A disk hardware interrupt level may well have higher priority than a parallel port hardware interrupt, so the disk may interrupt a parallel port interrupt.
Usually a driver has just one DIRQL, though there is no reason why it cannot support more, if that is how its hardware works. The interrupt level at which a driver is operating is very important, as it determines the types of operation that can be undertaken. For example, driver hardware interrupt service routines cannot access memory that might be paged out to a swap file.
A driver has to be aware that it could be interrupted by a higher priority task at any stage.
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This means that its own interrupt service routine could run in the middle of its own dispatch routines. For this reason, if a normal driver routine is about to interact with its own hardware, it uses a kernel function to raise its interrupt level temporarily to its DIRQL. This stops its own interrupt handler from running at the same time. In addition, in a multiprocessor system, another driver routine might be attempting the same task on another processor.
The KeSynchronizeExecution routine is used to run such critical sections in a multiprocessor-safe way. The Interrupt Level should not be confused with the scheduling priority. The scheduler uses the priority values to determine which thread to run next. Any other interrupt takes priority over a thread. Drivers need to be aware of scheduling priorities. When it completes an IRP, it can give the calling thread a temporary priority boost so that, for example, programs that interact with the mouse can continue their run more quickly.
This technique generally improves the perceived responsiveness of the system. Any routine servicing a hardware interrupt stops normal program execution.
Book: Writing Windows WDM Device Drivers
For this reason, it is best to make an interrupt service routine as quickly as possible. Any nonessential interrupt processing should be deferred until later. The interrupt routine requests that the driver's DPC post-interrupt routine DpcForIsr is called when things have calmed down. When writing your driver, be careful to comment the interrupt level at which each routine can be called. Similarly, make only kernel calls that are appropriate for the current interrupt level.
A device driver has to be very careful when allocating or accessing memory. However, it is not as gruesome to access memory as some other types of driver. For example, you do not need to worry about the murky x86 world of segments and selectors, as the kernel handles all this stuff. Windows implements virtual memory, where the system pretends that it has more memory than it really has. This allows more applications and the kernel to keep running than would otherwise be the case. Virtual memory is implemented by breaking each application's potential address space into fixed size chunks called pages.
A page can either be resident in physical memory, or not present and so swapped to hard disk. Drivers can allocate memory that can be paged out, called paged memory. Alternatively, it can allocate memory that is permanently resident, called nonpaged memory. Please do not make extravagant use of nonpaged memory. However, you will find that most of the memory your driver uses will be in nonpaged memory.
IN parameters contain information that is passed to the function, and vice versa. The other PoolType values are rarely used. When you are finished using the memory, you must release it using ExFreePool, passing the pointer you obtained from ExAllocatePool.
Use ExFreePool for all types of memory. If you forget to free memory, it will be lost forever, as the kernel does not pick up the pieces after your driver has unloaded. This makes it easier to analyze memory allocation in a debugger or in a crash dump. If your driver keeps on allocating and deallocating small amounts of pool memory then it will be inefficient and the available heap will fragment. The kernel helps in this case by providing lookaside lists  for fixed-size chunks of memory.
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A lookaside list still gets memory from the pool. However, when you free a chunk of memory, it is not necessarily returned to the pool. Instead, some chunks are kept in the lookaside list, ready to satisfy the next allocation request. The number of chunks kept is determined by the kernel memory manager. Lookaside lists can contain either paged or nonpaged memory. A similar set of functions is used for paged memory. Consult the DDK for full details of these functions.
The kernel stack is nonpaged memory for local variables . There is not room for huge data structures on the kernel stack because it is only 8KBKB long. Drivers need to be reentrant, so they can be called simultaneously on different processors. The use of global variables is, therefore, strongly discouraged.
However, you might read in some registry settings into globals, as they are effectively constant for all the code. Local static variables should also not normally be used for the same reason. Finally, you can reduce your driver's memory usage in other ways.
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Once the driver initialization routines have completed, they will not be needed again, so they can be discarded. Similarly, some routines may be put into a pageable code segment. There are two main techniques for accessing user data buffers. However, it is slightly harder to use and is usually only used by DMA drivers that transfer large amounts of data. While a driver can make the user buffer visible in the system address space, the MDL is usually passed to the DMA handling kernel routines.
The first implication is that DMA memory cannot be paged. Secondly, the DMA controller has to be programmed with a physical memory address, not a processor virtual address. The kernel provides routines to help with both these tasks. It is helpful to look at them briefly at this point, but they are described in full later.
In particular, this section looks at how IRP parameters are stored and how the IRP stack lets an irp be processed by a stack of drivers. Each driver gets its irp parameters from the current IRP stack location. If you pass an irp down the stack of drivers for the current device, you must set up the next stack location with the correct parameters. The parameters that you pass down may be different from the ones you are working with.
For a write, the IRP header contains the user buffer information. SystemBuffer field contains a pointer to the nonpaged copy of the user's buffer. The IRP stack location has the main write request parameters.