Moves the contents of a debug register (DR0, DR1, DR2, DR3, DR4, DR5, DR6, or DR7) to a general-purpose register or vice versa. The operand size for these instructions is always 32 bits, regardless of the operand-size attribute. (See Chapter 15, Debugging and Performance Monitoring, of the IA-32 Intel Architecture Software Developer's Manual, Volume 3, for a detailed description of the flags and fields in the debug registers.) The instructions must be executed at privilege level 0 or in real-address mode.
When the debug extension (DE) flag in register CR4 is clear, these instructions operate on debug registers in a manner that is compatible with Intel386 and Intel486 processors. In this mode, references to DR4 and DR5 refer to DR6 and DR7, respectively. When the DE flag in CR4 is set, attempts to reference DR4 and DR5 result in an undefined opcode (#UD) exception. (The CR4 register was added to the IA-32 Architecture beginning with the Pentium processor.) At the opcode level, the reg field within the ModR/M byte specifies which of the debug registers is loaded or read. The two bits in the mod field are always 11. The r/m field specifies the generalpurpose register loaded or read.