If you've ever been required to use Native API functions such as

**NtQuerySystemInformation/ZwQuerySystemInformation, NtSetSystemInformation/ZwSetSystemInformation, NtQueryInformationProcess/ZwQueryInformationProcess or NtSetInformationProcess/ZwSetInformationProcess**then you may have found yourself trying to find the correct class to pass in as a parameter for the function/s. The classes are part of an undocumented enum type definition and Microsoft tend to keep very quiet about these sorts of functions (and their internals).

**SYSTEM_INFORMATION_CLASS**

Code:

```
typedef enum _SYSTEM_INFORMATION_CLASS {
SystemBasicInformation = 0,
SystemProcessorInformation = 1,
SystemPerformanceInformation = 2,
SystemTimeOfDayInformation = 3,
SystemPathInformation = 4,
SystemProcessInformation = 5,
SystemCallCountInformation = 6,
SystemDeviceInformation = 7,
SystemProcessorPerformanceInformation = 8,
SystemFlagsInformation = 9,
SystemCallTimeInformation = 10,
SystemModuleInformation = 11,
SystemLocksInformation = 12,
SystemStackTraceInformation = 13,
SystemPagedPoolInformation = 14,
SystemNonPagedPoolInformation = 15,
SystemHandleInformation = 16,
SystemObjectInformation = 17,
SystemPageFileInformation = 18,
SystemVdmInstemulInformation = 19,
SystemVdmBopInformation = 20,
SystemFileCacheInformation = 21,
SystemPoolTagInformation = 22,
SystemInterruptInformation = 23,
SystemDpcBehaviorInformation = 24,
SystemFullMemoryInformation = 25,
SystemLoadGdiDriverInformation = 26,
SystemUnloadGdiDriverInformation = 27,
SystemTimeAdjustmentInformation = 28,
SystemSummaryMemoryInformation = 29,
SystemMirrorMemoryInformation = 30,
SystemPerformanceTraceInformation = 31,
SystemObsolete0 = 32,
SystemExceptionInformation = 33,
SystemCrashDumpStateInformation = 34,
SystemKernelDebuggerInformation = 35,
SystemContextSwitchInformation = 36,
SystemRegistryQuotaInformation = 37,
SystemExtendedServiceTableInformation = 38,
SystemPrioritySeparation = 39,
SystemVerifierAddDriverInformation = 40,
SystemVerifierRemoveDriverInformation = 41,
SystemProcessorIdleInformation = 42,
SystemLegacyDriverInformation = 43,
SystemCurrentTimeZoneInformation = 44,
SystemLookasideInformation = 45,
SystemTimeSlipNotification = 46,
SystemSessionCreate = 47,
SystemSessionDetach = 48,
SystemSessionInformation = 49,
SystemRangeStartInformation = 50,
SystemVerifierInformation = 51,
SystemVerifierThunkExtend = 52,
SystemSessionProcessInformation = 53,
SystemLoadGdiDriverInSystemSpace = 54,
SystemNumaProcessorMap = 55,
SystemPrefetcherInformation = 56,
SystemExtendedProcessInformation = 57,
SystemRecommendedSharedDataAlignment = 58,
SystemComPlusPackage = 59,
SystemNumaAvailableMemory = 60,
SystemProcessorPowerInformation = 61,
SystemEmulationBasicInformation = 62,
SystemEmulationProcessorInformation = 63,
SystemExtendedHandleInformation = 64,
SystemLostDelayedWriteInformation = 65,
SystemBigPoolInformation = 66,
SystemSessionPoolTagInformation = 67,
SystemSessionMappedViewInformation = 68,
SystemHotpatchInformation = 69,
SystemObjectSecurityMode = 70,
SystemWatchdogTimerHandler = 71,
SystemWatchdogTimerInformation = 72,
SystemLogicalProcessorInformation = 73,
SystemWow64SharedInformationObsolete = 74,
SystemRegisterFirmwareTableInformationHandler = 75,
SystemFirmwareTableInformation = 76,
SystemModuleInformationEx = 77,
SystemVerifierTriageInformation = 78,
SystemSuperfetchInformation = 79,
SystemMemoryListInformation = 80,
SystemFileCacheInformationEx = 81,
SystemThreadPriorityClientIdInformation = 82,
SystemProcessorIdleCycleTimeInformation = 83,
SystemVerifierCancellationInformation = 84,
SystemProcessorPowerInformationEx = 85,
