Since MultiBoot kernels initially run in physical memory, with paging disabled and segmentation effectively ``neutralized,'' the kernel must be linked at an address within the range of physical memory present on typical PCs. Normally the best place to link the kernel is at 0x100000, or 1MB, which is the beginning of extended memory just beyond the real-mode ROM BIOS. Since the processor is already in 32-bit protected mode when the MultiBoot boot loader starts the OS, running above the 1MB ``boundary'' is not a problem. By linking at 1MB, the kernel has plenty of ``room to grow,'' having essentially all extended memory available to it in one contiguous chunk.
In some cases, it may be preferable to link the kernel at a lower address, below the 1MB boundary, for example if the kernel needs to run on machines without any extended memory, or if the kernel contains code that needs to run in real mode. This is also allowed by the MultiBoot standard. However, note that the kernel should generally leave at least the first 0x500 bytes of physical memory untouched, since this area contains important BIOS data structures that will be needed if the kernel ever makes calls to the BIOS, or if it wants to glean information about the machine from this area such as hard disk configuration data.