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When such a condition is detected the bd flag in

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Unformatted text preview: rimary function of the debug registers is to set up and monitor from 1 to 4 breakpoints, numbered 0 though 3. For each breakpoint, the following information can be specified and detected with the debug registers: • • • • • The linear address where the breakpoint is to occur. The length of the breakpoint location (1, 2, or 4 bytes). The operation that must be performed at the address for a debug exception to be generated. Whether the breakpoint is enabled. Whether the breakpoint condition was present when the debug exception was generated. The following paragraphs describe the functions of flags and fields in the debug registers. 15.2.1. Debug Address Registers (DR0-DR3) Each of the four debug-address registers (DR0 through DR3) holds the 32-bit linear address of a breakpoint (refer to Figure 15-1). Breakpoint comparisons are made before physical address translation occurs. Each breakpoint condition is specified further by the contents of debug register DR7. 15.2.2. Debug Registers DR4 and DR5 Debug registers DR4 and DR5 are reserved when debug extensions are enabled (when the DE flag in control register CR4 is set), and attempts to reference the DR4 and DR5 registers cause an invalid-opcode exception (#UD) to be generated. When debug extensions are not enabled (when the DE flag is clear), these registers are aliased to debug registers DR6 and DR7. 15.2.3. Debug Status Register (DR6) The debug status register (DR6) reports the debug conditions that were sampled at the time the last debug exception was generated (refer to Figure 15-1). Updates to this register only occur when an exception is generated. The flags in this register show the following information: B0 through B3 (breakpoint condition detected) flags (bits 0 through 3) Indicates (when set) that its associated breakpoint condition was met when a debug exception was generated. These flags are set if the condition described for each breakpoint by the LEN n, and R/Wn flags in debug control register DR7 is true. They are set even if the breakpoint is not enabled by the Ln and Gn flags in register DR7. BD (debug register access detected) flag (bit 13) Indicates that the next instruction in the instruction stream will access one of the debug registers (DR0 through DR7). This flag is enabled when the GD (general detect) flag in debug control register DR7 is set. Refer to Section 15.2.4., “Debug Control Register (DR7)” for further explanation of the purpose of this flag. 15-4 DEBUGGING AND PERFORMANCE MONITORING BS (single step) flag (bit 14) Indicates (when set) that the debug exception was triggered by the single-step execution mode (enabled with the TF flag in the EFLAGS register). The singlestep mode is the highest-priority debug exception. When the BS flag is set, any of the other debug status bits also may be set. BT (task switch) flag (bit 15) Indicates (when set) that the debug exception resulted from a task switch where the T flag (debug trap flag) in the TSS of the target task was set (refer to Section 6.2.1., “Task-State Segment (TSS)”, in Section 6, “Task Management”, for the format of a TSS). There is no flag in debug control register DR7 to enable or disable this exception; the T flag of the TSS is the only enabling flag. Note that the contents of the DR6 register are never cleared by the processor. To avoid any confusion in identifying debug exceptions, the debug handler should clear the register before returning to the interrupted program or task. 15.2.4. Debug Control Register (DR7) The debug control register (DR7) enables or disables breakpoints and sets breakpoint conditions (refer to Figure 15-1). The flags and fields in this register control the following things: L0 through L3 (local breakpoint enable) flags (bits 0, 2, 4, and 6) Enable (when set) the breakpoint condition for the associated breakpoint for the current task. When a breakpoint condition is detected and its associated Ln flag is set, a debug exception is generated. The processor automatically clears these flags on every task switch to avoid unwanted breakpoint conditions in the new task. G0 through G3 (global breakpoint enable) flags (bits 1, 3, 5, and 7) Enable (when set) the breakpoint condition for the associated breakpoint for all tasks. When a breakpoint condition is detected and its associated Gn flag is set, a debug exception is generated. The processor does not clear these flags on a task switch, allowing a breakpoint to be enabled for all tasks. LE and GE (local and global exact breakpoint enable) flags (bits 8 and 9) (Not supported in the P6 family processors.) When set, these flags cause the processor to detect the exact instruction that caused a data breakpoint condition. For backward and forward compatibility with other Intel Architecture processors, Intel recommends that the LE and GE flags be set to 1 if exact breakpoints are required. GD (general detect enable) flag (bit 13) Enables (when set) debug-register protection, which causes a debug exception to be generated prior to any MOV...
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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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