Cleaning up the PReP patches for AIX

I've got surprisingly many requests on booting AIX under the QEMU 40p target. I've looked at the changes which need to be done both to QEMU and to OpenFirmWare, and it turned out it there was more than I thought.

In 2017 I used some original work from the OFW done by Mitch Bradley. After a brief discussion he kindly published this file today. So now I'm good to publish my changes.

At the moment I'm cleaning the code, then I'll publish it on GitHub, and hopefully the patches will make it upstream. Except maybe for OFW, since it has been decided that the official firmware is going to be OpenBIOS. But for the users it should not matter, it's just one additional file to download.

/ Stay tuned ;-)

The second bug in the qemu sparc CPU emulation

Mitch Bradley found a bug in the Sparc CPU emulation. I gave him access to my qemu session and he stepped through the code. Is sort of shame, I haven't done it myself, as I thought about it 2 weeks ago.

This bug is actually much more heavy than the previous one. While the previous one affected only the hand crafted assembly code, this one should hit the compiled code as well: the handling of carry flag in subxcc instruction is wrong. And, yes, it's RISC architecture, so this instruction is also used for comparison...

I'm really astonished that Linux/sparc is working under qemu since years. Of course Linux may be just more robust, but it also may mean that gcc doesn't use some sparcv8 instructions, and is therefore inefficient.

The OBP author is here and still cares

Mitch Bradley (OBP author) explained how OBP space* commands are working. While the emulation is meanwhile working properly for this commands, it is nevertheless great to know that the father still cares about his child.

And, speaking of children, Mitch is also the author of OLPC firmware and OLPC Forth tutorial I mentioned before.