Allocate DOS Memory Block

Allocates a block of memory from the DOS memory pool, i.e.
memory below the 1 MB boundary that is controlled by DOS.
Such memory blocks are typically used to exchange data with
real mode programs, TSRs, or device drivers.  The function
returns both the real mode segment base address of the block
and one or more descriptors that can be used by protected
mode applications to access the block.

Call With:

   AX     = 0100H
   BX     = number of (16-byte) paragraphs desired

Returns:

   if function successful
   Carry flag= clear
   AX     = real mode segment base address of allocated
           block
   DX     = selector for allocated block

   if function unsuccessful
   Carry flag= set
   AX     = error code
           0007H   memory control blocks damaged (also
                   returned by DPMI 0.9 hosts)
           0008H   insufficient memory (also returned by
                   DPMI 0.9 hosts).
           8011H   descriptor unavailable
   BX     = size of largest available block in paragraphs

Notes:

o  If the size of the block requested is greater than 64 KB
   (BX > 1000H) and the client is a 16-bit program,
   contiguous descriptors are allocated and the base
   selector is returned.  The consecutive selectors for the
   memory block can be calculated using the value returned
   by the Get Selector Increment Value function (Int 31H
   Function 0003H).  Each descriptor has a limit of 64 KB,
   except for the last which has a limit of blocksize MOD 64
   KB.
o  If the DPMI host is 32-bit, the client is 16-bit, and
   more than one descriptor is allocated, the limit of the
   first descriptor will be set to the size of the entire
   block.  Subsequent descriptors have limits as described
   in the previous Note. 16-bit DPMI hosts will always set
   the limit of the first descriptor to 64 KB even when
   running on an 80386 (or later) machine.
o  When the client is 32-bit, this function always allocates
   only one descriptor.
o  Client programs should never modify or free any
   descriptors allocated by this function.  The Free DOS
   Memory Block function (Int 31H Fn 0101H) will
   deallocate the descriptors automatically.
o  The DOS allocation function (Int 21H Fn 48H) is
   used.
o  Refer to the rules for descriptor usage in Appendix D.