Microtip: Stack corruption? windbg dps to the rescue.

Everyone encounters it sooner or later. A crash with a corrupted callstack. You know what the stack pointer is, but it is clearly not unwinding properly. With a little elbow grease I've found the WinDbg command "dps" to be extremely helpful in this situation.

With this, you're telling the debugger to dump data, in "pointer size" chunks, and also try to match against loaded symbols. You'll quickly start seeing *some* sort of callstack with this display. Now the trick is to sort through the various locations to find the actual callstack. Windbg can help with this as well, you can tell it where the stack actually starts using the kn = [base address] command to verify if what you think is a good stack really is one. The trick is to look at address offsets. Pretty much all real function return addresses will have a non-zero offset. Usually, there are a small enough number of these that you can just brute force it -- like you see me doing (although I skipped a few for brevity). If there are a ton of these potential addresses, it may require additional detective work. Also, one important gotcha -- there may appear to be more than one valid stack, so at this point you need to read the source code or disassembly, and determine what makes the most sense. Also, knowing the ABI of the platform you're working on is invaluable.

This is a little tedious, but the worst bugs only rarely repro and can hold up a product release...

0:000> kn
 # Child-SP          RetAddr           Call Site
00 0000006d`6dcdf310 0000006d`6dd80000 ntdll!RtlEnterCriticalSection+0x22
01 0000006d`6dcdf318 6f6e6d6c`402c0062 0x0000006d`6dd80000
02 0000006d`6dcdf320 00000000`00000470 0x6f6e6d6c`402c0062
03 0000006d`6dcdf328 00000000`00000480 0x470
04 0000006d`6dcdf330 00000000`00000000 0x480
0:000> * Oh noooooooo!!!!
0:000> dps 0000006d`6dcdf310
0000006d`6dcdf310  0000006d`6dd80000
0000006d`6dcdf318  6f6e6d6c`402c0062
0000006d`6dcdf320  00000000`00000470
0000006d`6dcdf328  00000000`00000480
0000006d`6dcdf330  00000000`00000000
0000006d`6dcdf338  0000006d`6dcdf428
0000006d`6dcdf340  77767574`73727170
0000006d`6dcdf348  00007fff`ba9738d8 KERNELBASE!VirtualQuery+0x28
0000006d`6dcdf350  87868584`83828180
0000006d`6dcdf358  8f9e8d9c`8b9a8988
0000006d`6dcdf360  97969594`93929190
0000006d`6dcdf368  ff9e9d9c`9b9a9998
0000006d`6dcdf370  00000000`00000030
0000006d`6dcdf378  0000006d`6dcdf3a8
0000006d`6dcdf380  0000006d`6dcdf440
0000006d`6dcdf388  00007fff`ba983552 KERNELBASE!SetUnhandledExceptionFilter+0x24a
0:000> dps
0000006d`6dcdf390  e7e6e5e4`e3e2e1e0
0000006d`6dcdf398  efeeedec`ebeae9e8
0000006d`6dcdf3a0  d7f6f5f4`f3f2f1f0
0000006d`6dcdf3a8  00000000`00000030
0000006d`6dcdf3b0  00000000`0000021a
0000006d`6dcdf3b8  00000000`00000000
0000006d`6dcdf3c0  00007ff6`d6290000 MicroLogUtil!__ImageBase
0000006d`6dcdf3c8  01000000`00000080
0000006d`6dcdf3d0  00000000`00095000
0000006d`6dcdf3d8  00007fff`bd24dcb7 ntdll!RtlDecodePointer+0x27
0000006d`6dcdf3e0  00000000`00000000
0000006d`6dcdf3e8  00000000`00000000
0000006d`6dcdf3f0  00000000`00000000
0000006d`6dcdf3f8  00000000`00000000
0000006d`6dcdf400  00000000`00000000
0000006d`6dcdf408  00007ff6`d62a5ec0 MicroLogUtil!__CxxUnhandledExceptionFilter
0:000> dps
0000006d`6dcdf410  000012b3`e0966000
0000006d`6dcdf418  00007fff`ba983476 KERNELBASE!SetUnhandledExceptionFilter+0x16e
