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7-4 PPC405 Core User’s Manual
The exact comparison depends on the page size, as shown in Table 7-1.
SIZE (page size, 3 bits)
Selects one of the eight page sizes, 1KB–16MB, listed in Table 7-1.
V (valid,1 bit)
Indicates whether a TLB entry is valid and can be used for translation.
A valid TLB entry implies read access, unless overridden by zone protection. TLB_entry[V] can be
written using a tlbwe instruction. The tlbia instruction invalidates all TLB entries.
TID (translation ID, 8 bits)
Loaded from the PID register during a tlbwe operation. The TID value is compared with the PID value
during a TLB access. The TID provides a convenient way to associate a translation with one of 255
unique software entities, typically a process or thread ID maintained by operating system software.
Setting TLBHI_entry[TID] = 0x00 disables TID-PID comparison and identifies a TLB entry as valid for
all processes; the value of the PID register is then irrelevant.
7.3.2.2 Translation Field
When a TLB entry is identified as matching an EA (and possibly the PID), TLBLO_entry[RPN] defines
how the EA is translated.
RPN (real page number, 22 bits)
Replaces some, or all, of EA
0:21
, depending on page size. For example, a 16KB page uses EA
0:17
for
comparison. The translation mechanism replaces EA
0:17
with TLBLO_entry[RPN]
0:17
to form the
physical address, and EA
18:31
becomes the real page offset, as illustrated in Figure 7-1.
Programming Note: Software must set all unused bits of RPN (as determined by page size) to 0.
See Table 7-1.
Table 7-1. TLB Fields Related to Page Size
Page
Size
SIZE
Field
n
Bits
Compared
EPN to EA
Comparison
RPN Bits
Set to 0
1KB 000 22 EPN
0:21
↔ EA
0:21
—
4KB 001 20 EPN
0:19
↔ EA
0:19
RPN
20:21
16KB 010 18 EPN
0:17
↔ EA
0:17
RPN
18:21
64KB 011 16 EPN
0:15
↔ EA
0:15
RPN
16:21
256KB 100 14 EPN
0:13
↔ EA
0:13
RPN
14:21
1MB 101 12 EPN
0:11
↔ EA
0:11
RPN
12:21
4MB 110 10 EPN
0:9
↔ EA
0:9
RPN
10:21
16MB 111 8 EPN
0:7
↔ EA
0:7
RPN
8:21