Книга: Microsoft Windows Embedded CE 6.0 Exam Preparation Kit

Communication between an ISR and an IST

Communication between an ISR and an IST

Because ISR and IST run at different times and in different contexts, you must take extra care of physical and virtual memory mappings if an ISR must pass data to an IST. For example, an ISR might copy individual bytes from a peripheral device into an input buffer, returning SYSINTR_NOP until the buffer is full. The ISR returns the actual SYSINTR value only when the input buffer is ready for the IST. The kernel signals the corresponding IST event and the IST runs to copy the data into a process buffer.

One way to accomplish this data transfer is to reserve a physical memory section in a .bib file. Config.bib contains several examples for the serial and debug drivers. The ISR can then call the OALPAtoVA function to translate the physical address of the reserved memory section into a virtual address. Because the ISR runs in kernel mode, the ISR can access the reserved memory to buffer data from the peripheral device. The IST, on the other hand, calls MmMapIoSpace outside the kernel to map the physical memory to a process-specific virtual address. MmMapIoSpace uses VirtualAlloc and VirtualCopy to map the physical memory into virtual memory, yet you can also call VirtualAlloc and VirtualCopy directly if you need more control over the address mapping process.

Another option to pass data from an ISR to an IST is to allocate physical memory in SDRAM dynamically by using the AllocPhysMem function in the device driver, which is particularly useful for installable ISRs loaded into the kernel on an as-needed basis. AllocPhysMem allocates a physically contiguous memory area and returns the physical address (or fails if the allocation size is not available). The device driver can communicate the physical address to the ISR in a call to KernelIoControl based on a user-defined IO control code. The ISR then uses the OALPAtoVA function to translate the physical address into a virtual address. The IST uses MmMapIoSpace or the VirtualAlloc and VirtualCopy functions, as already explained for statically reserved memory regions.

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