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12.3 Debug Exceptions

Two of the interrupt vectors of the 80386 are reserved for exceptions that relate to debugging. Interrupt 1 is the primary means of invoking debuggers designed expressly for the 80386; interrupt 3 is intended for debugging debuggers and for compatibility with prior processors in Intel's 8086 processor family.

12.3.1 Interrupt 1 -- Debug Exceptions

The handler for this exception is usually a debugger or part of a debugging system. The processor causes interrupt 1 for any of several conditions. The debugger can check flags in DR6 and DR7 to determine what condition caused the exception and what other conditions might be in effect at the same time. Table 12-2 associates with each breakpoint condition the combination of bits that indicate when that condition has caused the debug exception.

Instruction address breakpoint conditions are faults, while other debug conditions are traps. The debug exception may report either or both at one time. The following paragraphs present details for each class of debug exception.

Table 12-2. Debug Exception Conditions

Flags to Test              Condition

BS=1                       Single-step trap
B0=1 AND (GE0=1 OR LE0=1)  Breakpoint DR0, LEN0, R/W0
B1=1 AND (GE1=1 OR LE1=1)  Breakpoint DR1, LEN1, R/W1
B2=1 AND (GE2=1 OR LE2=1)  Breakpoint DR2, LEN2, R/W2
B3=1 AND (GE3=1 OR LE3=1)  Breakpoint DR3, LEN3, R/W3
BD=1                       Debug registers not available; in use by ICE-386.
BT=1                       Task switch Instruction Addrees Breakpoint

The processor reports an instruction-address breakpoint before it executes the instruction that begins at the given address; i.e., an instruction- address breakpoint exception is a fault.

The RF (restart flag) permits the debug handler to retry instructions that cause other kinds of faults in addition to debug faults. When it detects a fault, the processor automatically sets RF in the flags image that it pushes onto the stack. (It does not, however, set RF for traps and aborts.)

When RF is set, it causes any debug fault to be ignored during the next instruction. (Note, however, that RF does not cause breakpoint traps to be ignored, nor other kinds of faults.)

The processor automatically clears RF at the successful completion of every instruction except after the IRET instruction, after the POPF instruction, and after a JMP, CALL, or INT instruction that causes a task switch. These instructions set RF to the value specified by the memory image of the EFLAGS register.

The processor automatically sets RF in the EFLAGS image on the stack before entry into any fault handler. Upon entry into the fault handler for instruction address breakpoints, for example, RF is set in the EFLAGS image on the stack; therefore, the IRET instruction at the end of the handler will set RF in the EFLAGS register, and execution will resume at the breakpoint address without generating another breakpoint fault at the same address.

If, after a debug fault, RF is set and the debug handler retries the faulting instruction, it is possible that retrying the instruction will raise other faults. The retry of the instruction after these faults will also be done with RF=1, with the result that debug faults continue to be ignored. The processor clears RF only after successful completion of the instruction.

Real-mode debuggers can control the RF flag by using a 32-bit IRET. A 16-bit IRET instruction does not affect the RF bit (which is in the high-order 16 bits of EFLAGS). To use a 32-bit IRET, the debugger must rearrange the stack so that it holds appropriate values for the 32-bit EIP, CS, and EFLAGS (with RF set in the EFLAGS image). Then executing an IRET with an operand size prefix causes a 32-bit return, popping the RF flag into EFLAGS. Data Address Breakpoint

A data-address breakpoint exception is a trap; i.e., the processor reports a data-address breakpoint after executing the instruction that accesses the given memory item.

When using data breakpoints it is recommended that either the LE or GE bit of DR7 be set also. If either LE or GE is set, any data breakpoint trap is reported exactly after completion of the instruction that accessed the specified memory item. This exact reporting is accomplished by forcing the 80386 execution unit to wait for completion of data operand transfers before beginning execution of the next instruction. If neither GE nor LE is set, data breakpoints may not be reported until one instruction after the data is accessed or may not be reported at all. This is due to the fact that, normally, instruction execution is overlapped with memory transfers to such a degree that execution of the next instruction may begin before memory transfers for the prior instruction are completed.

If a debugger needs to preserve the contents of a write breakpoint location, it should save the original contents before setting a write breakpoint. Because data breakpoints are traps, a write into a breakpoint location will complete before the trap condition is reported. The handler can report the saved value after the breakpoint is triggered. The data in the debug registers can be used to address the new value stored by the instruction that triggered the breakpoint. General Detect Fault

This exception occurs when an attempt is made to use the debug registers at the same time that ICE-386 is using them. This additional protection feature is provided to guarantee that ICE-386 can have full control over the debug-register resources when required. ICE-386 uses the debug-registers; therefore, a software debugger that also uses these registers cannot run while ICE-386 is in use. The exception handler can detect this condition by examining the BD bit of DR6. Single-Step Trap

This debug condition occurs at the end of an instruction if the trap flag (TF) of the flags register held the value one at the beginning of that instruction. Note that the exception does not occur at the end of an instruction that sets TF. For example, if POPF is used to set TF, a single-step trap does not occur until after the instruction that follows POPF.

The processor clears the TF bit before invoking the handler. If TF=1 in the flags image of a TSS at the time of a task switch, the exception occurs after the first instruction is executed in the new task.

The single-step flag is normally not cleared by privilege changes inside a task. INT instructions, however, do clear TF. Therefore, software debuggers that single-step code must recognize and emulate INT n or INTO rather than executing them directly.

To maintain protection, system software should check the current execution privilege level after any single step interrupt to see whether single stepping should continue at the current privilege level.

The interrupt priorities in hardware 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 clears the TF bit. After saving the return address or switching tasks, the external interrupt input is examined before the first instruction of the single step handler executes. If the external interrupt is still pending, it is then serviced. The external interrupt handler is not single-stepped. To single step an interrupt handler, just single step an INT n instruction that refers to the interrupt handler. Task Switch Breakpoint

The debug exception also occurs after a switch to an 80386 task if the T-bit of the new TSS is set. The exception occurs after control has passed to the new task, but before the first instruction of that task is executed. The exception handler can detect this condition by examining the BT bit of the debug status register DR6.

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 cause the processor to enter an infinite loop.

12.3.2 Interrupt 3 -- Breakpoint Exception

This exception is caused by execution of the breakpoint instruction INT 3. Typically, a debugger prepares a breakpoint by substituting the opcode of the one-byte breakpoint instruction in place of the first opcode byte of the instruction to be trapped. When execution of the INT 3 instruction causes the exception handler to be invoked, the saved value of ES:EIP points to the byte following the INT 3 instruction.

With prior generations of processors, this feature is used extensively for trapping execution of specific instructions. With the 80386, the needs formerly filled by this feature are more conveniently solved via the debug registers and interrupt 1. However, the breakpoint exception is still useful for debugging debuggers, because the breakpoint exception can vector to a different exception handler than that used by the debugger. The breakpoint exception can also be useful when it is necessary to set a greater number of breakpoints than permitted by the debug registers.

up: Chapter 12 -- Debugging
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next: Chapter 13 -- Executing 80286 Protected-Mode Code