AMD-K6 Datasheet, PDF (238/346 Pages) Advanced Micro Devices – AMD-K6 Processor

AMD-K6^® Processor Data Sheet
Preliminary Information
20695H/0—March 1998
extensions are enabled (bit 3 of CR4 is set to 1), any attempt to
load DR5 or DR4 results in an undefined opcode exception.
Likewise, any attempt to store DR5 or DR4 also results in an
undefined opcode exception.
DR6. If a breakpoint is enabled in DR7, and the breakpoint
conditions as defined in DR7 occur, then the corresponding
B-bit (B3–B0) in DR6 is set to 1. In addition, any other
breakpoints defined using these particular breakpoint
conditions are reported by the processor by setting the
appropriate B-bits in DR6, regardless of whether these
breakpoints are enabled or disabled. However, if a breakpoint
is not enabled, a debug exception does not occur for that
breakpoint.
If the processor decodes an instruction that writes or reads
DR7 through DR0, the BD bit (bit 13) in DR6 is set to 1 (if
enabled in DR7) and the processor generates a debug
exception. This operation allows control to pass to the
debugger prior to debug register access by software.
If the Trap Flag (bit 8) of the EFLAGS register is set to 1, the
processor generates a debug exception after the successful
execution of every instruction (single-step operation) and sets
the BS bit (bit 14) in DR6 to indicate the source of the
exception.
When the processor switches to a new task and the debug trap
bit (T-bit) in the corresponding Task State Segment (TSS) is set
to 1, the processor sets the BT bit (bit 15) in DR6 and generates
a debug exception.
DR7. When set to 1, L3–L0 locally enable breakpoints 3 through
0, respectively. L3–L0 are set to 0 whenever the processor
executes a task switch. Setting L3–L0 to 0 disables the
breakpoints and ensures that these particular debug
exceptions are only generated for a specific task.
When set to 1, G3–G0 globally enable breakpoints 3 through 0,
respectively. Unlike L3–L0, G3–G0 are not set to 0 whenever
the processor executes a task switch. Not setting G3–G0 to 0
allows breakpoints to remain enabled across all tasks. If a
breakpoint is enabled globally but disabled locally, the global
enable overrides the local enable.
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Test and Debug
Chapter 11