The Old New Thing

Don’t save anything you can recalculate

December 20, 2004 Dec 20, 2004 12/20/04

Raymond Chen

Nowadays, a major barrier to performance for many classes of programs is paging. We saw earlier this year that paging can kill a server. Today, another example of how performance became tied to paging. The principle is "Don't save anything you can recalculate." This of course, seems counterintuitive: Shouldn't you save the answer so you ...

How did Windows 95 rebase DLLs?

December 17, 2004 Dec 17, 2004 12/17/04

Raymond Chen

Windows 95 handled DLL-rebasing very differently from Windows NT. When Windows NT detects that a DLL needs to be loaded at an address different from its preferred load address, it maps the entire DLL as copy-on-write, fixes it up (causing all pages that contain fixups to be dumped into the page file), then restores the read-...

The hunt for a faster syscall trap

December 15, 2004 Dec 15, 2004 12/15/04

Raymond Chen

The performance of the syscall trap gets a lot of attention. I was reminded of a meeting that took place between Intel and Microsoft over fifteen years ago. (Sadly, I was not myself at this meeting, so the story is second-hand.) Since Microsoft is one of Intel's biggest customers, their representatives often visit Microsoft to show off ...

Why do dialog editors start assigning control IDs with 100?

December 14, 2004 Dec 14, 2004 12/14/04

Raymond Chen

When you use a dialog editor and insert new controls, they typically are assigned control IDs starting at around 100. Why? Because the small numbers are already taken. /* * Dialog Box Command IDs */ #define IDOK 1 #define IDCANCEL 2 #define IDABORT 3 #define IDRETRY 4 #define IDIGNORE...

Why did Windows 95 run the timer at 55ms?

December 2, 2004 Dec 2, 2004 12/2/04

Raymond Chen

The story behind the 55ms timer tick rate goes all the way back to the original IBM PC BIOS. The original IBM PC used a 1.19MHz crystal, and 65536 cycles at 1.19MHz equals approximately 55ms. (More accurately, it was more like 1.19318MHz and 54.92ms.) But that just pushes the question to another level. Why 1.19...MHz, then? With that ...

What is the purpose of the bmPlanes member of the BITMAP structure?

December 1, 2004 Dec 1, 2004 12/1/04

Raymond Chen

Many bitmap-related structures in Windows have a field called "planes". For example the structure has a member (which must be set to 1). The structure has a field called . What's the deal with that field? The EGA video adapter supported 16 simultaneous colors. This was an enormous improvement over the CGA, which supported only four ...

If a program and a folder have the same name, the shell prefers the program

November 18, 2004 Nov 18, 2004 11/18/04

Raymond Chen

If you have both a folder named, say, and a program named and you type into the Start.Run dialog, you get the program and not the folder. Why is that? Because it is common to have where there is a setup program in the root, as well as a setup folder containing files needed by the setup program. Before Windows 95, you couldn't...

A history of GlobalLock, part 4: A peek at the implementation

November 9, 2004 Nov 9, 2004 11/9/04

Raymond Chen

On one of our internal discussion mailing lists, someone posted the following question: We have some code that was using DragQueryFile to extract file paths. The prototype for DragQueryFile appears as follows: UINT DragQueryFile( HDROP hDrop, UINT iFile, LPTSTR lpszFile, UINT cch ); In the code we have, instead of ...

A history of GlobalLock, part 3: Transitioning to Win32

November 8, 2004 Nov 8, 2004 11/8/04

Raymond Chen

Now that you know how the 16-bit memory manager handled the global heap, it's time to see how this got transitioned to the new 32-bit world. The function continued to emulate all its previous moveability rules, but the return value of was no longer a selector since Win32 used the processor in "flat mode". This means that the old trick ...