OpenBSD - i386 vs amd64 - RAM

[beiroot]

Hi,
I've a question I couldn't find the answer to googling.
When you have equal or more of 4GB RAM you need to install amd64?
I know it's true for the older versions of OpenBSD, but is it also true for 6.0?

What are the main reasons for this? I mean the technical answer - what are the architectural differences between those two architectures.

Thanks!

[e1-531g]

You don't need amd64, but your usable memory space would be limited to 4GB.
amd64 has 64-bit registers for addressing memory.
i386 has 32-bit registers for addressing memory.

2^32 [bytes] = 4294967296 [bytes] = 4 [GB]
2^64 [bytes] =(2^32)^2 [bytes] = 4294967296 * 4294967296 [bytes] = 16 Exabytes

There is something called PAE, but process address space remains expressed by 32-bit pointers, so you still have 4GB limit for every process in system.

[beiroot]

That's a great answer, thank you

But...
Is it 4GB or 3GB according to this thread (jggimi's posts)?
http://daemonforums.org/showthread.php?t=8892

AFAI understand:
- PAE (available since OpenBSD 5.0?) addresses virtual memory (per process) and not physical (overall memory) - do I understand this correctly?
- bigmem - what is it?

[jggimi]

Install and discover.

I have both i386 and amd64 systems; however none of the i386 systems have more than 512 MB of RAM.

[beiroot]

Now that's what I call japanese-style minimalistic

[jggimi]

They are Alix systems from PC Engines.