Unformatted text preview: ize of a stack-pointer increment or decrement, due to a CALL or RET Indirect-branch operand size In 64-bit mode, all of the above actions are forced to 64 bits regardless of operand size prefixes (operand size prefixes are silently ignored). However, the displacement field for relative branches is still limited to 32 bits and the address size for near branches is not forced in 64-bit mode. Address sizes affect the size of RCX used for JCXZ and LOOP; they also impact the address calculation for memory indirect branches. Such addresses are 64 bits by default; but they can be overridden to 32 bits by an address size prefix. Software typically uses far branches to change privilege levels. The legacy IA-32 architecture provides the call-gate mechanism to allow software to branch from one privilege level to another, although call gates can also be used for branches that do not change privilege levels. When call gates are used, the selector portion of the direct or indirect pointer references a gate descriptor (the offset in the instruction is ignored). The offset to the destination's code segment is taken from the call-gate descriptor. 64-bit mode redefines the type value of a 32-bit call-gate descriptor type to a 64-bit call gate descriptor and expands the size of the 64-bit descriptor to hold a 64-bit offset. The 64-bit mode call-gate descriptor allows far branches that reference any location in the supported linear-address space. These call gates also hold the target code selector (CS), allowing changes to privilege level and default size as a result of the gate transition. Because immediates are generally specified up to 32 bits, the only way to specify a full 64-bit absolute RIP in 64-bit mode is with an indirect branch. For this reason, direct far branches are eliminated from the instruction set in 64-bit mode. 64-bit mode also expands the semantics of the SYSENTER and SYSEXIT instructions so that the instructions operate within a 64-bit memory space. The mode also introduces two new instructions: SYSCALL and SYSRET (which are valid only in 64-bit mode). For details, see "SYSENTER--Fast System Call" and "SYSEXIT--Fast Return from Fast System Call" in Chapter 4, "Instruction Set Reference, N-Z," of the Intel 64 and IA-32 Architectures Software Developer's Manual, Volume 2B. 6-12 Vol. 1 PROCEDURE CALLS, INTERRUPTS, AND EXCEPTIONS 6.4 INTERRUPTS AND EXCEPTIONS The processor provides two mechanisms for interrupting program execution, interrupts and exceptions: An interrupt is an asynchronous event that is typically triggered by an I/O device. An exception is a synchronous event that is generated when the processor detects one or more predefined conditions while executing an instruction. The IA-32 architecture specifies three classes of exceptions: faults, traps, and aborts. The processor responds to interrupts and exceptions in essentially the same way. When an interrupt or exception is signaled, the processor halts execution...
View Full Document
- Winter '11
- X86, Intel corporation, 64-bit mode, fpu floating-point exception, FPU Control Instructions