[MIPS] RFE instruction not present.

The MIPS instruction set contains an instruction called “RFE”, for “Return From Exception”. Its encoding is well specified, with no parameters: http://people.cs.pitt.edu/~don/coe1502/current/Unit4a/Unit4a.html

This document translates “RFE” to the opcode 0x42000010

The point of the RFE instruction is to restore cop0 status flags atomically while returning to the user space. Its pseudo-code implementation can be seen as the following:

    COP0.Status = (COP0.Status & 0xfffffff0) | ((COP0.Status & 0x3c) >> 2);

The explanation of this behavior is simple: when triggering an exception, the MIPS CPU does the following atomically, in order to save usermode flags:

    COP0.Status = (COP0.Status & ~0x3f) | (CP0.Status & 0xf) << 2);

When disassembling a MIPS exception handler from a MIPS kernel with ghidra, it will currently fail to disassemble that instruction properly, nor recognize it as even an executable instruction:

[emteere]

Has been fixed and should be in patch and master soon. Thanks for fixing.

[nicolasnoble]

the _ is interesting, guessing its just because you might have already had the jr disassembled (which failed to do all the logic involved with delay since the delay slot instruction was broken) so when you disassembled the rfe, it had no clue it should be in the delay slot

Oh, that’s correct. I cleared the whole function and re-disassembled it, and now the underscore shows up.

submitted a PR for this fix

Thanks!