SystemRefTraceInformation = 86,
SystemSpecialPoolInformation = 87,
SystemProcessIdInformation = 88,
SystemErrorPortInformation = 89,
SystemBootEnvironmentInformation = 90,
SystemHypervisorInformation = 91,
SystemVerifierInformationEx = 92,
SystemTimeZoneInformation = 93,
SystemImageFileExecutionOptionsInformation = 94,
SystemCoverageInformation = 95,
SystemPrefetchPatchInformation = 96,
SystemVerifierFaultsInformation = 97,
SystemSystemPartitionInformation = 98,
SystemSystemDiskInformation = 99,
SystemProcessorPerformanceDistribution = 100,
SystemNumaProximityNodeInformation = 101,
SystemDynamicTimeZoneInformation = 102,
SystemCodeIntegrityInformation = 103,
SystemProcessorMicrocodeUpdateInformation = 104,
SystemProcessorBrandString = 105,
SystemVirtualAddressInformation = 106,
SystemLogicalProcessorAndGroupInformation = 107,
SystemProcessorCycleTimeInformation = 108,
SystemStoreInformation = 109,
SystemRegistryAppendString = 110,
SystemAitSamplingValue = 111,
SystemVhdBootInformation = 112,
SystemCpuQuotaInformation = 113,
SystemNativeBasicInformation = 114,
SystemErrorPortTimeouts = 115,
SystemLowPriorityIoInformation = 116,
SystemBootEntropyInformation = 117,
SystemVerifierCountersInformation = 118,
SystemPagedPoolInformationEx = 119,
SystemSystemPtesInformationEx = 120,
SystemNodeDistanceInformation = 121,
SystemAcpiAuditInformation = 122,
SystemBasicPerformanceInformation = 123,
SystemQueryPerformanceCounterInformation = 124,
SystemSessionBigPoolInformation = 125,
SystemBootGraphicsInformation = 126,
SystemScrubPhysicalMemoryInformation = 127,
SystemBadPageInformation = 128,
SystemProcessorProfileControlArea = 129,
SystemCombinePhysicalMemoryInformation = 130,
SystemEntropyInterruptTimingInformation = 131,
SystemConsoleInformation = 132,
SystemPlatformBinaryInformation = 133,
SystemThrottleNotificationInformation = 134,
SystemPolicyInformation = 134,
SystemHypervisorProcessorCountInformation = 135,
SystemDeviceDataInformation = 136,
SystemDeviceDataEnumerationInformation = 137,
SystemMemoryTopologyInformation = 138,
SystemMemoryChannelInformation = 139,
SystemBootLogoInformation = 140,
SystemProcessorPerformanceInformationEx = 141,
SystemSpare0 = 142,
SystemSecureBootPolicyInformation = 143,
SystemPageFileInformationEx = 144,
SystemSecureBootInformation = 145,
SystemEntropyInterruptTimingRawInformation = 146,
SystemPortableWorkspaceEfiLauncherInformation = 147,
SystemFullProcessInformation = 148,
SystemKernelDebuggerInformationEx = 149,
SystemBootMetadataInformation = 150,
SystemSoftRebootInformation = 151,
SystemElamCertificateInformation = 152,
SystemOfflineDumpConfigInformation = 153,
SystemProcessorFeaturesInformation = 154,
SystemRegistryReconciliationInformation = 155,
SystemEdidInformation = 156,
SystemManufacturingInformation = 157,
SystemEnergyEstimationConfigInformation = 158,
SystemHypervisorDetailInformation = 159,
SystemProcessorCycleStatsInformation = 160,
SystemVmGenerationCountInformation = 161,
SystemTrustedPlatformModuleInformation = 162,
SystemKernelDebuggerFlags = 163,
SystemCodeIntegrityPolicyInformation = 164,
SystemIsolatedUserModeInformation = 165,
SystemHardwareSecurityTestInterfaceResultsInformation = 166,
SystemSingleModuleInformation = 167,
SystemAllowedCpuSetsInformation = 168,
SystemDmaProtectionInformation = 169,
SystemInterruptCpuSetsInformation = 170,
SystemSecureBootPolicyFullInformation = 171,
SystemCodeIntegrityPolicyFullInformation = 172,
SystemAffinitizedInterruptProcessorInformation = 173,
SystemRootSiloInformation = 174,
SystemCpuSetInformation = 175,
SystemCpuSetTagInformation = 176,
MaxSystemInfoClass = 177,
} SYSTEM_INFORMATION_CLASS;
```

