Question about Double-Beat Rune?

[Sasarai10]

Hi it's been a while since i've posted here!

Started to play Suikoden 2 again and i have a question related with it.

I've managed to drop 3 Double-Beats in a row from the Cutrabbits in North Sparrow Pass. Was i just lucky or there's a certain pattern that triggers this actually?

[Omnigamer]

You were just lucky. Depending on which enemy groups you fought, your chances go up with the number of cutrabbits present. For example, a group of 6 CutRabbits have an effective drop rate of about 11.5%.

As far as patterns go, the random number generator used in the game is seeded from the beginning of the game, and then re-seeded on each load of a save file. Every step you take while running will advance the value, and after a while in areas where enemies are active it will just advance every frame regardless of what you do. So no, there's no realistic way to manipulate a drop or force a particular pattern.

[Pyriel]

It's sort of tough to do composite odds on drops. Each one is an independent event in some ways, and dependent in others. Compiling the odds for the entire enemy party is a convenient shorthand, but not all that realistic.I hate trying to work out complex probabilities when dealing with computers. The RNG in this game is so simple that I hesitate to even call the game rolling a random number stochastic. And driving down to that level is enough to make my head swim.

But yeah, you were just lucky. If you loaded up the game at the Journeyman's Crystal and you do that again and recreate the series of event exactly (down to how long you spent on the title screen and what you did to load the save), you might be able to repeat it. Almost certainly not, though.

[Omnigamer]

At least based on my own testing, you can "manipulate" certain events from a save in real time with some consistency. The problem is that it's still based on the actual game time of the save, so it's consistent for just that game time. You mash through title screen, and then the Continue/Load menu can be buffered so there's no issue there. But again, this is only for events from a save, and has to be redone for each and every save with a different game time. So it's not particularly useful in any scenario unless you have some program that will spit out the RNG progression along with relevant event triggers (like battles).

[Pyriel]

That's true. There's most likely a seed function in there that I missed when I was looking at the drop code, and if it goes off the game clock rather than the system clock, there's no way to manipulate it reliably without cheating. Based on the system clock, it would be hugely difficult, but not completely impossible.

[Omnigamer]

For reference, below is the relevant disassembly (I'm not sure which module it's from though):

Code: Select all

TEXT:8006DE10 init_srand:                              # DATA XREF: TEXT:8006B058o

TEXT:8006DE10                 lui     $s0, 0x8011

TEXT:8006DE14                 lw      $v0, dword_8010AF94

TEXT:8006DE18                 nop

TEXT:8006DE1C                 beqz    $v0, loc_8006DEE8       ## jump to 8006DEE8 ifeq (MEM32(0x8010AF94),0)



TEXT:8006DE20                 lw      $v0, dword_8010D704

TEXT:8006DE28                 nop

TEXT:8006DE2C                 sll     $a0, $v0, 5

TEXT:8006DE30                 subu    $a0, $v0

TEXT:8006DE34                 sll     $a0, 2

TEXT:8006DE38                 addu    $a0, $v0

TEXT:8006DE50                 sll     $a0, 3                  ## $a0 = 1000 * frame counter



TEXT:8006DE3C                 lui     $v0, 0x8011

TEXT:8006DE44                 li      $v0, 0x8000             ## $v0 = 0x80118000

TEXT:8006DE40                 lw      $v1, dword_8010D6D0     ## $v1 = MEM32(0x8010D6D0) 

TEXT:8006DE48                 addu    $v1, $v0                ## $v1 += $v0



TEXT:8006DE4C                 lhu     $a1, 0x1A56($v1)

TEXT:8006DE54                 sll     $v0, $a1, 2

TEXT:8006DE58                 addu    $v0, $a1

TEXT:8006DE5C                 sll     $v0, 1                  ## $v0 = 10 * game seconds



TEXT:8006DE60                 addu    $a0, $v0                ## $a0 += $v0



TEXT:8006DE64                 lhu     $a1, 0x1A54($v1)

TEXT:8006DE6C                 sll     $v0, $a1, 2

TEXT:8006DE70                 addu    $v0, $a1

TEXT:8006DE74                 addu    $a0, $v0                ## $a0 += 5*game minutes



TEXT:8006DE68                 lhu     $v1, 0x1A52($v1)

TEXT:8006DE7C                 addu    $a0, $v1                ## $a0 += game hours



TEXT:8006DE78                 jal     srand                   ## $v0 = srand($a0)



TEXT:8006DE80                 lw      $v1, -0x506C($s0)       ## $v1 = MEM32(0x8010AF94)

TEXT:8006DE84                 li      $v0, 2

TEXT:8006DE88                 bne     $v1, $v0, loc_8006DEA0

TEXT:8006DE8C                 lui     $v0, 0x8011

TEXT:8006DE90                 lui     $v1, 0x8011

TEXT:8006DE94                 sw      $0, dword_8010D614

TEXT:8006DE98                 j       loc_8006DEE4

TEXT:8006DE9C                 li      $v0, 1

The frame counter is constantly going and is not stored in the save file. Logic says this would mean you could get any one of 60 different seeds from the same save file, but in my testing there seems to be some frame rule that only allows you to load on every other frame.

[Pyriel]

That's just the plain old seed routine from the SLUS file. I never traced it, so I didn't realize what it was tied to.

Is it called frequently? Just to look at it, I'd guess it was part of the initialization state machine (it's part of the usual, massive, switch/case block anyhow), but it looks like it could also reseed every time an area is loaded.

[Omnigamer]

I've only investigated it in the context of loading from saves. It wouldn't surprise me if it loaded it at various points in time throughout gameplay, but I never bothered to set breakpoints on it.

[Metsu]

Glad I found this explanation. I've only debugged the "rng" mechanism in another playstation jRPG. It uses the system "rng" only, which is called zero to many times per frame. It technically isn't a time either, since it just keeps overflowing a number every time it is called. Found it weird too that it is initialized to 0x24040001 and is called 142 times IIRC on the bootup with the playstation logo. From there, I watched the entire intro of the game I was playing and it never calls it. But as soon as you start a new game, it starts calling the rng function.

I can't tell if it's the same as some of the things you're saying (outside of using the game clock method instead). I was always curious on if other games used this the same way. Interesting stuff!