Question

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 applications 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?

Answer 1

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.