[Vm-dev] 64bit FFI?

Bert Freudenberg bert at freudenbergs.de
Thu Feb 12 19:58:25 UTC 2009

On 12.02.2009, at 20:40, Eliot Miranda wrote:
>> 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
> 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.

It may not count for you but that's how we have been calling it. Since  
nobody expects 64 bit images to work "64-bit" VM refers to the host  
architecture. I'm not quite sure how we could phase out that  

> 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.

The point of this was not to be used in production but to *create* VM  
for running a 64 bit image without having to use a 64 bit host.

> 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 :)

Only if you want to use 64 bit images. Which nobody wants. Yet.

- Bert -

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