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Programming Block Address Translation (BAT) Registers

Q: What is the appropriate procedure for programming the Block Address Transla-

tion (BAT) registers in PowerPC?

Symptoms of not programming BAT registers correctly are random failures with the

MMU, incorrect BAT register values after writing to them and general system lockup.

Here are some rules for programmming the BAT registers that solve all of these problems:

* Use ISYNC before and after the write to a DBAT

* It is invalid to have two DBATs map to the same memory

* When validating the DBATs, write the Lower DBAT then the Upper DBAT.

* When invalidating the DBATs, write the Upper DBAT then the Lower DBAT.

These last two rules stem from the rule that states the values in BEPI and BRPN must

have at least as many low-order 0's as there are 1's in the BL.' In order to make certain

that nothing reads the DBATs until this case is true, the valid bits (Vs and Vp) in the

upper DBATs must be set last when turning the DBATs on. When disabling the DBATs

the ﬁrst thing to do is write the Upper DBAT to turn the valid bits off.

In addition to these rules, here are some guidelines to follow to ensure safe programming

of the DBATs:

* Turn Data Address translation off in the MSR before writing to the DBATs

* Write 0's into the DBATs in correct order (Upper then Lower) to ensure they

are invalidated before setting them.