9-8
MPC860 PowerQUICC UserÕs Manual
MOTOROLA
Part II. PowerPC Microprocessor Module
For pages larger than 4-Kbyte, set subpage validity ßags (bits 24-27) of the level-two
descriptor (and thus Mx_RPN) to 0b1111.
For 4-Kbyte pages, there are four separate entries with different encodings of
subpage validity ßags (bits 24Ð27) of the level-two descriptor (and thus Mx_RPN)
allowable for each entry.
For 4-Kbyte pages, the subpage validity ßags (bits 24Ð27) of the level-two
descriptor (and thus Mx_RPN) can be different for each of the four separate entries.
In this mode, the MMU page tables deÞned for the software tablewalk resolve to a
single level-two descriptor entry for a 1-Kbyte page. This is done by allowing
manipulation of the subpage validity ßags of a 4-Kbyte page. For example:
Ñ To deÞne a 4-Kbyte page with uniform protection, create four level-two
descriptors for the 4-Kbyte page, each with subpage validity ßags set to 0b1111.
All other Þelds of the level-two descriptors must also be the same for each of
these entries.
Ñ To deÞne four different 1-Kbyte pages, create four level-two descriptors, but set
the subpage validity ßags such that: entry one = 0b1000, entry two = 0b0100,
entry three = 0b0010, entry four = 0b0001. All other Þelds of the level-two
descriptor can be set differently for each of these entries.
Ñ To deÞne two different 2-Kbyte pages, create four level-two descriptors, but set
the subpage validity ßags in pairs such that: entry one = 0b1100, entry two =
0b1100, entry three = 0b0011, entry four = 0b0011. The other Þelds of the
ÔpairedÕ level-two descriptors must be the same for each of the pairs.
Other combinations are also possible.
This mode is the most complex and the most inefÞcient in memory size (that is,
MMU tables are approximately four times larger). However, it allows the most
detailed resolution of protection with full functionality.
IMMUs and DMMUs can use different modes; the IMMU could use mode 1 and the
DMMU could use mode 2, or vice versa. However, if mode 3 is desired, both MMUs must
be in mode 3.
9.6 Memory Attributes
Memory attributes deÞned by the PowerPC architecture are implemented as follows:
¥
Reference and change bit updatesÑThe MPC860 does not generate an exception for
an R (reference) bit update. In fact, there is no entry for an R bit in the TLB.
The change bit (C) is bit 23 in the level-two descriptor, described in Table 9-4.
Software updates C (changed) bits, but hardware treats the C bit (negated) as a
write-protect attribute. Therefore, attempting to write to a page marked unmodiÞed
invalidates that entry and causes an implementation-speciÞc DTLB error exception.
If change bits are not needed, set the C bit to one by default in the PTEs.
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