
6-34
G2 PowerPC Core Reference Manual
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MOTOROLA
Page Table Search Operation
The key bit saved in SRR1 is derived as follows:
Select KEY from segment register:
If MSR[PR] = 0, KEY = Ks
If MSR[PR] = 1, KEY = Kp
The rest of this section describes the format of the implementation-specific SPRs used by
the TLB miss exception handlers. These registers can be accessed by supervisor-level
instructions only. Because DMISS, IMISS, DCMP, ICMP, HASH1, HASH2, and RPA are
used to access the translation tables for software table search operations, they should only
be accessed when address translation is disabled (MSR[IR] = 0 and MSR[DR] = 0). Note
that MSR[IR] and MSR[DR] are cleared whenever an exception occurs.
6.5.2.1.1
Data and Instruction TLB Miss Address Registers (DMISS and
IMISS)
The DMISS and IMISS registers have the same format as shown in Figure 6-11. They are
loaded automatically on a data or instruction TLB miss. The DMISS and IMISS contain the
effective page address of the access which caused the TLB miss exception. The contents
are used by the processor when calculating the values of HASH1 and HASH2, and by the
tlbld
and
tlbli
instructions when loading a new TLB entry. Note that the core always loads
a big-endian address into the DMISS register. These registers are both read- and write-
accessible. However, great caution should be used when writing to these registers.
Figure 6-11. DMISS and IMISS Registers
6.5.2.1.2
Data and Instruction TLB Compare Registers (DCMP and ICMP)
The DCMP and ICMP registers are shown in Figure 6-12. These registers contain the first
word in the required PTE. The contents are constructed automatically from the contents of
the segment registers and the effective address (DMISS or IMISS) when a TLB miss
exception occurs. Each PTE read from the tables in memory during the table search process
should be compared with this value to determine whether or not the PTE is a match. Upon
execution of a
tlbld
or
tlbli
instruction, the contents of the DCMP or ICMP register is
loaded into the first word of the selected TLB entry.
Figure 6-12. DCMP and ICMP Registers
Effective Address
0
31
V
H
VSID
API
0 1
24 25 26
31
F
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