I do not understand why the base address of page tables and page frames can be extended from 20 to 24 bits. Can anyone explain how to do the calculation? Thank you.

software developers began to discover the memory limitations software memo of architectures, Intel adopted a page address extension (PAE, which allows 32-bit processors to access a physical address space larger than 4GB. The fundamental difference in by PAE support was that paging went from a two-level scheme (as shown in Figure 8.23) to a three-level scheme, where the top two bits refer to a page directory pointer table. Figure 8 illustrates a PAE system with 4-KB pages. (PAE also supports 2-MB pages.) page directory page table 12 11 3130 29 21 20 4-KB page CR3 register page directory page page pointer table directory table Figure 8.24 Page address extensions