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Considering how physical and device memory works, the driver needs to perform some
additional actions to transfer DMA memory to the device. Let’s take a look at how it’s possible
to transfer a user mode memory buffer to the device for DMA operations.
1. The memory utilized in user mode usually contains paged physical pages; therefore,
such memory should be fixed in RAM (to make it non-paged). This will ensure that
physical pages aren’t unloaded into the pagefile while the device is working with them.
2. Physical pages may be located outside the contiguous physical memory range, making
it necessary to obtain a device address for each of the pages or every continuous
region with region size.
3. After that, all acquired device memory addresses should be transferred to the device.
In order to maintain the same address format for the memory page, we’ll use a 64-bit
address for both the x86 and x64 versions of Windows.
Note that the physical address for 32-bit Windows doesn’t equal 32 bits because
there’s a Physical Address Extension (PAE), and Windows 64-bit uses only 44 bits
for the physical address, which allows addressing 2
44
= 16TB of physical memory.
At the same time, the first 12 bits describe the offset in the current memory
page (the address of one page of physical memory in Windows can be set by
using only 44 - 12 = 32 bits).
To simplify our implementation, we won’t wrap the addresses. Each memory page will be
described by an address of 64 bits, both for x86 and x64 versions of the driver.
There are two ways to transfer addresses of all pages or regions to the device:
a. Using the I/O memory. In this case, the device must contain enough memory to store
the entire array of addresses. The size of the I/O memory is usually fixed, adding some
restrictions on the maximum size of the DMA buffer.
b. Using a common buffer as an additional memory buffer that contains page addresses.
If the physical memory of this additional buffer is located in continuous physical or
device memory, it will be enough to transfer just the address of the beginning of the
buffer and its size to the device. The device can work with this memory as with a
regular data array.
Both approaches are used, and each has its pros and cons. Let’s consider approach b.
Windows has a family of special functions used to allocate contiguous device memory (or a
common buffer). Schematically, the user mode buffer transferred to the device for DMA
operation looks like this:
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