According to the docs from EspressIf, I couldn't find any documentation about Jtag port in Esp8266, so I think Jtag port is not supported in this chip. However, we can debug esp8266 with Openocd via Jtag port. Here is the forum discussion for this: viewtopic.php?f=9&t=1871
Accordingly, Esp8266 needs to connect to Jlink debugger through pins GPIO12, GPIO13, GPIO14, GPIO15 and RST. However, according to the datasheet, these pins only function as GPIO, HSPI, I2S or PWM.
So I don't understand why it is possible to debug Esp8266 with Openocd through these pins when the chip's hardware does not support it? And if the esp8266 hardware really doesn't support the Jtag port, how does the debugger interact with the esp8266? And how does OpenOcd work in this case?