Intel ITANIUM ARCHITECTURE - SOFTWARE DEVELOPERS MANUAL VOLUME 1 REV 2.3 Manual page 1593

MOV—Move to/from Debug Registers (Continued)
#DB
Real Address Mode Exceptions
#UD
#DB
Virtual 8086 Mode Exceptions
#GP(0)
Volume 4: Base IA-32 Instruction Reference
If any debug register is accessed while the GD flag in debug register
DR7 is set.
If the DE (debug extensions) bit of CR4 is set and a MOV instruction
is executed involving DR4 or DR5.
If any debug register is accessed while the GD flag in debug register
DR7 is set.
The debug registers cannot be loaded or read when in virtual 8086
mode.
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