IntelSoftwareDevelopersManual

Following reset the counter 15 14 debugging and

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Unformatted text preview: nto the LastBranchToIP MSR. The LastExceptionToIP and LastExceptionFromIP MSRs (also 32-bit registers) record the instruction pointers for the last branch that the processor took prior to an exception or interrupt being generated. When an exception or interrupt occurs, the contents of the LastBranchToIP and LastBranchFromIP MSRs are copied into these registers before the to and from addresses of the exception or interrupt are recorded in the LastBranchToIP and LastBranchFromIP MSRs. These registers can be read using the RDMSR instruction. 15.4.3. Monitoring Branches, Exceptions, and Interrupts When the LBR flag in the DebugCtlMSR register is set, the processor automatically begins recording branches that it takes, exceptions that are generated (except for debug exceptions), and interrupts that are serviced. Each time a branch, exception, or interrupt occurs, the processor records the to and from instruction pointers in the LastBranchToIP and LastBranchFromIP MSRs. In addition, for interrupts and exceptions, the processor copies the contents of the LastBranchToIP and LastBranchFromIP MSRs into the LastExceptionToIP and LastExceptionFromIP MSRs prior to recording the to and from addresses of the interrupt or exception. When the processor generates a debug exception (#DB), it automatically clears the LBR flag before executing the exception handler, but does not touch the last branch and last exception MSRs. The addresses for the last branch, interrupt, or exception taken are thus retained in the LastBranchToIP and LastBranchFromIP MSRs and the addresses of the last branch prior to an interrupt or exception are retained in the LastExceptionToIP, and LastExceptionFromIP MSRs. The debugger can use the last branch, interrupt, and/or exception addresses in combination with code-segment selectors retrieved from the stack to reset breakpoints in the breakpoint-address registers (DR0 through DR3), allowing a backward trace from the manifestation of a particular bug toward its source. Because the instruction pointers recorded in the LastBranchToIP, LastBranchFromIP, LastExceptionToIP, and LastExceptionFromIP MSRs are offsets into a code segment, software must determine the segment base address of the code segment associated with 15-13 DEBUGGING AND PERFORMANCE MONITORING the control transfer to calculate the linear address to be placed in the breakpoint-address registers. The segment base address can be determined by reading the segment selector for the code segment from the stack and using it to locate the segment descriptor for the segment in the GDT or LDT. The segment base address can then be read from the segment descriptor. Before resuming program execution from a debug-exception handler, the handler should set the LBR flag again to re-enable last branch and last exception/interrupt recording. 15.4.4. Single-Stepping on Branches, Exceptions, and Interrupts When the BTF flag in the DebugCtlMSR register and the TF flag in the EFLAGS register are both set, the processor generates a single-step debug exception the next time it takes a branch, generates an exception, or services an interrupt. This mechanism allows the debugger to singlestep on control transfers caused by branches, exceptions, or interrupts. This “control-flow single stepping” helps isolate a bug to a particular block of code before instruction single-stepping further narrows the search. If the BTF flag is set when the processor generates a debug exception, the processor clears the flag along with the TF flag. The debugger must reset the BTF flag before resuming program execution to continue control-flow single stepping. 15.4.5. Initializing Last Branch or Last Exception/Interrupt Recording The LastBranchToIP, LastBranchFromIP, LastExceptionToIP, and LastException-FromIP MSRs are enabled by setting the LBR flag in the DebugCtlMSR register. Control-flow single stepping is enabled by setting the BTF flag in the DebugCtlMSR register. The processor clears both the LBR and the BTF flags whenever a debug exception is generated. To re-enable these mechanisms, the debug-exception handler must thus explicitly set these flags before returning to the interrupted program. 15.5. TIME-STAMP COUNTER The Intel Architecture (beginning with the Pentium® processor) defines a time-stamp counter mechanism that can be used to monitor and identify the relative time of occurrence of processor events. The time-stamp counter architecture includes an instruction for reading the time-stamp counter (RDTSC), a feature bit (TCS flag) that can be read with the CPUID instruction, a timestamp counter disable bit (TSD flag) in control register CR4, and a model-specific time-stamp counter. Following execution of the CPUID instruction, the TSC flag in register EDX (bit 4) indicates (when set) that the time-stamp counter is present in a particular Intel Architecture processor implementation. (Refer to “CPUID—CPU Identification” in Chapter 3 of the Intel Architecture Software Developer’s Manual, Volume 2.) The time-stamp c...
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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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