ERL bit turned on at first because it's not initialized correctly, ERET raises Exception. Simple. Nah.....
The code from sgi port supposed to bootloader initialized it, but in this hardware, bootloader actually doesn't do that.
mfc0 v0, COP_0_STATUS_REG
- li v1, ~SR_INT_ENAB
+ li v1, ~(SR_INT_ENAB | SR_ERL | SR_EXL)
and v0, v1
mtc0 v0, COP_0_STATUS_REG