请教一个understanding the linux kernel 书中的关于分页的问题
在讲到为什么要用多级分页时，有这么一句话The aim of this two-level scheme is to reduce the amount of ram required for per-process Page Tables. If a simple one-level Page Table was used, then is would require up to 220 entries(i.e., at 4 bytes per entry, 4M of RAM) to represent the Page Table for each process(if the process used a full 4 GB linear address space), even though a process does not use all addresses in that range.
如果只用一级的话，需要1024*1024个表项来定位每一页，从而实现1024*1024*4K=4G的虚拟地址空间。每个表项4个字节，所以整个页表占用1024*1024*4byte = 4M内存，不明白文中的220 entries怎么算出来的。 是 2的20次方个entry.
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