These tables contain entries called segment

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Unformatted text preview: 4-KByte pages and the normal 32-bit physical address size. *Physical Address 1. MXCSR is new control/status register in the Pentium® III processor. Figure 2-1. System-Level Registers and Data Structures 2-2 SYSTEM ARCHITECTURE OVERVIEW 2.1.1. Global and Local Descriptor Tables When operating in protected mode, all memory accesses pass through either the global descriptor table (GDT) or the (optional) local descriptor table (LDT), shown in Figure 2-1. These tables contain entries called segment descriptors. A segment descriptor provides the base address of a segment and access rights, type, and usage information. Each segment descriptor has a segment selector associated with it. The segment selector provides an index into the GDT or LDT (to its associated segment descriptor), a global/local flag (that determines whether the segment selector points to the GDT or the LDT), and access rights information. To access a byte in a segment, both a segment selector and an offset must be supplied. The segment selector provides access to the segment descriptor for the segment (in the GDT or LDT). From the segment descriptor, the processor obtains the base address of the segment in the linear address space. The offset then provides the location of the byte relative to the base address. This mechanism can be used to access any valid code, data, or stack segment in the GDT or LDT, provided the segment is accessible from the current privilege level (CPL) at which the processor is operating. (The CPL is defined as the protection level of the currently executing code segment.) In Figure 2-1 the solid arrows indicate a linear address, the dashed lines indicate a segment selector, and the dotted arrows indicate a physical address. For simplicity, many of the segment selectors are shown as direct pointers to a segment. However, the actual path from a segment selector to its associated segment is always through the GDT or LDT. The linear address of the base of the GDT is contained in the GDT register (GDTR); the linear address of the LDT is contained in the LDT register (LDTR). 2.1.2. System Segments, Segment Descriptors, and Gates Besides the code, data, and stack segments that make up the execution environment of a program or procedure, the system architecture also defines two system segments: the task-state segment (TSS) and the LDT. (The GDT is not considered a segment because it is not accessed by means of a segment selector and segment descriptor.) Each of these segment types has a segment descriptor defined for it. The system architecture also defines a set of special descriptors called gates (the call gate, interrupt gate, trap gate, and task gate) that provide protected gateways to system procedures and handlers that operate at different privilege levels than application programs and procedures. For example, a CALL to a call gate provides access to a procedure in a code segment that is at the same or numerically lower privilege level (more privileged) than the current code segment. To access a procedure through a call gate, the calling procedure1 must supply the selector of the call gate. The processor than performs an access rights check on the call gate, comparing the CPL with the privilege level of the call gate and the destination code segment pointed to by the call gate. If access to the destination code segment is allowed, the processor gets the segment selector for the destination code segment and an offset into that code segment from the call gate. 1. The word “procedure” is commonly used in this document as a general term for a logical unit or block of code (such as a program, procedure, function, or routine). The term is not restricted to the definition of a procedure in the Intel Architecture assembly language. 2-3 SYSTEM ARCHITECTURE OVERVIEW If the call requires a change in privilege level, the processor also switches to the stack for that privilege level. (The segment selector for the new stack is obtained from the TSS for the currently running task.) Gates also facilitate transitions between 16-bit and 32-bit code segments, and vice versa. 2.1.3. Task-State Segments and Task Gates The TSS (refer to Figure 2-1) defines the state of the execution environment for a task. It includes the state of the general-purpose registers, the segment registers, the EFLAGS register, the EIP register, and segment selectors and stack pointers for three stack segments (one stack each for privilege levels 0, 1, and 2). It also includes the segment selector for the LDT associated with the task and the page-table base address. All program execution in protected mode happens within the context of a task, called the current task. The segment selector for the TSS for the current task is stored in the task register. The simplest method of switching to a task is to make a call or jump to the task. Here, the segment selector for the TSS of the new task is given in the CALL or JMP instruction. In switching tasks, the processor performs the following a...
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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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