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  DOS version 4's full-screen interface shell gives it a very different look
  from the "glass teletype" interface familiar to users of previous DOS
  versions. Nevertheless, from a programmer's perspective, DOS version 4 is
  quite similar to its predecessor, version 3.3.

  The most important changes in DOS version 4 are related to its ability to
  manage larger disk and memory resources. When the IBM PC and DOS were new,
  the lack of support for more than 1 megabyte (MB) of RAM or 32 MB of disk
  space was hardly a shortcoming. But by July 1988, when DOS version 4
  appeared, both of these limits had become important to many DOS users. DOS
  version 4 avoids the limitations of previous versions by improving the way
  it manages memory and disk space.

  DOS version 4 provides access to larger fixed-disk partitions by using
  32-bit logical sector numbers instead of the 16-bit sector numbers used in
  previous versions. With 16-bit logical sector numbers, the maximum number
  of sectors in a fixed-disk partition is 65,536. Thus, with a default
  512-byte sector size, the largest disk partition you can support is 32 MB.
  With DOS version 4's 32-bit logical sector numbers, the maximum number of
  logical sectors in a partition is not limited to 65,536. This means that
  DOS Version 4 can manage fixed-disk partitions larger than 32 MB without
  increasing the default sector size. (See Chapter 5 for more about logical
  sectors.)

  DOS version 4 supports expanded memory by incorporating the functionality
  of version 4.0 of the LIM (Lotus-Intel-Microsoft) Expanded Memory
  Specification, which consists of a set of function calls invoked through
  software interrupt 67H. (See Figure D-1.) Because DOS supports the LIM
  interface, you needn't install a separate device driver in order to use
  expanded memory in a PC or PS/2.

  One immediate benefit of EMS support is that DOS itself can use expanded
  memory for its internal buffers. (The /E switch with BUFFERS= in the
  CONFIG.SYS file places DOS disk buffers in expanded memory; the /E switch
  on FASTOPEN places FASTOPEN's directory/file cache in expanded memory.)
  This lets your applications use more conventional memory in the first 640
  KB of the 8086 address space.

  These features of DOS version 4 make the DOS application-programming
  interface different from previous versions in several ways. The
  differences are evident in several interrupt 21H functions, in the
  services provided through interrupts 25H and 26H, and in the way DOS
  version 4 formats disks. The following sections describe these
  differences.


  EMM             Function
  Function       Hex    Dec     Description
  --------------------------------------------------------------------------
   1             40H    64      Get status.
   2             41H    65      Get page frame address.
   3             42H    66      Get unallocated page count.
   4             43H    67      Allocate pages.
   5             44H    68      Map/unmap handle page.
   6             45H    69      Deallocate pages.
   7             46H    70      Get EMM version.
   8             47H    71      Save page map.
   9             48H    72      Restore page map.
  10                            (Reserved.)
  11                            (Reserved.)
  12             4BH    75      Get EMM handle count.
  13             4CH    76      Get EMM handle pages.
  14             4DH    77      Get all EMM handle pages.
  15             4EH    78      Get/set page map.
  16             4FH    79      Get/set partial page map.
  17             50H    80      Map/unmap multiple handle pages.
  18             51H    81      Reallocate pages.
  19             52H    82      Get/set handle attributes.
  20             53H    83      Get/set handle name.
  21             54H    84      Get handle directory.
  22             55H    85      Alter page map and jump.
  23             56H    86      Alter page map and call.
  24             57H    87      Move/exchange memory region.
  25             58H    88      Get mappable physical address array.
  26             59H    89      Get expanded memory hardware information.
  27             5AH    90      Allocate raw pages.
  28             5BH    91      Alternate page map register set.
  29             5CH    92      Prepare expanded memory for warm boot.
  30             5DH    93      Enable/disable operating system/environment
                                functions.
  --------------------------------------------------------------------------


  Figure D-1.  LIM Expanded Memory Support functions supported in DOS
  Version 4 through interrupt 67H.