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Unformatted text preview: e current stack is the one contained in the segment referenced by the SS register. Stack Segment Bottom of Stack (Initial ESP Value) Local Variables for Calling Procedure The Stack Can Be 16 or 32 Bits Wide Parameters Passed to Called Procedure Frame Boundary The EBP register is typically set to point to the return instruction pointer. Return Instruction Pointer EBP Register ESP Register Top of Stack Pushes Move the Top Of Stack to Lower Addresses Pops Move the Top Of Stack to Higher Addresses Figure 6-1. Stack Structure
The processor references the SS register automatically for all stack operations. For example, when the ESP register is used as a memory address, it automatically points to an address in the current stack. Also, the CALL, RET, PUSH, POP, ENTER, and LEAVE instructions all perform operations on the current stack. 6.2.1 Setting Up a Stack To set a stack and establish it as the current stack, the program or operating system/executive must do the following: 1. Establish a stack segment. 2. Load the segment selector for the stack segment into the SS register using a MOV, POP, or LSS instruction. 6-2 Vol. 1 PROCEDURE CALLS, INTERRUPTS, AND EXCEPTIONS 3. Load the stack pointer for the stack into the ESP register using a MOV, POP, or LSS instruction. The LSS instruction can be used to load the SS and ESP registers in one operation. See "Segment Descriptors" in of the Intel 64 and IA-32 Architectures Software Developer's Manual, Volume 3A, for information on how to set up a segment descriptor and segment limits for a stack segment. 6.2.2 Stack Alignment The stack pointer for a stack segment should be aligned on 16-bit (word) or 32-bit (double-word) boundaries, depending on the width of the stack segment. The D flag in the segment descriptor for the current code segment sets the stack-segment width (see "Segment Descriptors" in Chapter 3, "Protected-Mode Memory Management," of the Intel 64 and IA-32 Architectures Software Developer's Manual, Volume 3A). The PUSH and POP instructions use the D flag to determine how much to decrement or increment the stack pointer on a push or pop operation, respectively. When the stack width is 16 bits, the stack pointer is incremented or decremented in 16-bit increments; when the width is 32 bits, the stack pointer is incremented or decremented in 32-bit increments. Pushing a 16-bit value onto a 32-bit wide stack can result in stack misaligned (that is, the stack pointer is not aligned on a doubleword boundary). One exception to this rule is when the contents of a segment register (a 16-bit segment selector) are pushed onto a 32-bit wide stack. Here, the processor automatically aligns the stack pointer to the next 32-bit boundary. The processor does not check stack pointer alignment. It is the responsibility of the programs, tasks, and system procedures running on the processor to maintain proper alignment of stack pointers. Misaligning a stack pointer can cause serious performance de...
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- Winter '11