The very last function of the system BIOS POST after loading and validating the operating system boot sector is to transfer control. In an ISA system, control is transferred without any parameters. In a Plug and Play system BIOS, parameters will be passed to the operating system. The parameters are:
Entry:
ES:DI
Pointer to System BIOS Plug and Play Installation Check Structure (See section 4.4)
DL
Physical device number the OS is being loaded from (e.g. 80h, assuming the device supports INT 13H interface.)
In a non-Plug and Play operating environment this information will have no meaning. However, a Plug and Play operating system will look for a Plug and Play system BIOS and use any information it may need. The physical device number is passed to allow the operating system to continue to load from the current physical device, instead of assuming a physical device of 00h or 80h.