conditions that are global to all tasks. DR4 and DR5 Not real registers. On processors that support the CR4.DE bit (Intel Pentium and later), their behaviour is controlled by CR4.DE: CR4.DE=0 : DR4 and DR5 alias to DR6 and DR7, respectively. CR4.DE=1 : accessing DR4/5 results in #UD (invalid opcode) exception. On processor without CR4.DE, the behaviour is officially undefined - usually, DR4/5 are aliased to DR6/7, but exceptions exist and have been used for CPU detection. See also Control register Test register References Intel 80386 Programmer's Reference External links http://www.codeproject.com/KB/debug/hardwarebreakpoint.aspx Using the debug registers in Windows. X86 instructions Operating system technology Digital registers Debugging