if I want to use loopback, where is flag A, B, C, D on the FX3 chip if I use only 2 line address?
You can check the pin description section of the Fx3 datasheet: http://www.cypress.com/?docID=50647
FLAGA is GPIO21
FLAGB is GPIO22
You can use the unused CTL pins for FLAGC and FLAGD (GPIO's 23, 25, 26, 27).