Get Memory Block Size and Base

Returns the size of a memory block that was previously
allocated with Int 31H Fn 0501H or 0504H.

Call With:

   AX     = 050AH
   SI:DI  = memory block handle

Returns:

   if function successful
   Carry flag= clear
   SI:DI  = size of memory block (bytes)

   BX:CX  = base address of memory block if function
           unsuccessful
   Carry flag= set
   AX     = error code
           8023H   invalid handle