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Unformatted text preview: egments. When page-level protection is used with the flat memory model, it allows supervisor code and data (the operating system or executive) to be protected from user code and data (application programs). It also allows pages containing code to be write protected. When the segment- and page-level protection are combined, page-level read/write protection allows more protection granularity within segments. With page-level protection (as with segment-level protection) each memory reference is checked to verify that protection checks are satisfied. All checks are made before the memory cycle is started, and any violation prevents the cycle from starting and results in a page-fault 4-30 PROTECTION exception being generated. Because checks are performed in parallel with address translation, there is no performance penalty. The processor performs two page-level protection checks: • • Restriction of addressable domain (supervisor and user modes). Page type (read only or read/write). Violations of either of these checks results in a page-fault exception being generated. Refer to Chapter 5, Interrupt and Exception Handling for an explanation of the page-fault exception mechanism. This chapter describes the protection violations which lead to page-fault exceptions. 4.11.1. Page-Protection Flags Protection information for pages is contained in two flags in a page-directory or page-table entry (refer to Figure 3-14 in Chapter 3, Protected-Mode Memory Management): the read/write flag (bit 1) and the user/supervisor flag (bit 2). The protection checks are applied to both first- and second-level page tables (that is, page directories and page tables). 4.11.2. Restricting Addressable Domain The page-level protection mechanism allows restricting access to pages based on two privilege levels: • • Supervisor mode (U/S flag is 0)—(Most privileged) For the operating system or executive, other system software (such as device drivers), and protected system data (such as page tables). User mode (U/S flag is 1)—(Least privileged) For application code and data. The segment privilege levels map to the page privilege levels as follows. If the processor is currently operating at a CPL of 0, 1, or 2, it is in supervisor mode; if it is operating at a CPL of 3, it is in user mode. When the processor is in supervisor mode, it can access all pages; when in user mode, it can access only user-level pages. (Note that the WP flag in control register CR0 modifies the supervisor permissions, as described in Section 4.11.3., “Page Type”) Note that to use the page-level protection mechanism, code and data segments must be set up for at least two segment-based privilege levels: level 0 for supervisor code and data segments and level 3 for user code and data segments. (In this model, the stacks are placed in the data segments.) To minimize the use of segments, a flat memory model can be used (refer to Section 3.2.1., “Basic Flat Model” in Section 3, “Protected-Mode Memory Management”). Here, the user and supervisor code and data segments all begin at address zero in the linear address space and overlay each other. With this arrangement, operating-system code (running at the supervisor level) and application code (running at the user level) can execute as if there are no segments. Protection between operating-system and application code and data is provided by the processor’s page-level protection mechanism. 4-31 PROTECTION 4.11.3. Page Type The page-level protection mechanism recognizes two page types: • • Read-only access (R/W flag is 0). Read/write access (R/W flag is 1). When the processor is in supervisor mode and the WP flag in register CR0 is clear (its state following reset initialization), all pages are both readable and writable (write-protection is ignored). When the processor is in user mode, it can write only to user-mode pages that are read/write accessible. User-mode pages which are read/write or read-only are readable; supervisor-mode pages are neither readable nor writable from user mode. A page-fault exception is generated on any attempt to violate the protection rules. The P6 family, Pentium®, and Intel486™ processors allow user-mode pages to be writeprotected against supervisor-mode access. Setting the WP flag in register CR0 to 1 enables supervisor-mode sensitivity to user-mode, write-protected pages. This supervisor write-protect feature is useful for implementing a “copy-on-write” strategy used by some operating systems, such as UNIX*, for task creation (also called forking or spawning). When a new task is created, it is possible to copy the entire address space of the parent task. This gives the child task a complete, duplicate set of the parent's segments and pages. An alternative copy-on-write strategy saves memory space and time by mapping the child's segments and pages to the same segments and pages used by the parent task. A private copy of a page gets created only when one of the tasks writes t...
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This note was uploaded on 06/07/2013 for the course ECE 1234 taught by Professor Kwhon during the Spring '10 term at University of California, Berkeley.

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