Open in 32-bit mode

Under MacOS, you can change a little option for 32-bit executables called "Open in 32-bit mode". Wouldn't it work directly? And it works, but for some applicatio开发者_StackOverflow社区ns you had to select this option in order to run without problems. This was frequent in Safari, where some add-ons required a 32-bit environment.

I can't understand what makes an 32-bit executable not able to run directly in 64-bit, so what exactly changes in 32-bit mode?

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This is really only of historical interest. In the transition period from 32 bit to 64 bit many apps were built as universal with 3 or sometimes even 4 architectures combined into one fat binary (aka "Universal Binary"), typically ppc, x86 and x86-64. In a 32 bit x86 environment the 32 bit x86 executable would be used. In a 64 bit x86-64 bit environment the 64 bit executable would be used. However in some cases you might want to use the 32 bit x86 executable even in a 64 bit x86-64 environment, e.g. in the case you mentioned where you have older plug-ins which are 32-bit only and can not be used with a 64 bit executable. Hence the option to launch an app in 32 bit mode.

Obviously a 32 bit app uses 32 bit APIs and has a 32 bit address space, whereas a 64 bit app has a 64 bit address space and uses 64 bit APIs.