INT 21 - Phar Lap 386/DOS-Extender - ALLOCATE MS-DOS MEMORY BLOCK
        AX = 25C0h
        BX = number of 16-byte paragraphs of MS-DOS memory requested
Return: CF clear if successful
            AX = real-mode paragraph address of memory
        CF set on error
            AX = error code
                07h MS-DOS memory control blocks destroyed
                08h insufficient memory
            BX = size in paragraphs of largest available memory block
SeeAlso: AX=25C2h