In reference to bit 0 of the ‘flags’ parameter in the Multiboot information structure, if the bootloader in question uses older bios interfaces, or the newest ones are not available (see description about bit 6), then a maximum of either 15 or 63 megabytes of memory may be reported. It is highly recommended that boot loaders perform a thorough memory probe.
In reference to bit 1 of the ‘flags’ parameter in the Multiboot information structure, it is recognized that determination of which bios drive maps to which device driver in an operating system is non-trivial, at best. Many kludges have been made to various operating systems instead of solving this problem, most of them breaking under many conditions. To encourage the use of general-purpose solutions to this problem, there are 2 bios device mapping techniques (see BIOS device mapping techniques).
In reference to bit 6 of the ‘flags’ parameter in the Multiboot information structure, it is important to note that the data structure used there (starting with ‘BaseAddrLow’) is the data returned by the INT 15h, AX=E820h — Query System Address Map call. See See Query System Address Map, for more information. The interface here is meant to allow a boot loader to work unmodified with any reasonable extensions of the bios interface, passing along any extra data to be interpreted by the operating system as desired.