0000006d`6dcdf420  00000000`00000000
0000006d`6dcdf428  00000000`959f04b3
0000006d`6dcdf430  0000006d`6dd86480
0000006d`6dcdf438  00007ff6`d6295a90 MicroLogUtil!_lock+0x50
0000006d`6dcdf440  00007ff6`d6290000 MicroLogUtil!__ImageBase
0000006d`6dcdf448  00000000`00095000
0000006d`6dcdf450  03810381`01000000
0000006d`6dcdf458  00000000`005a0058
0000006d`6dcdf460  0000006d`6dcdf468
0000006d`6dcdf468  00007ff6`d6299232 MicroLogUtil!_heap_alloc_dbg_impl+0x32
0000006d`6dcdf470  00000000`00000004
0000006d`6dcdf478  00007fff`bd2e1ebc ntdll!RtlZeroHeap+0x6e8
0000006d`6dcdf480  006b0073`00690064
0000006d`6dcdf488  0075006c`006f0056
0:000> dps
0000006d`6dcdf490  005c0032`0065006d
0000006d`6dcdf498  005c0070`006d0074
0000006d`6dcdf4a0  00720063`0069004d
0000006d`6dcdf4a8  0067006f`10070017
0000006d`6dcdf4b0  00000000`6dd86310
0000006d`6dcdf4b8  00007fff`bd2be5e7 ntdll!memset+0x1fa27
0000006d`6dcdf4c0  00000000`00000000
0000006d`6dcdf4c8  00000000`00000001
0000006d`6dcdf4d0  0000006d`6dd80000
0000006d`6dcdf4d8  00007ff6`d6299ab9 MicroLogUtil!_nh_malloc_dbg_impl+0x39
0000006d`6dcdf4e0  00000000`00000030
0000006d`6dcdf4e8  00000000`00000001
0000006d`6dcdf4f0  00000000`00000000
0000006d`6dcdf4f8  00007fff`00000000
0000006d`6dcdf500  0000006d`6dcdf560
0000006d`6dcdf508  00000000`00000000
0:000> dps
0000006d`6dcdf510  00000000`00000000
0000006d`6dcdf518  02100210`02100210
0000006d`6dcdf520  00000000`00000101
0000006d`6dcdf528  00007ff6`d6299a49 MicroLogUtil!_nh_malloc_dbg+0x49
0000006d`6dcdf530  00000000`00000030
0000006d`6dcdf538  00007ff6`00000000
0000006d`6dcdf540  00000000`00000001
0000006d`6dcdf548  00000000`00000000
0000006d`6dcdf550  00000000`00000000
0000006d`6dcdf558  0000006d`6dcdf560
0000006d`6dcdf560  00000000`00000000
0000006d`6dcdf568  00000000`00000000
0000006d`6dcdf570  00000000`00000000
0000006d`6dcdf578  00007ff6`d6294b7a MicroLogUtil!malloc+0x2a
0000006d`6dcdf580  00000000`00000030
0000006d`6dcdf588  0000006d`00000000
0:000> dps
0000006d`6dcdf590  0000006d`00000001
0000006d`6dcdf598  00000000`00000000
0000006d`6dcdf5a0  00000000`00000000
0000006d`6dcdf5a8  00007ff6`d6295db2 MicroLogUtil!_unlock+0x22
0000006d`6dcdf5b0  00000000`00000000
0000006d`6dcdf5b8  02100302`00000030
0000006d`6dcdf5c0  00007ff6`00000000
0000006d`6dcdf5c8  00007ff6`d62915e6 MicroLogUtil!AstNodeCreate+0x26
0000006d`6dcdf5d0  00000000`00000030
0000006d`6dcdf5d8  00007ff6`d62999b4 MicroLogUtil!_msize_dbg+0x234
0000006d`6dcdf5e0  0000006d`00000004
0000006d`6dcdf5e8  00007ff6`d62aa677 MicroLogUtil!_setmbcp_nolock+0x447
0000006d`6dcdf5f0  00000000`00000000
0000006d`6dcdf5f8  00007fff`bd265a63 ntdll!RtlEncodePointer+0x27
0000006d`6dcdf600  00000000`00000100
0000006d`6dcdf608  00007ff6`d62919e8 MicroLogUtil!AstStateInit+0x48
0:000> dps
0000006d`6dcdf610  00000000`00000000
0000006d`6dcdf618  00007ff6`d631bec0 MicroLogUtil!pairNode `RTTI Type Descriptor'+0x5a0
0000006d`6dcdf620  00000000`00000001
0000006d`6dcdf628  0000006d`6dcdf680
0000006d`6dcdf630  00000000`00000000
0000006d`6dcdf638  00007ff6`d6291115 MicroLogUtil!ProcessFile+0x85
0000006d`6dcdf640  0000006d`6dcdf6c0
0000006d`6dcdf648  0000006d`6dd827c8
0000006d`6dcdf650  00000000`00000000
0000006d`6dcdf658  00007fff`bd265a63 ntdll!RtlEncodePointer+0x27
0000006d`6dcdf660  00000000`00000002
0000006d`6dcdf668  00007ff6`d6307020 MicroLogUtil!`string'
0000006d`6dcdf670  0000006d`00000000
