INT 31 P - DPMI 0.9+ - ALLOCATE DOS MEMORY BLOCK
        AX = 0100h
        BX = number of paragraphs to allocate
Return: CF clear if successful
            AX = real mode segment of allocated block
            DX = first selector for allocated block
        CF set on error
            AX = DOS error code (07h,08h) (see INT 21/AH=59h)
                 (DPMI 1.0+) DPMI error code (8011h) (see AX=0000h)
            BX = size (in paragraphs) of largest available block
Notes:  multiple contiguous selectors are allocated for blocks of more than 64K
          if the caller is a 16-bit program
        never modify or deallocate returned descriptors
        not supported by MS Windows 3.0 in Standard mode
SeeAlso: AX=0501h