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Unformatted text preview: a page are read/write or read/only. The Í Ë bit, which determines whether the page can be accessed in user mode, protects code and data in the operating system kernel from user programs. 512 CHAPTER 10. VIRTUAL MEMORY As the MMU translates each virtual address, it also updates two other bits that can be used by the kernel’s page fault handler. The MMU sets the bit, which is known as a reference bit, each time a page is accessed. The kernel can use the reference bit to implement its page replacement algorithm. The MMU sets the bit, or dirty bit, each time the page is written to. A page that has been modiﬁed is sometimes called a dirty page. The dirty bit tells the kernel whether or not it must write-back a victim page before it copies in a replacement page. The kernel can call a special kernel-mode instruction to clear the reference the dirty bits.
Aside: Execute permissions and buffer overﬂow attacks. Notice that a Pentium page table entry lacks an execute permission bit to control whether the contents of a pag...
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