0000006d`6dcdf678  00000000`00000000
0000006d`6dcdf680  00000000`00000000
0000006d`6dcdf688  00007ff6`d62a5414 MicroLogUtil!__crtSetUnhandledExceptionFilter+0x14
0:000> dps
0000006d`6dcdf690  00000000`00000000
0000006d`6dcdf698  00000000`00000000
0000006d`6dcdf6a0  00000000`00000000
0000006d`6dcdf6a8  00000000`00000000
0000006d`6dcdf6b0  00007ff6`d62a7810 MicroLogUtil!_RTC_Terminate
0000006d`6dcdf6b8  00007ff6`d62a5f50 MicroLogUtil!__CxxSetUnhandledExceptionFilter+0x10
0000006d`6dcdf6c0  00000000`00000000
0000006d`6dcdf6c8  00000000`00000000
0000006d`6dcdf6d0  00000000`00000000
0000006d`6dcdf6d8  00000000`00000000
0000006d`6dcdf6e0  00000000`00000000
0000006d`6dcdf6e8  00000000`00000000
0000006d`6dcdf6f0  00000000`00000000
0000006d`6dcdf6f8  00000000`00000000
0000006d`6dcdf700  00000000`00000000
0000006d`6dcdf708  00000000`00000000
0:000> * Hmmm, that looks a little more promising...
0:000> * Looking for offsets that make sense... is it the VirtualQuery+0x28???
0:000> kn = 0000006d`6dcdf348
 # Child-SP          RetAddr           Call Site
00 0000006d`6dcdf348 00007fff`ba9738d8 ntdll!RtlEnterCriticalSection+0x22
01 0000006d`6dcdf350 00007fff`ba983552 KERNELBASE!VirtualQuery+0x28
02 0000006d`6dcdf390 00007fff`ba983476 KERNELBASE!SetUnhandledExceptionFilter+0x24a
03 0000006d`6dcdf420 00007ff6`d62a5414 KERNELBASE!SetUnhandledExceptionFilter+0x16e
04 0000006d`6dcdf690 00007ff6`d62a5f50 MicroLogUtil!__crtSetUnhandledExceptionFilter+0x14
05 0000006d`6dcdf6c0 00000000`00000000 MicroLogUtil!__CxxSetUnhandledExceptionFilter+0x10
0:000> * Hmm, doesn't look like it.  How about  KERNELBASE!SetUnhandledExceptionFilter+0x24a?
0:000> kn = 0000006d`6dcdf388
 # Child-SP          RetAddr           Call Site
00 0000006d`6dcdf388 00007fff`ba983552 ntdll!RtlEnterCriticalSection+0x22
01 0000006d`6dcdf390 00007fff`ba983476 KERNELBASE!SetUnhandledExceptionFilter+0x24a
02 0000006d`6dcdf420 00007ff6`d62a5414 KERNELBASE!SetUnhandledExceptionFilter+0x16e
03 0000006d`6dcdf690 00007ff6`d62a5f50 MicroLogUtil!__crtSetUnhandledExceptionFilter+0x14
04 0000006d`6dcdf6c0 00000000`00000000 MicroLogUtil!__CxxSetUnhandledExceptionFilter+0x10
0:000> * Nope.  Okay... skipping ahead to the correct one:  MicroLogUtil!_lock+0x50
0:000> kn = 0000006d`6dcdf438
 # Child-SP          RetAddr           Call Site
00 0000006d`6dcdf438 00007ff6`d6295a90 ntdll!RtlEnterCriticalSection+0x22
01 0000006d`6dcdf440 00007ff6`d6299232 MicroLogUtil!_lock+0x50
02 0000006d`6dcdf470 00007ff6`d6299ab9 MicroLogUtil!_heap_alloc_dbg_impl+0x32
03 0000006d`6dcdf4e0 00007ff6`d6299a49 MicroLogUtil!_nh_malloc_dbg_impl+0x39
04 0000006d`6dcdf530 00007ff6`d6294b7a MicroLogUtil!_nh_malloc_dbg+0x49
05 0000006d`6dcdf580 00007ff6`d62915e6 MicroLogUtil!malloc+0x2a
06 0000006d`6dcdf5d0 00007ff6`d62919e8 MicroLogUtil!AstNodeCreate+0x26
07 0000006d`6dcdf610 00007ff6`d6291115 MicroLogUtil!AstStateInit+0x48
08 0000006d`6dcdf640 00007ff6`d6291473 MicroLogUtil!ProcessFile+0x85
09 0000006d`6dcdf730 00007ff6`d629549c MicroLogUtil!wmain+0x33
0a 0000006d`6dcdf770 00007ff6`d62955de MicroLogUtil!__tmainCRTStartup+0xec
0b 0000006d`6dcdf7c0 00007fff`bcc916ad MicroLogUtil!wmainCRTStartup+0xe
0c 0000006d`6dcdf7f0 00007fff`bd2734a5 KERNEL32!BaseThreadInitThunk+0xd
0d 0000006d`6dcdf820 00000000`00000000 ntdll!RtlUserThreadStart+0x1d
0:000> * Hey yo!  That's looking like a correct stack!

