With the intel386 and later intel architecture

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Unformatted text preview: lag is set to 1, 2, or 3). The processor generates the exception after it executes the instruction that made the access, so these breakpoint condition causes a trap-class exception to be generated. Because data breakpoints are traps, the original data is overwritten before the trap exception is generated. If a debugger needs to save the contents of a write breakpoint location, it should save the original contents before setting the breakpoint. The handler can report the saved value after the breakpoint is triggered. The address in the debug registers can be used to locate the new value stored by the instruction that triggered the breakpoint. 15-9 DEBUGGING AND PERFORMANCE MONITORING The Intel486™ and later Intel Architecture processors ignore the GE and LE flags in DR7. In the Intel386™ processor, exact data breakpoint matching does not occur unless it is enabled by setting the LE and/or the GE flags. The P6 family processors, however, are unable to report data breakpoints exactly for the REP MOVS and REP STOS instructions until the completion of the iteration after the iteration in which the breakpoint occurred. For repeated INS and OUTS instructions that generate an I/O-breakpoint debug exception, the processor generates the exception after the completion of the first iteration. Repeated INS and OUTS instructions generate an I/O-breakpoint debug exception after the iteration in which the memory address breakpoint location is accessed. GENERAL-DETECT EXCEPTION CONDITION When the GD flag in DR7 is set, the general-detect debug exception occurs when a program attempts to access any of the debug registers (DR0 through DR7) at the same time they are being used by another application, such as an emulator or debugger. This additional protection feature guarantees full control over the debug registers when required. The debug exception handler can detect this condition by checking the state of the BD flag of the DR6 register. The processor generates the exception before it executes the MOV instruction that accesses a debug register, which causes a fault-class exception to be generated. SINGLE-STEP EXCEPTION CONDITION The processor generates a single-step debug exception if (while an instruction is being executed) it detects that the TF flag in the EFLAGS register is set. The exception is a trap-class exception, because the exception is generated after the instruction is executed. (Note that the processor does not generate this exception after an instruction that sets the TF flag. For example, if the POPF instruction is used to set the TF flag, a single-step trap does not occur until after the instruction that follows the POPF instruction.) The processor clears the TF flag before calling the exception handler. If the TF flag was set in a TSS at the time of a task switch, the exception occurs after the first instruction is executed in the new task. The TF flag normally is not cleared by privilege changes inside a task. The INT n and INTO instructions, however, do clear this flag. Therefore, software debuggers that single-step code must recognize and emulate INT n or INTO instructions rather than executing them directly. To maintain protection, the operating system should check the CPL after any single-step trap to see if single stepping should continue at the current privilege level. The interrupt priorities guarantee that, if an external interrupt occurs, single stepping stops. When both an external interrupt and a single-step interrupt occur together, the single-step interrupt is processed first. This operation clears the TF flag. After saving the return address or switching tasks, the external interrupt input is examined before the first instruction of the singlestep handler executes. If the external interrupt is still pending, then it is serviced. The external interrupt handler does not run in single-step mode. To single step an interrupt handler, set a break point inside the handler and then set the TF flag. 15-10 DEBUGGING AND PERFORMANCE MONITORING TASK-SWITCH EXCEPTION CONDITION The processor generates a debug exception after a task switch if the T flag of the new task’s TSS is set. This exception is generated after program control has passed to the new task, and after the first instruction of that task is executed. The exception handler can detect this condition by examining the BT flag of the DR6 register. Note that, if the debug exception handler is a task, the T bit of its TSS should not be set. Failure to observe this rule will put the processor in a loop. 15.3.2. Breakpoint Exception (#BP)—Interrupt Vector 3 The breakpoint exception (interrupt 3) is caused by execution of an INT 3 instruction (refer to Section 5.12., “Exception and Interrupt Reference” in Chapter 5, Interrupt and Exception Handling). Debuggers use break exceptions in the same way that they use the breakpoint registers; that is, as a mechanism for suspending program execution to examine registers and memory locations. With earlier Intel Architecture processors, breakpoint exceptions are used extensively for setting instruction breakpoints. With the Intel3...
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