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The key is the Unix system call – mmap function.

The idea is to use mmap function to map a memory-mapped components on FPGA, or a contiguous non-cacheable region of physical memory (not managed by the OS virtual memory) to the current software process's virtual memory space.

Here is an example C++ code.

1. The main function starts with opening the /dev/mem device file.
2. Then the first mmap function call requests to map a region of kSdramSize bytes of physical memory starting at kSdramPhysicalAddress, to the current program's virtual memory (referenced by sdram_mmap).
3. The second mmap function maps konChipBufferSize bytes of on-chip buffer starting at kOnChipBufferPhysicalAddressL3, to the virtual memory (referenced by ocb_mmap).

The rest of the program then moves data back-and-forth between SDRAM and on-chip buffer using the 'memcpy' system call.