INT 21 - Phar Lap 386/DOS-Extender - MAP PHYSICAL MEMORY AT END OF SEGMENT
        AX = 250Ah
        ES = segment selector in the Local Descriptor Table (LDT) of segment
             to modify
        EBX = physical base address of memory to map (multiple of 4K)
        ECX = number of physical 4K pages to map
Return: CF clear if successful
            EAX = 32-bit offset in segment of mapped memory
        CF set on error
            EAX = error code
                08h insufficient memory to create page tables
                09h invalid segment selector