What makes a good job?

Lately I've been reflecting on previous positions I've held and thought about the good and bad of each. It's probably a good idea to think about what job traits lead to satisfaction or dissatisfaction. There seem to be a few common properties which effect me most.

Teammates

I've pretty much always stayed in a team well after the expiration date because of great team members. I often find myself missing peers more than any other positive aspects of a job. Because of this I feel my peers are an extremely important factor in job satisfaction.

Good teammates turn into family friends and lifelong contacts. I recently had lunch with a guy I worked with over 3 years ago. It was the highlight of my week and I'd love to work with him again. If you want to know if you're in a cohesive team you only need to count the number of times team members have done things together away from work.

Technology

If I'm honest with myself I think I have to admit something: What I'm working on seems to be much more of a deciding factor when choosing the next job than contributing to actual happiness once there. I get caught up looking for the next shiny thing. However, there is some practicality in allowing technology to influence my next role. Usually, I try to choose a technology area that I believe will be most useful for my long term career. This is when I try to make sure I don't get stuck or pigeon holed into one area. I remember an engineer I worked with who was getting progressively bitter about always working on the same thing. He hated what he was being asked to work on but didn't seem to want to change positions. I suppose this is the most important reason to consider technology when choosing a job. You don't want to whither away as an expert of a dead-end technology.

Manager

My manager is probably the single most important influence on job satisfaction. Unfortunately, companies (and their managers) don't seem to understand this fact very often. I've worked in company cultures where the only way to grow was to enter management. This is horrible. It takes a company's most productive employees and mashes them into a completely different area of expertise -- one in which they most likely are not qualified. One great thing about Microsoft is the dual path they support. You can be a "people manager" or you can be an "individual contributor" (IC) and obtain the same compensation.

In my opinion, even worse than the employee who enters management because company culture doesn't permit IC growth is the employee who enters management for the prestige of it. These "prestige managers" seem to think being a boss is a more important role than being an IC. The reason this is the worst possible situation is these people don't understand a fundamental rule of leading: The leader works for the team.

Someone looking for prestige in a leadership role is under the impression the team works for the leader. Paradoxically, the only time the team truly will work for the leader is when the leader has won the respect and admiration of the team members. This does not happen through silly mandates, make-work meetings and tasks, managing appearances rather than results, or overbearing "guidance." It does not come from business buzzwords or MBA degrees. It comes from facilitating the team members. It comes from getting out of the way when you're a hindrance, proactively removing obstacles, and internalizing team member requirements. A bad leader will say, "I hope you're empowered to fix this." A good leader will not need to talk about empowerment because their ICs already are.

