AMD-K6-2E/400AFR AMD [Advanced Micro Devices], AMD-K6-2E/400AFR Datasheet - Page 262

• Manufacturer Part Number: AMD-K6-2E/400AFR
• Manufacturer: AMD [Advanced Micro Devices]

AMD-K6™-2E Processor Data Sheet
244
enabled (bit 3 of CR4 is 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 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 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 cleared to 0 whenever the processor
executes a task switch. Clearing 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 cleared to 0
whenever the processor executes a task switch. Not clearing
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.
Preliminary Information
Test and Debug
22529B/0—January 2000
Chapter 12