[blog] MMU mini-series (2/4)

[ssakash]

Actually, that doesn't matter here. It was just a simple typo, just needs to replace the sdd with ssd.

[ssakash]

great, it's fixed. though, I have one doubt. how does keeping the MMU reduce cost over a hardwired implementation ?

[gregory]

Actually the real story is that my login server at work is called sdd (really a dump name). So I had to get used to type sdd instead of ssd

[gregory]

great, it's fixed. though, I have one doubt. how does keeping the MMU reduce cost over a hardwired implementation ?

MMU is builtin in the CPU. If you want to replace it with a hardwired implementation you need to hire some developers (to code it), some verifiers (to ensure the work of the former), and finally some guys to check/fix delay propagations. Architecture is just a matter of pen and paper, then there is real life

[ssakash]

Thanks for the explanation. though, I'm still wondering how you made Goemon playable with this knowledge. I'm trying to figure it out before, you yourself explain it in the next mini-series. ( it would be great if, you could provide any hint )

[willkuer]

Well first you need to know what goemon tried to do what all other games haven't done. I guess debugging is still needed. You can not only theoretically solve this.

[ssakash]

Please notice that Goemon is crashing with GSdx-DX-Windows.

actually, Bouken Jidai Katsugeki Goemon is playable in GSdx-rev1286 according to the post of parotaku. I think, you missed his final reply.

Well first you need to know what goemon tried to do what all other games haven't done. I guess debugging is still needed. You can not only theoretically solve this.

Debugging is pretty much impossible right now. I have to start getting the game from any local store near my house. but, this game can't be found anywhere near the stores at my area.

[gregory]

actually, Bouken Jidai Katsugeki Goemon is playable in GSdx-rev1286 according to the post of parotaku. I think, you missed his final reply.

Oh I saw it but from my point of view the game is currently broken.

By the way, if someone is able to test this patch. With a hell of luck, it could fix the crash. Otherwise a nice backtrace would be very helpful

Code:

diff --git a/plugins/GSdx/GSState.cpp b/plugins/GSdx/GSState.cpp
index c09eb8c..ea1f1d9 100644
--- a/plugins/GSdx/GSState.cpp
+++ b/plugins/GSdx/GSState.cpp
@@ -852,6 +852,8 @@ template<int i> void GSState::GIFRegHandlerTEX0(const GIFReg* RESTRICT r)
        if((TEX0.TBW & 1) && (TEX0.PSM == PSM_PSMT8 || TEX0.PSM == PSM_PSMT4))
        {
                ASSERT(TEX0.TBW == 1); // TODO // Bouken Jidai Katsugeki Goemon
+               if (TEX0.TBW == 1)
+                       TEX0.TBW = 2;

                TEX0.TBW &= ~1; // GS User 2.6
        }

Debugging is pretty much impossible right now. I have to start getting the game from any local store near my house. but, this game can't be found anywhere near the stores at my area.

Studying PS2 concept (ie this post and the previous one) is one side of the coin. Then you need to check the real game behavior and the way PCSX2 works (dump various log/use PCSX2 as a dissablembler tools). Times + small progress is the key of success.

Post 3 will explain the MMU fix for the interpreter.
Post 4 will explain the gamefix (hack) for the recompiler. And the reason why I choose a hack instead of a real solution.

[willkuer]

Can you maybe es well elaborate why applying bad cheats result in TLB miss? First is this message correct or just wrong exception handling? Second why does it kill the game? If I understood correctly TLB miss is bad for performance but should not be game breaking.

[Blyss Sarania]

Can you maybe es well elaborate why applying bad cheats result in TLB miss? First is this message correct or just wrong exception handling? Second why does it kill the game? If I understood correctly TLB miss is bad for performance but should not be game breaking.

I think it depends. Cheats writing to the wrong address could cause all sorts of havoc. TLB miss may only be one symptom, separate from breaking the game itself.