At one point I naively thought these misguided managers simply never had the benefit of an excellent manager mentor. I don't believe that's the case anymore. I've personally observed multiple examples of ICs with solid examples of good leadership turn into some of the worst prestige managers. They are incapable of observing what makes the solid leaders excellent. Since they are blind to these traits they are also incapable of applying the principles in their team. It's easy to spot these managers. Their teams bleed talent and whither. Team members who stick around are demoralized, have low job satisfaction, and through apathy eventually give up trying to make any positive change. A bad leader can poison a team or organization the way cancer moves through the body.

Partner teams

Much like the culture of a team, the culture of partner teams is extremely important. For the same reasons a bad manager introduces a cancer to the team partner teams do the same. Partner teams are by definition peers or customers. Nobody wants to work with a jerk and nobody wants a jerk for a customer. Talented ICs will eventually leave a team forced to deal with horrible partner teams.

Work-Life balance

"I don't live to work, I work to live." I believe a balanced personal life creates many more motivated employees and directly impacts the success of a company, organization, or team. For me, when work cuts into my personal time I end up despising work. More importantly, when work-life balance shifts too far toward work it's an indicator the team, organization, or company will eventually fail. ICs are asked to work too much because of scheduling / planning issues or staffing issues.

If a team, organization, or company has poor planning the quality of the final product will eventually suffer. Quality will suffer because bugs are not caught (or are caught too late to fix and reset testing to make a release date). Quality will also eventually suffer because the best employees will eventually leave. Common "fire drills" point to a process or cultural issue which the team, organization, or company cannot or is not motivated enough to change.

If a team, organization, or company has a staffing issue one must ask why. Do they suffer from high attrition? Do they not compensate properly? Do they expect too much of ICs? Can they not find qualified candidates? All of these are related to the simple fact that it's not a good place to work. Most of the issues behind the questions above are obvious, but what about the lack of qualified candidates? I believe there are quite a few extremely smart people out there looking for something new. If you can't entice quality candidates how do you expect to keep current talent?

Impact

I want to change the world. I want to work on something wonderful that makes the world a better place. I believe life is too short to not try to make the world better while we are here. There are many ways to make the world better. Entertainment, creating tools for others, basically anything that helps someone else. Someone also pointed out to me that developers typically want to work on things that ship. I fully agree -- it's pretty hard to have much impact on the world if your project doesn't ship.

Conclusion

I started writing this to help me decide what I value the most should I look for a change. It appears leadership is a top influence for me at this time. However, I think leadership is perhaps a proxy for general team, organization, or company culture. The leaders will dictate the culture of a workplace.

DarkOS: Hobbyist OS architecture taking shape.

I've been tinkering with some things in my "hobbyist OS" for a bit now. There was a bit of hiatus when my son was born but he's getting into a sleeping schedule more conducive to projects.

First, I've been messing around with a few emulators. VirtualBox seems to be the best for my work. A few key bits of functionality make it favorable:

• It doesn't require some of the more advanced CPU virtualization extensions needed by HyperV.
• It does run using some virtualization, so it's much faster than something like Bochs.
• It has a "debugger" mode with windbg style commands to dump registers, gdt, etc.

Bochs is still pretty useful, but it started throwing odd crashes during protected-to-real transitions (more on this in a bit).

http://wiki.osdev.org/ has been very helpful in all my development.

I've got a plan of attack for the OS now:

• Using long mode (amd64).
• Using ELF x86-amd64 Linux ABI. Cross compiler running in Cygwin has been built.
• No real/protected mode ping-ponging.
  
  • The real/protected transitions were proving more annoying (and variable) between Bochs and VirtualBox. I'd get one working and then the other would fail. Since this whole idea of switching back and forth is full of issues I'm just not doing it.
  • This requires slightly odd bootstrapping of components (loading kernel, drivers from disk but not processing the ELF format at first).
  • Doing this, I hope to spend more time focused on the OS and less on the quirkiness of various BIOS implementations.
• POSIX user mode environment with a Windows NT -style kernel.

