Calculating Free System Resources in Microsoft Windows

The information in this article applies to:

• Microsoft Windows Software Development Kit (SDK) for Windows versions 3.1 and 3.0

SUMMARY

In the Microsoft Windows graphical environment, the About dialog box in the Program Manager displays a value that indicates the amount of free system resources available at a given time. The text below documents this calculation and the methods that an application can use to calculate this figure.

MORE INFORMATION

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)

                 Current free heap assuming 64K
   % free DS =  -------------------------------- * 100
                   Maximum heap assuming 64K

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);