Eric Auer schrieb:
> Hi,
> 
>> I always wanted to know this, couldn't find it on google.
>> Is it possible to switch from long mode back to let's say real mode?
> 
> Probably yes, but very slow. Until people are able to have
> more than 64 GB RAM in their PC, using PAE is a lot faster
> and easier even though it is not "the technology of the
> future".
> 
>>> which apps would use it and for what?
> 
>> The ram dumper! Or maybe a ram tester?
> 
> Ramdumper makes sense, maybe even switching to/from long
> mode every few MB would be bearable there. Ramtesters do
> already work best without OS, see memtest86+ and memtest86.
> 
>> Future applications? :D
> 
> Which?

Who know what the future brings...

>> Don't we have more phantasm? Otherwise poor performance. :p
> 
> Do you know any DOS app which is RAM bound and forces
> you to have more than, say, 500 MB RAM?

Not yet.

> Are you sure you
> should not write a multi core CPU DOS extender first? ;-)

Probable a great idea. =)

>> Don't you think it's theoretically possible to modify an
>> existing DOS extender to
>> - save the state
>> - switch to long mode
>> - do something
> 
> While DOS and BIOS are frozen, IRQ cannot be handled.
> 
>> - return to state
>> - do something in v86
> 
> You probably have to switch all down to real mode and
> it adds complexity to resurrect the v86 of your EMM386
> or "normal DOS extender mode" from that point again.
> 
>> - loop
>> while staying compatible with all that stuff?
> 
> Possible, yes. Useful, probably no.
> 
>> For example DOS_extender in wikipedia.
>>
>> It says it switches all time from v86 OR realmode
>> (means not limited to v86) to protected mode and back.
> 
> Because v86 TASKS and protected mode TASKS can
> exist simultaneously in a 32 bit multitasking
> environment. The DOS extender does exactly that.
> 
>> Why a switch to long mode shouldn't work?
> 
> Because v86 cannot coexist with long mode. You
> cannot simply switch tasks, you have to switch
> the mode of the whole CPU, much much slower and
> much more complex. Even more complex than the
> annoying way 286 extenders had to follow to
> "reboot" the CPU into real mode when they had
> to leave protected mode, I would say.
> 
>> Well, for a forensic tool (a ram dumper) a full blown 64 bit
>> os is like "operating success, patient dead". Not much sense.
> 
> That is true.


> Otoh, how do you avoid your BIOS destroying
> RAM contents already while you boot the forensic tool?

BIOS doesn't destroy to much, might be also a good idea to stop the ram
test.

> The to-be-analyzed software could put the to-be-hidden
> data into the first 1 MB deliberately, as that is very
> often checked and wiped at boot to ensure stable boot.

Unfortunately it's working very good.

>>>> There are some but dd as command line tool is nicer in some cases.
> 
>> Mostly graphical tools and on their homepages I don't
>> find the term "raw image" or something similar, mostly
>> proprietary formats.
> 
> How about Linux ghost clones, g4l or so? I also meant
> that Heise had a DOS tool for disk imaging which was
> almost certainly text mode or at most DEFRAG style text
> mode GUI as far as I can remember...

I need to check it out but I preferred to stick to DOS, simpler and less
black magic for me.

> 
> Eric
> 
> 
> 
> 
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