I suppose that internal system ROM in the chip is located at 40000000 and contains basic system functions. In case if you have sdk please find a file "eagle.rom.addr.v6.ld" and it lists all the functions in there. That allows uart and download routines to program an external flash. I think It reads the 3 GPIO boot mode pins.
Newbies Start Here
Are you new to ESP8266?
Unsure what to do?
Dunno where to start?
Start right here!
We also have a RTOS version and a MESH version too!
Complete listing of the official ESP8266 related documentation release by ESPRESSIF!
Must read here!