CreateBitmap() API - Bitmap Size Limits Introduced in Windows NT 4.0

• Microsoft Windows NT Workstation version 4.0
• Microsoft Windows NT Server version 4.0

SYMPTOMS

If you have an application that uses the CreateBitmap() application programming interface (API) to create bitmaps, you may find it impossible to create bitmaps greater than 48 MB (or less, on computers with limited RAM) when you run the application in Windows NT 4.0.

This limit is not apparent when the application is run in Windows NT 3.51.

CAUSE

This is a side effect of moving display drivers from user mode in Windows NT 3.51 to kernel mode in Windows NT 4.0.

In Windows NT 3.51, the CreateBitmap() API allocates the bitmap in user- mode paged memory, and it is therefore possible to create bitmaps up to the maximum size of the pagefile. Bitmap size is therefore limited only by the amount of disk space that you can allocate to your pagefile(s) in Windows NT 3.51.

In Windows NT 4.0, the CreateBitmap() API allocates the bitmap in kernel- mode paged memory. As Kernel-mode paged memory is a scarce resource, and exhausting it can adversely affect system performance, application usage of Kernel-mode paged memory is restricted to smaller allocations.

RESOLUTION

To circumvent this limit, the application must be recoded to create bitmaps by calling the CreateDIBSection() API instead of the CreateBitmap() API.

CreateDIBSection() API allocates the bitmap in user-mode paged memory, and bitmaps created using this API are therefore not affected by the limit imposed on kernel-mode paged memory allocations.

Keywords : ntgeneral NTSrvWkst
Version : WinNT:4.0
Platform : winnt
Issue type : kbprb