[rayiner]

Before you get that deep, you need to step back for a bit. The CPU is itself several different processors and controllers. Look at a modern Intel CPU: https://www.anandtech.com/show/3922/intels-sandy-bridge-arch.... The individual x86 cores are connected via a ring bus to a system agent. The ring bus is a kind of parallel bus. In general, a parallel bus works by having every device on the bus operating on a clock. At each clock tick (or after some number of clock ticks), data can be transferred by pulling address lines high or low to signify an address, and pulling data lines high or low to signify the data value to be written to that address.

The system agent then receives the memory operation and looks at the system address map. If the target address is PCI-E memory, it generates a PCI-E transaction using its built-in PCI-E controller. The PCI-E bus is actually a multi-lane serial bus. Each lane is a pair of wires using differential signaling (https://en.wikipedia.org/wiki/Differential_signalling). Bits are sent on each lane according to a clock by manipulating the voltages on the differential pairs. The voltage swings don't correspond directly to 0s and 1s. Because of the data rates involved and the potential for interference, cross-talk, etc., an extremely complex mechanism is used to turn bits into voltage swings on the differential pairs: https://pcisig.com/sites/default/files/files/PCI_Express_Ele...

From the perspective of software, however, it's just bits sent over a wire. The bits encode a PCI-E message packet: https://www.semisaga.com/2019/07/pcie-tlp-header-packet-form.... The packet has headers, address information, and data information. But basically the packet can encode transactions such as a memory write or read or register write or read.