Explanation of System Resources in Windows 95

• Microsoft Windows 95

SUMMARY

The use of system resources in Windows 95 is improved over that in Windows 3.x and Windows for Workgroups 3.x. The 32-bit architecture of Windows 95 makes this improvement possible.

MORE INFORMATION

NOTE: In this article, the term "Windows" refers to Windows 3.x and Windows for Workgroups 3.x.

In Windows 95, large portions of the graphics device interface (GDI) and USER heaps reside in the shared 32-bit virtual flat-address space of the system. This address space is shared by all cached objects in Windows 95 (for example, the disk I/O cache, the network cache, the GDI cache, and third-party shared application data). This region of memory is as large as your physical memory plus your swap file.

Windows 95 incorporates the Windows 64-kilobyte (K) system-resource limit for better performance when it is providing backward compatibility. Items that remain in one or more 64K segment(s) are mostly for use in the GDI heap and include logical pens, logical brushes, logical fonts, bitmaps, and palettes. To improve the use of system resources, Windows 95 moves the following items into the 32-bit shared address space: device contexts, physical pens, physical brushes, and so on. Another improvement in Windows 95 is that the limit for device contexts is now over 4,000 system wide. In contrast, device contexts are limited to between 150 and 200 system wide in Windows.

The USER heap in Windows has a limit of 200 menu and window handles (combined) for the entire system. Some programs tend to allocate many of these handles when they are performing large tasks, which causes the system to run out of resources quickly. To avoid such problems, Windows 95 moves the data for these menu and window handles to the 32-bit shared address space, thereby raising the total limit to 32,767 each per process (compared to the system-wide limit of 200 in Windows). In addition, the USER heap data for list boxes now resides in the 32-bit shared address space. The USER heap in Windows 95 also provides a new 32-bit edit control that imposes no limits on the size of the data that you can store in it.

The following table is only approximate because it is difficult to list exact limits for GDI objects in Windows 3.1 because all regions, physical objects, logical objects, DCs, and installed fonts have to fit in a single 64K segment. Moving these into the 32-bit heap in Windows 95 provides more room for the remaining (small) items such as logical pens and brushes. The remaining items in the Windows 95 local heap are all less than 10-20 bytes each.

   Resource                Windows              Windows 95    Windows NT
   ---------------------------------------------------------------------
   Window/Menu Handles     about 200            32K (each)    Unlimited
   Timers                  32                   Unlimited     Unlimited
   COM/LPT ports           4 each               Unlimited     Unlimited
   Listbox items           8K                   32K           Unlimited
    (per listbox)
   Listbox data            64K                  Unlimited     Unlimited
    (per listbox)
   Edit control data       64K                  Unlimited     Unlimited
    (per control)
   Regions                 All in 64K segment   Unlimited     Unlimited
   Physical pens, brushes  All in 64K segment   Unlimited     Unlimited
   Logical fonts           All in 64K segment   750-800       Unlimited
   Installed fonts         250-300 (best case)  1000          Unlimited
   Device Contexts         200 (best case)      16K           Unlimited
   Logical pens, brushes   All in 64K segment   64K segment   Unlimited