Findings researched by Uwe Bonnes, Ulrich Weigand and Marcus Meissner.
(Extracted from wine/documentation/accelerators)
Some notes concerning accelerators.
There are three differently sized accelerator structures exposed to the user. The general layout is:
BYTE fVirt;
WORD key;
WORD cmd;
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We now have three different appearances:
Accelerators in NE resources. These have a size of 5 byte and do not have any padding. This is also the internal layout of the global handle HACCEL (16 and 32) in Windows 95 and WINE. Exposed to the user as Win16 global handles HACCEL16 and HACCEL32 by the Win16/Win32 API.
Accelerators in PE resources. These have a size of 8 byte. Layout is:
BYTE fVirt;
BYTE pad0;
WORD key;
WORD cmd;
WORD pad1;
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They are exposed to the user only by direct accessing PE resources.
Accelerators in the Win32 API. These have a size of 6 bytes. Layout is:
BYTE fVirt;
BYTE pad0;
WORD key;
WORD cmd;
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These are exposed to the user by the CopyAcceleratorTable and CreateAcceleratorTable functions in the Win32 API.
Why two types of accelerators in the Win32 API? We can only guess, but my best bet is that the Win32 resource compiler can/does not handle struct packing. Win32 ACCEL is defined using #pragma(2) for the compiler but without any packing for RC, so it will assume #pragma(4).