Создание и работа с DLL

library CPUUsage;

uses Windows;

const
    SystemBasicInformation = 0;
    SystemPerformanceInformation = 2;
    SystemTimeInformation = 3;

type
    TPDWord = ^DWORD;

    TSystem_Basic_Information = packed record
        dwUnknown1: DWORD;
        uKeMaximumIncrement: ULONG;
        uPageSize: ULONG;
        uMmNumberOfPhysicalPages: ULONG;
        uMmLowestPhysicalPage: ULONG;
        uMmHighestPhysicalPage: ULONG;
        uAllocationGranularity: ULONG;
        pLowestUserAddress: Pointer;
        pMmHighestUserAddress: Pointer;
        uKeActiveProcessors: ULONG;
        bKeNumberProcessors: byte;
        bUnknown2: byte;
        wUnknown3: word;
    end;

    TSystem_Performance_Information = packed record
        liIdleTime: LARGE_INTEGER; {LARGE_INTEGER}
        dwSpare: array[0..75] of DWORD;
    end;

    TSystem_Time_Information = packed record
        liKeBootTime: LARGE_INTEGER;
        liKeSystemTime: LARGE_INTEGER;
        liExpTimeZoneBias: LARGE_INTEGER;
        uCurrentTimeZoneId: ULONG;
        dwReserved: DWORD;
    end;

var
    NtQuerySystemInformation: function(infoClass: DWORD;
                                       buffer: Pointer;
                                       bufSize: DWORD;
                                       returnSize: TPDword): DWORD; stdcall = nil;

    liOldIdleTime: LARGE_INTEGER = ();
    liOldSystemTime: LARGE_INTEGER = ();

function Li2Double(x: LARGE_INTEGER): Double;
begin
    Result := x.HighPart * 4.294967296E9 + x.LowPart
end;

function GetCPUUsage: Double;
var
    SysBaseInfo: TSystem_Basic_Information;
    SysPerfInfo: TSystem_Performance_Information;
    SysTimeInfo: TSystem_Time_Information;
    status: Longint; {long}
    dbSystemTime: Double;
    dbIdleTime: Double;

    bLoopAborted: boolean;

begin
    if @NtQuerySystemInformation = nil then
        NtQuerySystemInformation := GetProcAddress(GetModuleHandle('ntdll.dll'), 'NtQuerySystemInformation');

    // get number of processors in the system
    status := NtQuerySystemInformation(SystemBasicInformation, @SysBaseInfo, SizeOf(SysBaseInfo), nil);
   
    if status <> 0 then
    begin
        Result:=-1;
        Exit;
    end;
   
    bLoopAborted := False;
   
    while not bLoopAborted do
    begin
        // get new system time
        status := NtQuerySystemInformation(SystemTimeInformation, @SysTimeInfo,    SizeOf(SysTimeInfo), 0);
        if status <> 0 then
        begin
            Result:=-1;
            Exit;
        end;

        // get new CPU's idle time
        status := NtQuerySystemInformation(SystemPerformanceInformation, @SysPerfInfo, SizeOf(SysPerfInfo), nil);
        if status <> 0 then
        begin
            Result:=-1;
            Exit;
        end;

        // if it's a first call - skip it
        if (liOldIdleTime.QuadPart <> 0) then
        begin
            // CurrentValue = NewValue - OldValue
            dbIdleTime := Li2Double(SysPerfInfo.liIdleTime) - Li2Double(liOldIdleTime);
            dbSystemTime := Li2Double(SysTimeInfo.liKeSystemTime) - Li2Double(liOldSystemTime);

            // CurrentCpuIdle = IdleTime / SystemTime
            dbIdleTime := dbIdleTime / dbSystemTime;

            // CurrentCpuUsage% = 100 - (CurrentCpuIdle * 100) / NumberOfProcessors
            dbIdleTime := 100.0 - dbIdleTime * 100.0 / SysBaseInfo.bKeNumberProcessors + 0.5;

            if dbIdleTime > 0 then
            begin
                bLoopAborted:= True;
            end;
        end;
        liOldIdleTime := SysPerfInfo.liIdleTime;
        liOldSystemTime := SysTimeInfo.liKeSystemTime;
    end;
    Result:=dbIdleTime;
end;

exports
    GetCPUUsage;
   
end.

if @NtQuerySystemInformation = nil then
  begin
    Result := -1;
    Exit;
  end;