size of pointers and architecture

Question

By conducting a basic test by running a simple C++ program on a normal desktop PC it seems plausible to suppose that sizes of pointers of any type (including pointers to fun

Answer 1

It's not correct, for example DOS pointers (16 bit) can be far (seg+ofs).

However, for the usual targets (Windows, OSX, Linux, Android, iOS) then it's correct. Because they all use the flat programming model which relies on paging.

In theory, you can also have systems which uses only the lower 32 bits when in x64. An example is a Windows executable linked without LARGEADDRESSAWARE. However this is to help the programmer avoid bugs when switching to x64. The pointers are truncated to 32 bits, but they are still 64 bit.

In x64 operating systems then this assumption is always true, because the flat mode is the only valid one. Long mode in CPU forces GDT entries to be 64 bit flat.

One also mentions a x32 ABI, I believe it is based on the same paging technology, forcing all pointers to be mapped to the lower 4gb. However this must be based to the same theory as in Windows. In x64 you can only have flat mode.

In 32 bit protected mode you could have pointers up to 48 bits. (Segmented mode). You can also have callgates. But, no operating system uses that mode.