ba482b3950
userlevel; this is necessary due to the 601, unlike other 6xx, having no concept of separated Valid_user vs. Valid_supervisor for BATs. * When crossing the kernel/userlevel boundary, have platform-provided hooks set up the two fixed BAT entries, and possibly additional segment registers to redeem the 601's BAT limitations. Both of the above are only built if the $MACHINE provides these hooks, sparing others the pain.