Key components and TODO list:

• Bootloader
  
  • Enable A20 support.
  • Get RAM configuration from BIOS.
  • Read kernel boot options from disk or perhaps keyboard.
  • Load kernel stub, kernel, and all modules indicated as required for boot. (The kernel and modules are streamed into higher address RAM directly from disk, no ELF image loading done yet).
  • Switch video to VESA linear frame buffer mode (preferably 1024x768 24/32bit color.
  • Fill out a table of boot and machine parameters for use by the kernel stub.
  • Setup basic identity paging for long mode (amd64).
  • Enter long mode and jump to the kernel stub.
• Kernel Stub
  
  • Load kernel from raw file images the Bootloader copied from disk to full on ELF relocated modules.
  • Convert the Bootloader "boot and machine parameters" to a table suitable for calling KeMain(...) with.
  • Calling KeMain(...) to initiate kernel startup.
• Kernel
  
  • Initialize a very basic set of kernel debugger stubs accessed through a serial port.
  • Initialize the memory manager based on Kernel Stub provided parameters (used memory, etc.)
  • Initialize the object manager.
  • Create a "system" process (virtual address space, etc.)
  • Call the loader functions in the kernel for each of the Bootloader provided boot critical modules.
  • Call ModuleInit(...) for each boot critical modules.
  • Call ModuleEnter(...) for each of the boot critical modules to initiate PCI/PnP/Hardware enumeration.
  • Using file system and storage modules loaded as part of the boot critical modules group, load and start additional modules required by the system.
  • Create a user mode process ("init") to bootstrap the rest of the system from user mode startup scripts.

Once I get the basic kernel booting I hope to crib quite a few module tidbits from a free *nix distribution like FreeBSD or NetBSD. I'll have to evaluate the licenses I'm choosing for my project and the ones provided by FreeBSD and/or NetBSD.

Some modules will be pretty straight forward. For example, the VESA module will mainly just be a set of helper functions for drawing and outputting text to the linear frame buffer. Other modules like EHCI/XHCI will be much more in-depth and probably won't every be tackled.

Ideally I'd like to be able to boot from USB thumb drive, access the network card, and if the POSIX environment works out, perhaps get enough libraries and components running in user mode for a web browser and self hosting a build of the OS.

Well, that's the plan anyway. I hope to make gradual progress with a few planned breaks in the next few months (moving houses... maybe a vacation...).

Why is my default timezone Monrovia, Reykjavik?

I recently sent a friend an email from work and he pointed out my email timezone is set to Monrovia, Reykjavik and that I should refer to http://support.microsoft.com/kb/2298834.

Here is my response...

Oh, I gave up running Outlook and I'm accessing email via the Office365 interface.

Outlook had (has) some nasty bug where it takes a UI lock in a thread and then posts blocking web calls from the same thread. When the blocking calls actually take a bit of time (because the company isn't good at running data centers?) my dev machine would lose the ability to CHANGE WINDOW FOCUS or otherwise process windows messages.

I tried debugging this issue and after attaching windbg to Outlook.exe and discovering ~150 CLR threads I decided this effort was hopeless. I'm still unclear why they use the UI thread to post blocking web calls in an environment where they already have 149 OTHER threads they could use for this. Thus, I decided to use the https://outlook.com/owa/XXXX.com site instead for corporate email.

HOWEVER, more comedy ensued since MSFT apparently isn't very good with cookie management and continually tried to use my XXXX.X.XXXX@live.com tokens to access my corporate email -- the concept that I'd want to be logged in as XXXX.X.XXXX@live.com and NNNN@XXXX.com at the SAME TIME to different Microsoft website properties is completely alien.

Thus I've settled on my current solution, a shortcut: "iexplore.exe -private https://outlook.com/owa/XXXX.com"

This works very well since it ignores any persistent cookies on the system via InPrivate mode. However, I guess persistent settings like my actual timezone cannot be accounted for and I'm not even prompted for them (some UX designer probably focus grouped this and convinced themselves it was the optimal solution to just use the null timezone setting).

I thought about using Chrome exclusively for my Outlook.com needs but alas, Google seems to think the key to a successful browser is launching FIVE Chrome.exe processes utilizing ~75MB OF MEMORY to provide access to their useless "Hang Outs" feature. Which I hear now supports (THANKFULLY, the world was empty without this feature) Mustaches and Hats so I guess 75MB of RAM is a small price to pay for MUSTACHES AND HATS in a failing social media landing point that replaced the highly effective and useful googletalk.

