Int 31 Fn FF21  P - Causeway - "getmemdos" - Allocate Conventional Memory  [E]

   AX = FF21h
   BX = number of paragraphs to allocate

Return: CF clear if successful
       AX = real-mode segment of allocated block
       DX = initial selector for block
   CF set on error
       AX = DOS error code (see #1366 at INT 21/AH=59h/BX=0000h)
       BX = size of largest available block

Note:  if the requested size is greater than 64K, contiguous selectors will
     be allocated, one for each 64K; for 32-bit applications, the first
     selector's limit will be set to the size of the entire block, while
     that selector will have a limit of 64K for 16-bit applications.  All
     selectors after the first one have a limit of 64K except the final
     one

See Also: AX=FF22h,AX=FF23h,INT 21/AH=48h