INT 21 - DOS 2+ - ALLOCATE MEMORY
        AH = 48h
        BX = number of paragraphs to allocate
Return: CF clear if successful
            AX = segment of allocated block
        CF set on error
            AX = error code (07h,08h) (see AH=59h)
            BX = size of largest available block
Notes:  DOS 2.1-5.0 coalesces free blocks while scanning for a block to
          allocate
        .COM programs are initially allocated the largest available memory
          block, and should free some memory with AH=49h before attempting any
          allocations
        under the FlashTek X-32 DOS extender, EBX contains a protected-mode
          near pointer to the allocated block on a successful return
SeeAlso: AH=4Ah,AH=58h,AH=83h