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24 lines
1.1 KiB
Markdown
24 lines
1.1 KiB
Markdown
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## Disabling BAT translation
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BAT translation can be disabled by invalidating BAT registers. This is somewhat CPU specific.
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MPC601 implements its own format for BAT registers that differs from the PowerPC specification.
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MPC601-specific lower BAT registers has the "V" bit. If it's cleared, the corresponding BAT pair
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is invalid and won't be used for address translation. To invalidate BATs on MPC601, it's enough
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to write NULL to lower BAT registers. That's exactly what PowerMac 6100 ROM does:
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```
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li r0, 0
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mtspr ibat0l, r0
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mtspr ibat1l, r0
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mtspr ibat2l, r0
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```
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PowerPC CPUs starting with 603 uses the BAT register format described in the PowerPC specification.
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The upper BAT registers contain two bits: Vs (supervisor state valid bit) and Vp (problem/user state valid bit).
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PowerPC Architecture First Edition from 1993 gives the following code:
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```BAT_entry_valid = (Vs & ~MSR_PR) | (Vp & MSR_PR)```
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If neither Vs nor Vp is set, the corresponding BAT pair isn't valid and doesn't participate in address translation.
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To invalidate BATs on non-601, it's sufficient to set the upper BAT register to 0x00000000.
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