Bochs "disk images" / raw devices

bewing · Post by **bewing** » Mon Jun 09, 2008 1:39 pm

The versions of my kernel that run on bochs and on real hardware are somewhat different -- mostly having to do with the difference between the bochs disk image, and the real hard disk sizes. Sometimes, like now, I have an issue where the bochs version runs fine, and the version running on hardware has some bug. Since I still do not have a debugger running on the real hardware, this becomes a very tedious debugging process -- because I can't use bochs to track the bug down.

I had a momentary brainstorm, and said "Oh! I can attach the hard drive to the other PC, and tell bochs to use that hard drive as the disk image, and run bochs to debug the REAL hard drive!" -- And then I realized that it wouldn't work ... at least with a Windoze system, AFAIK.

The disk is not formatted with a filesystem that Windoze recognizes ... so Windoze will not assign a drive letter. So, AFAIK, bochs has no way of accessing the drive. Even if Windoze did assign a drive letter, I'm betting that it would not let bochs have full access the whole logical drive (FAT tables and all)?

And then I started wondering, if I had done all this development under Linux, could I have done it then? So, this is my question to all of you: do any of you know if bochs can access a disk image, under linux, that is a raw blockmode (or character) device? Or alternately, am I full of sh*t, and there IS a way to get bochs to access an unrecognized "physical drive" on a Windoze system?

Combuster · Post by **Combuster** » Mon Jun 09, 2008 2:14 pm

There is one way: Cygwin - it allows you to access /dev/sd(xx) as a file directly (!)

Which suggests that if you compile bochs from source under cygwin you can point it directly to a physical drive like you would under linux.

Enjoy your d'oh moment

bewing · Post by **bewing** » Mon Jun 09, 2008 2:32 pm

Heh.

I never even bothered looking at the /dev directory in cygwin ... I assumed there couldn't possibly be anything interesting in there.

Hmmm. Well, if I reinstall all 40K+ files of cygwin, at least it would give me a good reason for erasing and reusing my old Linux (which isn't working right anyway) disk for better purposes.

Brynet-Inc · Post by **Brynet-Inc** » Mon Jun 09, 2008 2:33 pm

Surprisingly, it may be possible to directly access devices under NT without using the Cygwin solution Combuster provided.

http://support.microsoft.com/kb/100027

No clue if it actually works or not... but the QEMU related page below claims it does.

http://www.h7.dion.ne.jp/~qemu-win/HowT ... alharddisk

But yes, it probably would have been easier on a Unix system, for one, most don't try to hide direct drive access from the user.

I haven't had the need, but under OpenBSD I could access a USB attached hard drive as /dev/rsd0c if I needed too.

AJ · Post by AJ » Tue Jun 10, 2008 1:58 am

It does work.

I used to use this method back in the days of Win95 to allow my programs to copy and back up floppy disks sector-by sector.

Cheers,
Adam

bewing · Post by **bewing** » Tue Jun 10, 2008 4:14 pm

Huh! I had no idea. Thanks Brynet! (PS. It doesn't say WinNT ... it says ALL Win32 versions.) I am now going to bet that is how Norton Diskedit does all the magic that I find so surprising. -- And, of course, one more gaping security hole in Windoze ....

I don't know how a *NIX could be any easier than that!

It's just the difference between being documented and undocumented.

Maybe now I can write a utility of my own to modify my disk images without having to exit to DOS ....

Brynet-Inc · Post by **Brynet-Inc** » Tue Jun 10, 2008 4:18 pm

bewing wrote:(PS. It doesn't say WinNT ... it says ALL Win32 versions.)

That was an assumption I made, in the QEMU document it claims that method isn't supported by Windows 98 and ME, so I assumed it was an NT exclusive thing.

My bad, still, gotta love Google.

inx · Post by **inx** » Wed Jun 11, 2008 12:08 am

If you use the \\.\<dev> method, it is supported by 98/ME. That's the Win32 subsystem's raw device referencing. The \Device\PhysicalDriveN method is the NT core referencing, so that only works on NT 3.1+ (Maybe OS/2? Who knows, I lost my OS/2 2.1 disks forever ago, but I guess it's not important.)

bewing · Post by **bewing** » Wed Jun 11, 2008 6:55 pm

Sadly, bochs doesn't seem to open the "file" correctly, to make the \\.\ thing work directly. It gives a "cannot open" error. Maybe I'll file an upgrade suggestion with the bochs guys. In the meantime, I'm thinking of using \\.\ as the basis of a dll, to make "raw disks" available to me and bochs.

Ooops. Damn. Just checked the bochs documentation, and their dll access method isn't going to help me either. So I guess I've gotta run with Combuster's cygwin suggestion!

... Either that ... or modify the bochs source, maybe .... Hmmmm .....

bewing · Post by **bewing** » Wed Jun 11, 2008 10:59 pm

Crap. The cygwin method that Combuster suggested only works on WinNT or XP (maybe Vista -- but access to physical drives on vista has been curtailed). Cygwin will return a -1 if you try to open /dev/sdX on anything else.