virtualbox.org

This isn't really a vbox question (more of an OS question) - so a bit off topic.

The mem requirement for windows 7 32bit is 1GB (already a lot! even solaris is only 512MB with its ZFS file caching).
For windows 7 64bit the req is 2 GB. I realize the instructions are going to be twice as large, but I would have though that the 64bit instruction set has increased efficiency, so maybe somewhere between 1 and 2GB would have been my guess (between 1x and 2x). Maybe MS is just saying this is a requirement to be safe.

So my first question is - what are the memory hog aspects of the system in windows 7? And, why does the 64bit version require double the RAM?

Windows 7 RC states to have the 1 gig -2 gig requirement but this is a host requirement.
Cut it in half and you have what it take for a guest to survive, albeit a little slow.

And to answer you questions,
"So my first question is - what are the memory hog aspects of the system in windows 7? And, why does the 64bit version require double the RAM?"
Hey its Windows. That's the way they do it. There are more requirements for 64 bit but double? No. (IMHO)

Jondice wrote:I realize the instructions are going to be twice as large

64 bits is the size of a register, not an instruction (which in any case do not have a fixed length on Intel architectures). I have not looked, but I expect the instruction opcodes are virtually the same. It does mean however that whenever an application needs to store the contents of a register (whether a pointer or data register), then that storage needs to be twice as large as before. Efficient programming would require that all data fields be aligned on the processors natural word length, so even if the data doesn't need 64 bits it will often be given 64 bits. Also, it is easier for MS to simply set a compile flag and compile everything with a new assumption that "int" is now 64 bits... so yes, this could add up to a lot of extra space taken, even though no new functionality is added.