On Thu, Feb 12, 2009 at 11:25 AM, Bert Freudenberg
<bert@freudenbergs.de> wrote:
On 12.02.2009, at 19:29, Eliot Miranda wrote:
Um, I have (studied my Agrippa), and I quote:
Does my 64-bit VM run both 32-bit and 64-bit images? No. Any VM will run either 32-bit or 64-bit images
Read it like this:
Does my 64-bit VM run *both* 32-bit and 64-bit images? No. Any VM will run *either* 32-bit *or* 64-bit images
Apologies for being so persistent :)
What is a 64-bit image?
A 64-bit image is an image in which the object memory uses a 64-bit word size for object pointers
What is a 64-bit VM?
A 64-bit VM is one which is compiled with the LP64 or ILP64 data model. This means, in C terms, that pointers and longs are 64-bits wide.
These two are independent. There are four kinds of VMs:
1) a 32-bit VM compiled to run 32-bit images
2) a 32-bit VM compiled to run 64-bit images
3) a 64-bit VM compiled to run 32-bit images
4) a 64-bit VM compiled to run 64-bit images
Wether the VM can run 32 or 64 bit *images* is governed by the checkbox in VMMaker.
Wether the VM can run on a 32 or 64 bit host is governed by the C compiler.
Also, nobody uses 64 bit images yet.
Please suggest how the FAQ could make this more clear (assuming you actually studied the whole thing and not just skimmed it).
Speaking through a self-imposed gag let me suggest the FAQ distinguish clearly between a 64-bit VM (meaning a VM that runs 64-bit images that necessarly is compiled as a 64-bit executable) and a 32-bit VM compiled to run as a 64-bit executable. The latter doesn't count as a 64-bit VM for me. 64-bit clean perhaps.
Let me further suggest that 2) is pointless, in that it can't scale beyond the 32-bit address space but uses about 1.5 times the space of a 32-bit image as it tries to do so. i.e. it'll fail a lot earlier than a 32-bit image. Further, if a 64-bit image is larger than will fit in a 32-bit address space one *cant* run it on 2 because it won't fit. However, one can derive a 32-bit image from a 64-bit image that isn't too big using the SystemTracer (don't know if tis has been implemented; we only implemented 32-bit to 64-bit conversion in VsualWorks, uing something akin to the system tracer).
So I would suggest the FAQ explain tat there are really three different VMs, two 32-bit ones and one 64-bit one. One of the 32-bit VMs is compiled to run as a 64-bit executable on 64-bit systems but only runs a 32-bit image. That at least accords with the planet I grew up on :)
- Bert -