Chris Cant - Writing Windows WDM Device Drivers

Most of the commands are fairly self-explanatory. They are best explained using examples in the context of a sample application, WdmIoTest, that outputs data to the LPT printer parallel port.

Table 15.2 WdmIo and PhdIo commands

The WdmIoTest Win32 application uses the WdmIo driver to output a couple of lines of text to a printer on the old Centronics LPT1 parallel printer port. The source and executable for this program are in the book software WdmIo\Exe directory. However, you will not be able to run this program until you have installed the WdmIo driver and configured your system correctly, as described later.

A parallel port responds at three addresses in I/O space in an x86 PC. It also generates an interrupt (when enabled) on one interrupt line. The LPT1 port traditionally lives at I/O space addresses 0x378 to 0x37A and generates ISA interrupt IRQ7.

A basic parallel port has the three registers as listed in Table 15.3. Signals with #at the end of their name use negative logic. For example, if the Status BUSY# bit is read as zero (low), the printer is busy.

The printer is ready to accept data if the Status BUSY# and ONLINE bits are high.

To write data to the printer, write the byte to the Data port. Wait 1µs for the data lines to settle. Set the Control port STROBE output high for at least 0.5µs. The printer signals that it is busy as BUSY# is read as low. When it is ready for another byte, it pulses ACK# low briefly and BUSY# goes high again.

The ACK# pulse signals the parallel port electronics to interrupt the processor (provided ENABLE_INT is high). I have no clear documentation to say what is exactly supposed to happen now. One source says that bit 2 of the Status register is set low to indicate that it has caused the interrupt. However, the two printers that I tried did not set this bit low. It seems that you just have to assume that if the right interrupt arrives (e.g., IRQ7), the correct parallel port caused the interrupt. It seems that reading the Status register clears the interrupt.

A printer may sometimes also pulse ACK# low and so generate an interrupt at other times, such as when it has finished initializing itself, when it is switched off, when it goes off-line, or when it runs out of paper. A dedicated parallel port driver might well take special action for these circumstances. The code here just needs to ensure that the WdmIo driver reads the Status port to clear the interrupt.

As you may be aware, more sophisticated versions of the parallel port are available. For example, when configured in the appropriate mode, a parallel port can input information via the Data port. In addition, some non-standard devices can be connected to a basic parallel port. These might allow input of 4-bits at a time using the Status port. Dongles are hardware devices that are used to verify that some software is licensed to run on a computer. They are designed to respond when their manufacturer's software talks to them in some special way. At all other times, they are designed to pass all parallel port signals to the printer and vice versa.

Table 15.3 Parallel port registers

The WdmIoTest application uses the WdmIo driver to write a brief message to the printer. It outputs information to the console screen to indicate its progress as it performs the following steps. Steps 2-8 all involve DeviceIoControl or WriteFile calls to the WdmIo driver.

1. Open a handle to the WdmIo device. The GetDeviceViaInterface routine is used as before to open a handle to the first device with a WDMIO_GUID device interface.

2. Disable the interrupts and connect to an interrupt.

3. Initialize the printer.

4. Send commands for writing each byte.

5. Read the status every second until the printer is ready. Time-out after 20 seconds.

6. Write a message to the printer.

7. Get the write results.

8. Disable the interrupt.

9. Close the handle.

PHDIoTest does exactly the same job as WdmIoTest, except that it obtains a handle to the PHDIo device in a different way. It calls CreateFile directly, passing the required resources in the filename, as follows.

HANDLE hPhdIo = CreateFile("\\\\.\\PHDIo\\isa\\io378,3\\irq7\\override", GENERIC_READ|GENERIC_WRITE, 0, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);

Table 15.4 shows the different resource specifiers that may be used as a part of the PHDIo filename. The first specifier must be \isa. The other specifiers can be in any order. An I/O port specifier must be given. Use the \override specifier with caution and never use it in a commercial release. All letters in the filename must be in lower case.

Table 15.4 PHDIo filename resource elements

Listing 15.1 shows the commands that initialize the printer. It also shows how DeviceIoControl is called with the IOCTL_PHDIO_RUN_CMDS IOCTL code to run these commands.