A PCIe connection consists of one or more data-transmission lanes, connected serially. Each lane consists of two pairs of wires, one for receiving and one for transmitting. You can have one, four, eight, or sixteen lanes in a single consumer PCIe slot–denoted as x1, x4, x8, or x16.
A BAR size: it can be get by writing 0FFFF FFFFH to a physical BAR register first, then read it. Assume that the register’s value to read is A, the BAR size is: ~(A & ~0FH) + 1