There was a discussion here that is now closed: https://stackoverflow.com/questions/25871579/what-is-the-difference-between-rtos-and-embedded-linux#:~:text=Many%20RTOS%20are%20not%20full,essentially%20the%20scheduling%20kernel%20only.&text=Critically%20Linux%20is%20not%20real%2Dtime%20capable
The accepted answer could be summarised as "Critically Linux is not real-time capable".
However, I think this is misleading. When it comes to an OS like linux, you have to distinguish between applications, the kernel, and device drivers.
According to my understanding (having written many Unix device drivers and worked on the Unix kernel decades ago), the device drivers and the kernel are real-time capable.
Device drivers respond to hardware events (interrupts) and the kernel, in turn, can perform actions based on these events.
So, yes, while it's true that certain real-time features are not available to applications, if you have the capability of writing a device driver, you can circumvent this limitation by moving your application logic into a specialised device driver or kernel module.
(Of course it has to make logical sense to do that...)
Have I missed something? Does anyone disagree with this?
Related
I've been on a little adventure learning about how Windows platforms deal with sounds. Now I know a couple of things from taking an Operating System class and reading this:
IO devices such as a sound card communicate with the OS through a device driver. Microsoft supplies a standard so that in kernel mode the OS can make certain system calls (in this case Win32 api calls) to make an I/O device do something. Bytes or characters are streamed to over buses to whatever port the I/O device resides in. These are interpreted by the device controller as instructions for the physical hardware.
There are OS calls to prepare audio devices and stream data to them. Actually for some reason their are 2 ways to do this: MMSystem and DirectSound. I think the reason there are 2 has something to do with gaming? Like DIrectSound was more geared toward that while MMsystem was more standard apps that need audio?
Beyond that their are also other audio APIs NOT written by microsoft for the Win32 platform. ASIO and GSIF.
How can these non Microsoft standards exist on the Windows platform? How can they possibly communicate with hardware or the OS? With something like openGL(something else I toy with), a version of Windows supports a version openGL standard and so does you GPU. BAM, easy-peasy no problems, everything checks out.
How I see it can be is:
MMsystem and/or DirectSound are not part of Win32, but there is some even lower level windows kernel driver everyone is keeping a secret because it's really ugly or
These other driver API's are built on top of the Microsoft supplied ones
Can someone please clarify? Where am I mistaken, I am missing something. I own my old OS textbook still(Modern Operating Systems 3rd Ed. by Tanenbaum) so if you want to point at something in there, go for it.
I'm working with various (mostly Bluetooth) development boards (ConnectBlue, Ubertooth, USRPs etc.) in order to research about Bluetooth communication behaviour at PHY level. In order to get some more insights I'm looking for a way to debug the Bluetooth stack on a Windows 7 Desktop computer. My use-case is relatively simple: I have custom baseband implementations, which establish connections with the Windows computer. I'd like to see everything the Bluetooth hardware/driver does.
I'm not sure how to approach this: I'd like to see when the Bluetooth Chip/Windows driver receives a Signal, and how it (the message) gets interpreted/formatted/passed through the various APIs concerned. Mostly this relates to kernel debugging.
Is there a way to display the state of the attached hardware in Windows in WinDBG? Maybe to perform (Kernel) API logging on the Bluetooth kernel service?
I hope somebody more familiar with device driver debugging and Windows Kernel services can give me some pointers here.
Since you don't appear to have gotten any hits on this, I'll post what I can.
I don't have any definite answers, but on the NTDebugging blog they often do hardware level debugging in windbg.
I.e.
http://blogs.msdn.com/b/ntdebugging/archive/2007/06/22/where-the-rubber-meets-the-road-or-in-this-case-the-hardware-meets-the-probe.aspx
To be honest this is going to require extensive knowledge not only of your hardware, but also of the deep internals of windows, and how the bluetooth stack is written, but the WDK would probably be a good place to start for understanding the bluetooth stack. I would also check out the blog for tips and tricks.
The other place to check and ask questions is http://osronline.com/ It's one of the better communities about device drivers, so they should have some reasonable tips on doing what you're trying to do.
I'm creating an application that needs to use some kernel level modules, for which I've divided the app into 2: one user-level program and one kernel level program.
After reading about device drivers and walking through some tutorials, I'm a little confused.
Can there be a device driver without any specific device associated with it? Is there anything other than the device driver (kernel code or something) which works in kernel mode?
How do anti-virus programs and other such applications work in kernel mode? Is device driver the correct way or am I missing something?
Yes, device drivers can work without an actual piece of hardware (i.e. the device) attached to the machine. Just think of the different programs that emulate a connected SCSI drive (CD-ROM, whatever) for mounting ISO images. Or think about TrueCrypt, which emulates (removable) drives using containers, which are nothing more than encrypted files on your hard drive.
A word of warning, though: Driver development requires much more thought and has to be done more carefully, no shortcuts, good testing and in general expects you to know quite a good deal about the Windows driver model. Remember that faulty and poor drivers put the whole system's stability in jeopardy.
Honestly, I don't think reading a tutorial is sufficient here. You might want to at least invest in a decent book on that subject. Just my 2 cents, though.
Sorry, but the Windows Internals book is more of a general reading for the curious. I cannot recommend it if you want to engage in driver development - or at most as prerequisite reading to understand the architecture. There are plenty of other books around, although most of them are a bit older.
Depending on your goal, you may get away with one of the simpler driver models. That is not to say that driver development is trivial - in fact I second all aspects of the warning above and would even go further - but it means that you can save some of the more tedious work, if instead of writing a legacy file system filter you'd write one based on the filter manager. However, Windows XP before SP2 did not have it installed by default and Windows 2000 would require SP4+SRP+patch if I remember correctly. WDF (Windows Driver Foundation) makes writing drivers even easier, but it is not suitable for all needs.
