List

   the following applications/DOS extenders are verified to run with HDPMI:

   - DOS4/GW Rational/Tenberry DOS extender
   - PMODE/W DOS extender
   - CauseWay DOS extender
   - DOS/32 Advanced DOS extender
   - Borland Powerpack (16 + 32-bit)
   - WDOSX extender
   - RSX DOS extender
   - RAW32 DOS extender
   - X-32 DOS extender (Digital Mars)
   - Pharlab TNT DOS extender (i.e. MS VC 1.52)
   - DJGPP applications.
   - MS Windows 3.1 (standard mode)
   - MS Windows for Workgroups 3.11 (standard mode)
   - MS DOSX16/DOSX32 applications (Codeview for DOS, MS C++ 7.0)
   - MicroFocus COBOL applications (16 + 32-bit)
   - kernel debugger WDEB386.EXE/WDEB98.EXE/DEBUGGER.EXE
   - 386SWAT, Qualitas' ring 0 debugger 

   HDPMI itself has been tested to run under the following emulators:

   - Bochs
   - Qemu
   - VMWare
   - VirtualBox
   - DosBox (version must be >= 0.70)

   HDPMI has also been tested to run with the following VCPI hosts:

   - MS-DOS Emm386 (V7.x, V6.x, V5.0)
   - Jemm
   - FreeDOS Emm386 (V2.x)
   - Quarterdeck Qemm386 (V6, V8, V9)
   - Qualitas 386Max (V6.02, V7.02)


 10. License

   HDPMI is part of the HX DOS extender runtime, which is freeware. It may
   be used for any purpose. Copyright Japheth 1993-2020. 

   Japheth ( Andreas Grech )

