As discussed in #11
Known limitations:
- Only supports l8ui/l16ui
- Unoptimised load routine
- Not called from DoubleExceptionHandler but should be
- Doesn't restore state properly when falling through to fatal exception handler
Not actually a problem with the current linker script as the address is
predetermined there at start of IRAM, but might as well be accurate
about it.
As per advice from @foogod on esp8266-re mailing list:
https://groups.google.com/d/msg/esp8266-re/1ljIY3Zttbo/HvzHSWOLCgAJ
