Calls to any API or DDI that takes an affinity mask or processor number as an argument, but not a group number, is limited to affecting or reporting on those processors in the calling thread's group. By default, threads of the process can run on all logical processors of the same group, although the thread affinity can be explicitly changed. When a process is created, the process is assigned to a specific group.
Any computer with more than 64 logical processors will necessarily have more than one group. Typically, a group's processors are represented by an affinity mask, which is 64 bits long. Note The concept of processor groups, introduced with Windows 7, allows existing APIs and DDIs to continue to work on computers with more than 64 logical processors. This means you can test drivers and components for multiple processor group compatibility on computers that have 64 or fewer logical processors. For testing purposes, you can configure any computer that has multiple logical processors to have multiple processor groups by limiting the group size. This support is made possible by introducing Processor groups. Windows 7 and Windows Server 2008 R2 provide support for computers with more than 64 processors.
Boot Parameters to Test Drivers for Multiple Processor Group Support
