What is function of GPR ?

[JeanLink20]

Hi! everyone.:)

I will try to ask about problem, as I can not find report about my problem in this forum.

I'm starting to make a simple BD Player for Menu.
So, I read a index file and movie object file for FirstPlayBack.
And , I have many problems to show the menu, when I executed some Navigation Command.

First, I want to ask about a general purpose register.
There is a detail explanation of PSRs on Blu-ray description draft 2.02.
But, I can not find a detail explanation for GPRs on it.

I explain navigation command that is executing.
1]MOVE immediate value of 1 to GPR[0].
2]MOVE immediate value of 0 to GPR[1].
3]PLAY PlayList playlist id of immediate value 0.
4]JUMP Title from GPR[3].

Navigation Command is using GPR[0],GPR[1].
What function is this ?

FirstPlayBack Navigation Command use a value of GPR[3] for title number of "Jump Title".
But,GPR[3] has a value of default 0.
Nobody write down a value to the GPR[3] before Navigation Command use.

I know a m2ts file name of IG stream.
But, I can not reach to m2ts file that is including IG stream by Navigation Command.

What should I do ???:confused:
Could you please give me some hint about function of GPRs.

Thanks a lot.:thanks: