KAFFINITY_EX

The KAFFINITY_EX structure is used by the kernel to represent an arbitrary boolean property per processor. It is essentially a dressing-up of a bitmap that allows one bit per processor. Outside the kernel, the structure is opaque. The kernel exports functions that support a range of operations such as setting, clearing and counting bits. All date from version 6.1 unless otherwise noted:

KeAddGroupAffinityEx
KeAddProcessorAffinityEx
KeAndAffinityEx
KeAndGroupAffinityEx
KeCheckProcessorAffinityEx
KeComplementAffinityEx
KeCopyAffinityEx
KeCountSetBitsAffinityEx
KeFindFirstSetLeftAffinityEx
KeFindFirstSetRightAffinityEx (6.3 and higher)
KeFirstGroupAffinityEx
KeInitializeAffinityEx
KeInterlockedClearProcessorAffinityEx
KeInterlockedSetProcessorAffinityEx
KeIsEmptyAffinityEx
KeIsEqualAffinityEx
KeIsSingleGroupAffinityEx
KeIsSubsetAffinityEx
KeOrAffinityEx
KeQueryGroupAffinityEx
KeRemoveGroupAffinityEx
KeRemoveProcessorAffinityEx
KeSubtractAffinityEx

Documentation Status

The KAFFINITY_EX structure is not documented. Neither are the functions, though they are declared in the NTOSP.H from the Enterprise edition of the Windows Driver Kit (WDK) for Windows 10 Version 1511.

Layout

The KAFFINITY_EX expands as successive Windows versions allow more processor groups:

Version	Size (x86)	Size (x64)
6.1	0x0C	0x28
6.2 to 10.0	0x0C	0xA8

Sizes, offsets, types and names in the table that follows are from Microsoft’s symbol files for the kernel.

Offset	Definition
0x00	USHORT Count;
0x02	USHORT Size;
0x04	ULONG Reserved;
0x08	KAFFINITY Bitmap [MAX_PROC_GROUPS];

Here, a macro, MAX_PROC_GROUPS, is presumed for the maximum number of processor groups:

1 for 32-bit Windows;
4 for 64-bit Windows in version 6.1;
20 for 64-bit Windows in version 6.2 and higher.

The Count and Size count processor groups. They are initially 1 and MAX_PROC_GROUPS. The Count grows, up to the Size, as bits get set further into the bitmap.