The term device is somewhat of bad choice here. Device has a meaning in drivers as well, and it does not necessarily refer to the hardware device (as pointed out). Roughly there is a distinction between PDOs (physical device objects) and CDOs (control device objects). The latter are usually what you get to see in user mode and what can be accessed by means of CreateFile, ReadFile, WriteFile, DeviceIoControl and friends. CDOs are usually made visible to the Win32 realm by means of symbolic links (not to be confused with the file system entities of the same name). Drive letter assignments like C: are actually symbolic links to an underlying device. It depends on the driver whether that'd be a CDO or PDO. The distinction is more of a conceptual one taught as such in classes.
And that's what I would actually recommend. Take a class about Windows driver development. Having attended two seminars from OSR myself, I can highly recommend it. Those folks know what they're talking about. Oh, and sign up to their mailing lists over at OSR Online.
Use Sysinternals' WinObj to find out more about the device and driver objects and symlinks.
As for the question about AVs, yes they use file system filter drivers (briefly mentioned above). The only alternative to a full-fledged legacy FSFD is a mini-filter.
It is possible to load a special kind of DLL in kernel mode, too. But in general a driver is the way into the kernel mode and well documented as such.
Books you may want to consider (by ISBN): Most importantly "Programming the Windows Driver Model" (0735618038), "Windows NT Device Driver Development" (1578700582), "Windows NT File System Internals" (0976717514 (OSR's new edition)), "Undocumented Windows NT" (0764545698) and "Undocumented Windows 2000 Secrets" (0201721872) - and of course "Windows NT/2000 Native API Reference" (9781578701995) (classic). Although the last three more or less give you a better insight and are not strictly needed as reading for driver developers.
Anti-virus (and system recovery) software generally make use of file-system filter drivers. A device can have multiple filter drivers arranged like a stack, and any event/operation on this device has to pass through all the stacked up drivers. For example, anti-viruses install a filter driver for disk device so that they can intercept and scan all file system (read/write) operation.
As mentioned in above post, going through a good book would be a nice way to start. Also, install DDK/WDK and refer the bundled examples.
I need some advice on windows programming, MIDI and WDM. I am trying to write a small application that will sit in the sys tray and be advertised to the system as a MIDI In/Out device so that MIDI programs can send to it and it will convert the messages into a different format. I have been reading Cant's WDM book and scouring for information about writing device drivers, but don't know if I'm going down the right path.
I don't see yet how to:-
a) register my driver as MIDI capable (do I stick a ref to it in the registery and let the OS direct MIDI calls to the functionality in a dll?)
b) direct MIDI data through the my driver to my app, which is probably going to be too large to be a driver itself.
Any advice on where to start would be much appreciated.
thanks,
Pete
Windows MIDI drivers do not need to be implemented in the kernel, they can be implemented entirely in userspace as DLLs.
MSDN has some information about the functions you need to implement -
Audio Device Messages for MIDI - unfortunately it is somewhat lacking.
There used to be sample code for this kind of driver, as part of the NT4 DDK, but more recent releases of the DDK / WDK unfortunately don't include it any more.
Some better (though older) documentation and sample code can still be found after some searching:
Introduction to Multimedia Drivers (From NT4 DDK)
Sample MIDI Wine Driver for Mac OS X
Devices are enumerated (or simulated) by device drivers, not applications. What you see in the sys tray is an application icon. Hence, you will need to have both a driver and an app - you can't have one bit of compiled code acting as both.
On the driver side, you probably want to have a peek at the MSDN docs. This will answer part (a) of yopur question.
Assuming that you still would like to continue, (b) is best don by letting your application pull the data from the driver. That's far easier than the other way around - an application can trivially find a driver, but a driver has big problems finding a specific app (process)
If you are looking for a bit easier way to get started, there is a MIDI loopback driver out there, and the folks that make it also offer (or used to offer) a version of it that allows your program to communicate directly with the driver. This gives you the behavior you are looking for, where a program appears as a MIDI device. The loopback driver is at http://nerds.de/en/loopbe1.html. I don't see the developer page anymore, but if you contact them, you might be able to purchase a license to a driver that you can access directly without the loopback.
I am very curious in messing up with HW. But my top level "messing" so far was linked or inline assembler in C program. If my understanding of CPU and ring mode is right, I cannot directly from user mode app access some low level CPU features, like disabling interrupts, or changing protected mode segments, so I must use system calls to do everything I want.
But, if I am right, drivers can run in ring mode 0. I actually don´t know much about drivers, but this is what I ask for. I just want to know, is learning how to write your own drivers and than call them the way I should go, to do what I wrote?
I know I could write whole new OS (at least to some point), but what I exactly want to do is acessing some low level features of HW from standart windows application. So, is driver the way to go?
Short answer: yes.
Long answer: Managing access to low-level hardware features is exactly the job of the OS kernel and if you only want access to a single feature there's no need to start your own OS from scratch. Most modern OSes, such as WIndows, Linux, or the BSDs, allow you to add code to the kernel through kernel modules.
When writing a kernel module (or device driver), you write code that is going to be executed inside the OS kernel and will thus be running in CPU ring 0. Great power comes with great responsibility, which in this case means that you should really know what you're doing as there will be no pre-configured OS interface to prevent you from doing the wrong things. You should therefore study the manuals of your hardware (e.g., Intel's x86 software developer's manuals, device specs, ...) as well as standard operating systems development literature (where you're also going to find plenty on the web -- OSDev, OSDever, OSR, Linux Device Drivers).
If you want to play with HW write some programs for 16-bit real-mode (or even with your own transition to protected-mode). There you have to deal with ASM, BIOS interrupts, segments, video memory and a lot of other low-level stuff.