* swapper_pg_dir is the virtual address of the initial page table.

* We place the page tables 16K below KERNEL_RAM_VADDR.Therefore, we must

* make sure that KERNEL_RAM_VADDR is correctly set.Currently, we expect

* the least significant 16 bits to be 0x8000, but we could probably

* relax this restriction to KERNEL_RAM_VADDR >= PAGE_OFFSET + 0x4000.

大致的意思是，在KERNEL_RAM_VADDR（內(nèi)核的在RAM中的地址）下面16K（16bit，也就是說是32Kb，0x8000）的地方放著初始化頁表的虛擬地址，因此至少在RAM的起始地址還有偏移0x8000才能開始放內(nèi)核。