Confused with 32/64 bit HW, OS and Binary, Listed few queries in description

Question

I am confused with all 32/64 bit architecture for Hardware/OS/Binary, Can someone please help in clarifying this queries?

1. Of'course 64 bit hardware is faster than 32 bit, still there are people who prefer 32 bit over 64 bit hardware, Why so?
2. If I have 32 bit Hardware then On that can I run 64 bit OS? (and vice versa)
3. If I have a 64 bit Hardware and 32 bit OS then what will be the speed of my OS, as per 32 bit or as per 64 bit? (and vice versa)
4. If I have 32 bit OS then can I run binary of 64 bit on that? (and vice versa)
5. Is it necessary to compile binary with 32/64 bit both or we can compile with any and use it in both?
6. Is the speed of the process also depends on type of binary that I have? For example, If I have 64 bit hardware and 64 bit OS but I have a binary which is compiled with 32 bit compiler then what will be the speed difference?

P.S. I know its basic and silly queries but lets clear now before it gets too late. :)

Answer 1

If you have a 32-bit computer, you have no choice but to install a 32-bit OS. On the opposite side you can run a 32bit OS on 64 bit machine and if you are running a 32bit os on a 64 bit machine, you are under utilizing your processor.

When running 32bit binaries on 64bit machines (assuming the OS is running in 64bit mode), you have the following limitations:
The binary cannot take advantage of 64bit arithmetic instructions. This might reduce performance.
The binary cannot address more than 4GB of virtual memory.
On the reverse of it i.e. running the 64bit binary on the 32bit OS (64 bit hardware) is a OS dependent as it does require a little more work inside the kernel, because it must manage 64-bit pointers to user space together with 32-bit pointers to kernel space. (I doubt anyone must be supporting it)

Ideal situation is to have a 64bit binary for the 64bit OS running on 64bit Hardware so that OS need to work less.

32but compiler can not address more than 4GB of virtual memory. So running a 32but binary on a 64bit OS may have an impact on the performance on case to case basis.

Coming to first question - really I dont know....