INFO: Calculating Free System Resources in Microsoft Windows (61803)

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Windows uses the same method to calculate the percentage of free system resources in each of its operating modes (real mode, standard mode, and enhanced mode under Windows 3.0, and standard mode and enhanced mode under Windows 3.1).

In the Windows environment, system resources are the amount of space available in the Data Segment (DS) for the user Dynamic-Link Library (DLL) or the GDI DLL. Because system resources are exhausted if either of these data segments fills up, Windows reports the smaller of the two percentages for the system. The following formula describes the calculation for Windows 3.0 and 3.1; this may or may not be accurate in future versions of Windows:

   % free = min (% free of user DS, % free of GDI DS)

               (64K - current DS size) + free blocks in Heap

   % free DS = --------------------------------------------- * 100

                     64K - (size of statics and stack)
				

Or, in other words:

                 Current free heap assuming 64K

   % free DS =  -------------------------------- * 100

                   Maximum heap assuming 64K
				

Under Windows 3.1, an application can call the GetFreeSystemResouces function with the GFSR_SYSTEMRESOURCES value to obtain the percentage of free system resources. Because this function is not present in Windows 3.0, an application that runs in both environments must explicitly import this function from the user Dynamic-Link Library (DLL) when the application runs under Windows 3.1.

Under Windows 3.0, an application can use the GetHeapSpaces function to determine the percentage of free system resources as follows:

   DWORD cbFree;
   WORD  wFreeK, wHeapK;
   WORD  wUserPercent, wGDIPercent, wSysPercent;

   cbFree = GetHeapSpaces(GetModuleHandle("USER"));
   wFreeK = LOWORD(cbFree) / 1024;
   wHeapK = HIWORD(cbFree) / 1024;
   wUserPercent = MulDiv(wFreeK, 100, wHeapK);

   cbFree = GetHeapSpaces(GetModuleHandle("GDI"));
   wFreeK = LOWORD(cbFree) / 1024;
   wHeapK = HIWORD(cbFree) / 1024;
   wGDIPercent = MulDiv(wFreeK, 100, wHeapK);

   wSysPercent = min(wUserPercent, wGDIPercent);
				

Because this technique does not provide accurate results under Windows 3.1, an application must determine the version of Windows with which it is running and perform the calculation accordingly.