Gone are the days of a trim gtalk.exe client and Outlook.exe (not .com) just working (for various levels of "working" exceedingly greater than the current implementation). I realize I'm becoming a crusty fossil in the industry; (already?!) clinging to concepts like trim code, easy to understand and debug projects, and my generate-the-object-code-I-intend C coding methodology...

Dispatch Objects

It seems like the next step in my OS development is going to be laying down some kernel work. I thought I'd be able to delay this, but doing so would only result in less useful work being done. A kernel is responsible for many things. I'm sure many more things than I even know about right now (such is the nature of this whole undertaking). However, my experience writing Windows drivers has offered a decent skeleton to start with.

One piece of this skeleton is dispatch objects. Basically, this is any sort of thing that can be waited upon. A timer, an event, a lock, a thread (joining a thread, etc.). I figure this is as decent a place to start as any. The way a dispatch object will work in my kernel is it will be a light weight structure containing a list of waiting threads. Of course, this means I need to start defining a thread structure as well. I imagine I'll continue to flesh out the guts of the kernel (or at least their structures) by jumping off on this one spot.

Since a dispatch object is essentially a list, the first structure I need to create is a list structure. There are many ways to create a list. You can create a linked list, a vector, or various other structures. Again, leaning on my Windows background I'll choose something very similar to the LIST_ENTRY structure in the NT kernel. In more data structure books I've seen a doubly-linked list defined as:

typedef struct _LIST_NODE
{
    struct _LIST_NODE* pPrev;
    struct _LIST_NODE* pNext;
    void* pData;
} LIST_NODE;

Here, I'll rely on something clever that Windows does with it's list structure -- there's no "data" pointer. The structure is intended to be embedded directly into any other data structure:

typedef struct _LIST_NODE
{
    struct _LIST_NODE* pPrev;
    struct _LIST_NODE* pNext;
} LIST_NODE;

typedef struct
{
    LIST_NODE  ThreadListHead;
    ...
} DISPATCH_HEADER;

typedef struct
{
    UINT32     ThreadId;
    ...
    LIST_NODE  DispatchListNode;
    ...
} THREAD_OBJECT;

The way this works is the list pointers are offset from to find the original object the list points to. In the DISPATH_HEADER above, this is simply the same address. However, say I had additional members before the ThreadListHead member, you'd then SUBTRACT back to get the original object pointer. This is done using a member offset macro, where the offset of a member into a structure is computed and then subtracted off the list pointer. While this may appear clunky at first it has one very nice advantage - you don't need to dynamically allocate small chunks of memory. In the example above, you would simply have a list insertion function that you'd pass a pointer to the THREAD_OBJECT::DispatchListNode member variable to.

There are multiple reasons this is a good thing. First, the previously mentioned avoidance of small memory allocations. Second, by not relying on any sort of memory allocation this data structure can be used in the kernel before memory allocation is even possible -- obviously, you need to write an allocator before you can use one. Third (but related to the first), the absence of smaller allocations for what is a widely used construct can add up to some pretty substantial memory savings. Any memory allocation will require a chunk of memory for bookkeeping. This is usually going to be at least the size of a few pointers. For small allocations of only 3 pointers (in the first list structure definition) the overhead of the allocation may very well be larger than the actual data usage. Considering how widely used the LIST_NODE structure will be, this is a substantial savings in memory as well as access speed (there isn't an indirect memory access to get to the actual structure, if you have the pointer to the list, it's only a subtraction to get the actual pointer versus another read).

So, there it is, the beginnings of a DISPATCH_HEADER, a THREAD_OBJECT, and a LIST_NODE. This kernel is practically writing itself...

Rolling Your Own Operating System

I started an extremely ambitious project a while back -- working on my own operating system. This isn't one that I ever intend to gain any ground or even boot on real hardware. (I'm targeting VirtualBox, VirtualPC/HyperV, and Bochs emulator environments right now.) It's just a place for me to play around with OS concepts. At work I write driver code for windows, but this code relies on a system of interfaces and mechanisms that have been designed and implemented by people much smarter than I. I'd like to start working on those more basic concepts. Sort of like peeling back the layers of an onion by recreating it. I anticipate I will learn much not only about how my OS works and can work, but about the tradeoffs that most OS designers have encountered along the way.

