文件列表:

com_vdd\com_vdd.c (5549, 2002-03-25)
com_vdd\com_vdd.def (77, 2002-03-25)
com_vdd\com_vdd.h (507, 2002-03-25)
com_vdd\com_vdd.htm (5305, 2002-03-25)
com_vdd\makefile (252, 2002-03-25)
com_vdd\nt_com.c (32338, 2002-03-25)
com_vdd\nt_com.h (5613, 2002-03-25)
com_vdd\pc_com.c (25487, 2002-03-25)
com_vdd\pc_com.h (2906, 2002-03-25)
com_vdd\pc_def.h (924, 2002-03-25)
com_vdd\sources (317, 2002-03-25)
com_vdd\uart.h (3484, 2002-03-25)
com_vdd\util_def.h (324, 2002-03-25)
com_vdd (0, 2008-06-13)

This VDD sample implements serial port support for the Virtual DOS Machine (VDM) under Windows NT. The Windows NT VDM has a built-in Virtual Device Driver for serial communications, so this sample driver is completely redundant. It illustrates how serial support might have been implemented using Win32 and the Windows NT DDK. This sample is based in part on the built-in serial driver, but it has been significantly modified and works differently. You should not rely on any feature of this driver when writing serial I/O code that might be used under Windows NT. To install this VDD, use regedit to modify HKEY_LOCAL_MACHINE \ SYSTEM \ CurrentControlSet \ Control \ VirtualDeviceDrivers \ VDD to include ddkroot\lib\*\com_vdd.dll from your DDK tree. Shutdown and restart Windows NT to let this change take effect. Note: once you install the DLL as a VDD you can only modify (rebuild) it when no VDM's are running. Use regedit to uninstall the VDD from HKEY_LOCAL_MACHINE\...\VDD if you want to use the built-in serial support. An optional feature of a VDD is the direct dispatch function. Your application can call this function directly instead of through the I/O instruction trap. A sample direct dispatch function is included (VDDDispatch), but it has not been completed or tested at this time. You may use it as a guide, but be aware of its status. Also, there is no equivalent to this function in the built-in serial support. The code is split into three modules. The first module (COM_VDD.C) is the VDD interface. This holds the code to install the VDM hooks and initialize the virtual UARTs, and to close and release them. The second module (PC_COM.C) implements the DOS interface to the virtual UARTs. It handles IN and OUT instructions to the UART ports and generates simulated interrupts as appropriate. The final module (NT_COM.C) implements the interface to Win32. Unlike the built-in serial support there is no provision for this VDD to ever release the serial port once it is used. As long as the VDM remains active you will be able to access the serial port from it, but otherwise the serial port is locked out. You should consider whether this is the way you want your VDD to act, and how you might deal with this problem. One option would be to dynamically load and unload your VDD rather than installing it permanently. This VDD is multithreaded. Besides the thread that executes the DOS code there is a thread that monitors the serial port status and an optional thread for transmit buffering. Multithreading is one way you can get better performance and reduce the drag on other apps. Transmit buffering is an example of how this works. Through testing it was found that byte transmission offered the most room for improvement. Each transmitted byte was being output using a WriteFile call. By collecting several byt es into a buffer before doing the WriteFile the overhead can be reduced. A separate transmit thread helps this work by delaying the actual write for a few milliseconds while trying to fill the buffer. Transmit buffering also shows an example of how queues can be implemented using Win32. Queues are not supported directly by Win32, but they are easy to implement using a mutual exclusion object and a semaphore object. The TX_enqueue and TX_dequeue functions and TX_queue structure show one way to do this. Overlapped I/O is another point that this VDD illustrates well. Under Win32 you must specify overlapped I/O or any file operation will block all others until it completes. For serial communications this includes WaitCommEvent as well as ReadFile and WriteFile. In the VDD this would have prevented any reads or writes while the status monitoring thread was waiting for a comm event. See the wait_comm, read_comm and write_comm routines for details. A CRITICAL_SECTION object is used to control access to the UART state. This allows the status monitoring thread to update the virtual UART state asynchronously. A Mutex object could have been used as with the transmit buffers, but a critical section is more appropriate in this context.

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