**PROCESSINFOCLASS**

Code:

```
typedef enum _PROCESSINFOCLASS {
ProcessBasicInformation = 0,
ProcessQuotaLimits = 1,
ProcessIoCounters = 2,
ProcessVmCounters = 3,
ProcessTimes = 4,
ProcessBasePriority = 5,
ProcessRaisePriority = 6,
ProcessDebugPort = 7,
ProcessExceptionPort = 8,
ProcessAccessToken = 9,
ProcessLdrInformation = 10,
ProcessLdtSize = 11,
ProcessDefaultHardErrorMode = 12,
ProcessIoPortHandlers = 13,
ProcessPooledUsageAndLimits = 14,
ProcessWorkingSetWatch = 15,
ProcessUserModeIOPL = 16,
ProcessEnableAlignmentFaultFixup = 17,
ProcessPriorityClass = 18,
ProcessWx86Information = 19,
ProcessHandleCount = 20,
ProcessAffinityMask = 21,
ProcessPriorityBoost = 22,
ProcessDeviceMap = 23,
ProcessSessionInformation = 24,
ProcessForegroundInformation = 25,
ProcessWow64Information = 26,
ProcessImageFileName = 27,
ProcessLUIDDeviceMapsEnabled = 28,
ProcessBreakOnTermination = 29,
ProcessDebugObjectHandle = 30,
ProcessDebugFlags = 31,
ProcessHandleTracing = 32,
ProcessIoPriority = 33,
ProcessExecuteFlags = 34,
ProcessTlsInformation = 35,
ProcessCookie = 36,
ProcessImageInformation = 37,
ProcessCycleTime = 38,
ProcessPagePriority = 39,
ProcessInstrumentationCallback = 40,
ProcessThreadStackAllocation = 41,
ProcessWorkingSetWatchEx = 42,
ProcessImageFileNameWin32 = 43,
ProcessImageFileMapping = 44,
ProcessAffinityUpdateMode = 45,
ProcessMemoryAllocationMode = 46,
ProcessGroupInformation = 47,
ProcessTokenVirtualizationEnabled = 48,
ProcessConsoleHostProcess = 49,
ProcessWindowInformation = 50,
MaxProcessInfoClass // always last one so no need to add a value manually
} PROCESSINFOCLASS;
```

I have posted a "tutorial" here which demonstrates how to call NtQuerySystemInformation to enumerate the running processes on the system and extract the Process ID based on the Process Name passed in as a parameter to the function. You can find it here: Tutorial - NtQuerySystemInformation and Process IDs (it makes use of the SYSTEM_INFORMATION_CLASS for SystemProcessInformation which has a class ID of 5).

For the record, when using functions exported by NTDLL (NTDLL.DLL), it will not matter whether you use the Nt* or Zw* function stub. All the Nt*/Zw* function prologues are identical because they are designed to do the exact same thing: move the identifier for the function being called into the EAX register and then perform a system call (however on older versions of Windows there may be other functions involved like KiFastSystemCall, and on 64-bit environments any 32-bit processes will be required to go through emulation (WOW64 - Windows On Windows 64) which will end in the 64-bit NTDLL being used internally).

If you think you are missing an enum value or need one for a new version of Windows, you can try using WinDbg to extract them yourselves. A faster alternate would be trying to search if someone else has already found them (e.g. ReactOS can be useful for finding the class IDs for older versions of Windows).

Thank you for reading.