My current "OS" is very limited. It uses the standard DOS (well, Windows 7 -- because there are differences...) bootloader from the partition table. I created my own volume boot record to receive control from there. I'm able to load from the beginning of the partition. I'm still working on the loader. This is not be choice, but actually some unexpected first bit of learning. Here's the deal...

The BIOS loads and calls into the MBR code. The MBR code loads and calls into my volume boot record code. Now the fun starts. There are numerous extremely important interfaces the BIOS provides to early boot code. Before protected mode, paging, multi-tasking, and all the other things that really make an operating system an operating system you could just happily keep calling these BIOS interfaces to get this functionality. However, the BIOS code is legacy... It's so legacy that it isn't all that compatible with the way a modern OS sets up the CPU. (There are a few more compatible extensions -- VESA graphics BIOS stuff for example, but even this is severely limited)

So, it's already decision time. A lot sooner than I thought it would be. How do I start laying the ground work for the more modern features of my OS when I need to use the BIOS routines to talk to the hardware? Well, the short answer is I need to stop at some point, and probably the sooner the better. I don't want to litter my kernel code with a bunch of BIOS dependencies -- anything I write that requires this cannot really be reused anywhere else. Ideally, I would switch over from BIOS interfaces to my own all at once, however that will be difficult. Instead, I've decided a better approach is to rely on the BIOS interfaces until I'm ready to create my own. So, what does the BIOS give me that I need to reproduce?

1. VGA/VESA support -- in order to know my OS is doing something, I should probably have a way to see it working. ¹
2. Keyboard support.
3. Disk IO support.
4. ...

I'm sure there are many more but these are the interfaces I'm immediately aware of. The first item, VGA support was easiest. This is probably because it's also the only one that's moved past legacy support. I am able to select a video mode which allows me to draw 24/32 bit RGB directly into a flat buffer space. This buffer space is directly mapped to the screen buffer, so I'm set as far as rudimentary drawing is concerned. I don't get any hardware acceleration and I'm stuck having to implement all my own line drawing, bit-blitting, and other APIs, but at least it's doable. The most annoying thing about this is I cannot adjust for changing the monitor resolution, but if I look at my above goals of not really caring if this runs on real hardware this is acceptable.

Now comes disk IO. This is a bit sticky, since there is no easy way to go about this. I'm probably going to have to support at least IDE and AHCI. I'm not exactly sure if I can get away with IDE only and rely on hard disks to support the legacy commands... I think so, but not sure. I also cannot keep thunking back and forth between BIOS code for this support -- every time I do that I'll probably have to tear down the things I really wanted to learn about -- memory paging, scheduling, etc. I see some work on a storage driver in my near future... It does indeed look like I can just go with IDE. Hopefully, IDE will be pretty easy to work with. It looks like the specs are open as well -- http://www.t10.org/t13/technical/d98120r0.pdf

Here's what my volume boot record code looks like so far: http://www.darkautomata.com/blog/os/vbr_2013_11_29.asm.txt

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¹ I can just use a COM port interface for logging what is actually happening in the OS, but there is going to come a time relatively early in the OS where I'll actually want to SEE what I've created. Maybe this isn't as big of a requirement as I think it is right now...

Screen DPI vs Remoting issues

It seems to me the user experience for high DPI displays like the Apple Retina display will be horrible for certain scenarios.

If I'm remoting into a windows machine using Remote Desktop or displaying an X application using X11 remoting then the host DPI settings are probably going to result in a bad experience. I already see this in when using my work laptop to remote into my desktop.

At work I have multiple 24" 1920x1080 monitors. When I RDP into the machine from my 14" 1920x1080 work laptop I get text in the code editor that is barely legible. I can't imagine how bad this would be on the Retina display!

The Remote Desktop and other remote display clients should enable a pass-through mechanism for client DPI. This way the host can probably format the display for remoting.

To expand on this idea, if all UI settings relating to size were stored in a device independent way (for example: twips, 1/1440 of an inch) then a translation could take place to always match the physical dimensions of the UI. You could even have a constant scaling factor so you can change this for a device. For example, I may not want my cell phone displaying at full physical dimensions, maybe it's okay to go with 50% or 25% and I know I have to squint at it, but I want my laptop or tablet to use 80-90% physical dimensions, etc...

If no one has thought of this yet well it's here first... prior art recorded.