I "accidentally" went to firmware 2.00.
So, I thought I'd just write a bit about what I did when I had 1.51, for reference to others perhaps giving them some ideas.
Of course there's the possibility that the techniques will vary if you're using something other than 1.51.
First, let's prepare some software that you will need to do your analysis (if you want to call it that).
Warning: We can't get anywhere if you don't have the necessary update files (1.50, 1.52). If you don't have them, get them from someone else. Don't ask me for them, because I can't help you.
1. Necessary software
PBP Unpacker: http://pdc.me.uk/pbp/
2. Begin downgrade method analysis!
Before we being, I suggest you make a backup copy of the update files in case something goes wrong; you'll have a "way out."
Based on the knowledge of our predecessors, we'll first try it just using two Memory Sticks (hereafter: MS). Or that's what I thought, because other people have already tried this. Anyway, we'll give it a shot.
Put the 1.52 update on one MS, and the 1.50 on the other MS.
Insert the MS with version 1.52 on it, and launch the update. While the PSP logo is showing, quickly swap the MS. And...
It will get to the update screen, but then it will show "a newer version is already installed."
So that didn't work. Here, we see that the version check is actually done twice.
Hows about we just try and change the version number, then?
We'll use PBP Unpacker. Use "Open" to open the 1.52 update file. Then, designate an "Extract" folder. Four files in the list will be output to that directory. Open up PARAM.SFO. And you'll say to yourself, "holy hell it's an update!"
Here you'll want to double-click on the area that says UPDATER_VER. And change 1.52 to 1.50. If you feel like it, you may also want to change the TITLE portion (if that's what it was called, don't remember exactly).
Choose Save to File from Basic Options and push Save. After a bunch of crap comes up, the PARAM.SFO save dialog will appear. Save it to wherever you want.
Next, choose "New." Choose the files we created earlier, at the beginning of all this. Then, choose the PARAM.SFO that we "fixed." Leave everything else the same.
Choose Save and save the file. We're now done with our "imposter" (a 1.52 update file disguised as a 1.50 update).
Put this imposter on your MS. You're able to run it without any problems, single-MS style. But then...problems.
From this result, we can see that the initial check only looks at one thing: the numerical value of the version that is contained in the information of the update file. What prevents the update from starting is the version information that was read from the actual data portion of the file, during the "second check."
The question that we can ask ourselves, then, is why not do the swap after the second check is carried out? That's what I tried -- I launched the 1.52 update, and when I got as far as the update screen, I tried swapping the MS. The instant I pulled the card out, it got pissed off at me.
Let's use Stirling.
Of the four outputted files, open up DATA.PSP. Output the same four files from the v1.50 update, and open up its DATA.PSP file as well.
From the Window Menu, choose Cascade Vertically. Then from the Search/Move menu, choose Compare.
You'll get warned about the sizes being different, but don't worry about that. When you push the OK button, the results will be displayed.
Put checks in "Stress" and "Synchro-scroll." Scroll through the page and take a look, without closing the discrepancy point summary. It is red for the most part, but where there are white areas, you can see that the data is the same.
There's something mysterious about this. Seems like there's some sort of hint here...
But this is as far as I have gotten. In closing, I'll leave you with a thought (since actually executing it would be quite difficult).
From these results, it appears that there is some sort of hole in the MS swap method. And there's some sort of switch that detects when you remove a MS.
Because of this, if you were able to create some sort of external device that could switch between two MSs, when the update has gotten as far as the update screen, would you not be able to run the old update from the swapped stick?
Update August 18, 2005
I made a new discovery.
It doesn't matter if other people already know this or not. To each his own.
So I'm just going to write about it. I may also add stuff in the future.
Inside the DATA.PSP file that you end up with after splitting the update with PBP Unpacker, I've found a part that is the same between the two files.
Even saying "a part" is a too generous. But even such a small piece is good. If everyone finds their own parts, then if a lot of people do it, something will come out of it.
You've read this far, but the result was a bit gloomy. Analysis is difficult, even for those who aren't amateurs.
In other news, I've had all sorts of freezes since going to 2.00 -- is this how it's supposed to be?