I once made a PCI interface, I used a dual port memory (DPM). I used an off the shelf PCI interface chip to write and read from the PC to the DPM, which, with the spec of the PCI interface chip in hand, is fairly simple. I split the DPM in 3 sections
1) PC to uC
2) uC to PC
3) sentinels

When the PC had data for the uC it wrote to section 1 and then set a BYTE (NOT bit) in section 3. The uc would periodically check that byte and if set would copy the information to its own memory and clear the byte.
The transfer the other way was done in a similar fashion.
The reason for using a byte as a sentinel is that with a coincidental write and read of the same location in a DPM the only certain thing is that zero or non-zero will be read. One bit can be wrong in that case.
The specifics are the property of a customer and thus not available here.

Have fun,

Erik