73 initializing paging before the gdt can be used the

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Unformatted text preview: s must be loaded into physical memory to support further initialization of the processor, as described in Section 8.6., “Software Initialization for Real-Address Mode Operation”. Before the processor can be switched to protected mode, the software initialization code must load a minimum number of protected mode data structures and code modules into memory to support reliable operation of the processor in protected mode. These data structures include the following: • • • • • • • • • • • A protected-mode IDT. A GDT. A TSS. (Optional.) An LDT. If paging is to be used, at least one page directory and one page table. A code segment that contains the code to be executed when the processor switches to protected mode. One or more code modules that contain the necessary interrupt and exception handlers. Software initialization code must also initialize the following system registers before the processor can be switched to protected mode: The GDTR. (Optional.) The IDTR. This register can also be initialized immediately after switching to protected mode, prior to enabling interrupts. Control registers CR1 through CR4. (Pentium® Pro processor only.) The memory type range registers (MTRRs). With these data structures, code modules, and system registers initialized, the processor can be switched to protected mode by loading control register CR0 with a value that sets the PE flag (bit 0). 8-11 PROCESSOR MANAGEMENT AND INITIALIZATION 8.7.1. Protected-Mode System Data Structures The contents of the protected-mode system data structures loaded into memory during software initialization, depend largely on the type of memory management the protected-mode operatingsystem or executive is going to support: flat, flat with paging, segmented, or segmented with paging. To implement a flat memory model without paging, software initialization code must at a minimum load a GDT with one code and one data-segment descriptor. A null descriptor in the first GDT entry is also required. The stack can be placed in a normal read/write data segment, so no dedicated descriptor for the stack is required. A flat memory model with paging also requires a page directory and at least one page table (unless all pages are 4 MBytes in which case only a page directory is required). Refer to Section 8.7.3., “Initializing Paging” Before the GDT can be used, the base address and limit for the GDT must be loaded into the GDTR register using an LGDT instruction. A multisegmented model may require additional segments for the operating system, as well as segments and LDTs for each application program. LDTs require segment descriptors in the GDT. Some operating systems allocate new segments and LDTs as they are needed. This provides maximum flexibility for handling a dynamic programming environment. However, many operating systems use a single LDT for all tasks, allocating GDT entries in advance. An embedded system, such as a process controller, might pre-allocate a fixed number of segments and LDTs for a fixed number of application programs. This would be a simple and efficient way to structure the software environment of a real-time system. 8.7.2. Initializing Protected-Mode Exceptions and Interrupts Software initialization code must at a minimum load a protected-mode IDT with gate descriptor for each exception vector that the processor can generate. If interrupt or trap gates are used, the gate descriptors can all point to the same code segment, which contains the necessary exception handlers. If task gates are used, one TSS and accompanying code, data, and task segments are required for each exception handler called with a task gate. If hardware allows interrupts to be generated, gate descriptors must be provided in the IDT for one or more interrupt handlers. Before the IDT can be used, the base address and limit for the IDT must be loaded into the IDTR register using an LIDT instruction. This operation is typically carried out immediately after switching to protected mode. 8.7.3. Initializing Paging Paging is controlled by the PG flag in control register CR0. When this flag is clear (its state following a hardware reset), the paging mechanism is turned off; when it is set, paging is enabled. Before setting the PG flag, the following data structures and registers must be initialized: 8-12 PROCESSOR MANAGEMENT AND INITIALIZATION • Software must load at least one page directory and one page table into physical memory. The page table can be eliminated if the page directory contains a directory entry pointing to itself (here, the page directory and page table reside in the same page), or if only 4-MByte pages are used. Control register CR3 (also called the PDBR register) is loaded with the physical base address of the page directory. (Optional) Software may provide one set of code and data descriptors in the GDT or in an LDT for supervisor mode and another set for user mode. • • With this paging initialization complete, paging is enabled and the processor is switched to protected mode at the same time by loading control register CR0 with...
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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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