We will use SRP (supervisor root pointer) register to hold the current thread pointer.
Its use by the pmmu is optional and I don't plan on using it.
There are 32 other bits left.
Wonder if weshouldn't use it for system_time() instead... will see, but there are no other usable regs.
git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@22696 a95241bf-73f2-0310-859d-f6bbb57e9c96
