This article lists the various Reset causes of Esp8266

esp8266 reset causes and common fatal exception causes.pdf

Here is a quick list based on Off’s work here http://www.esp8266.com/viewtopic.php?p=2096#p2112

reset causes:
0:
1: normal boot
2: reset pin
3: software reset
4: watchdog reset

boot device:
0:
1: ram
3: flash