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7-16 PPC405 Core User’s Manual
Setting ZPR[Z
n
] = 00 for a ZPR field is the only way to deny read access to a page defined by an
otherwise valid TLB entry. TLB_entry[EX] and TLB_entry[WR] do not support read protection. Note
that the icbi instruction is considered a load with respect to access protection; executed in user
mode, it causes a data storage interrupt if MSR[DR] = 1 and ZPR[Z
n
] = 00 is associated with the EA.
For a given ZPR field value, a program in supervisor state always has equal or greater access than a
program in the problem state. System software can never be denied read (load) access for a valid
TLB entry.
7.7.2 Access Protection for Cache Control Instructions
Architecturally the instructions dcba, dcbi, and dcbz are treated as “stores” because they can
change data, or cause loss of data by invalidating a dirty line (a modified cache block).
Table 7-2 summarizes the conditions under which the cache control instructions can cause data
storage interrupts.
If data address translation is enabled, and write permission is denied (TLB_entry[WR] = 0), dcbi and
dcbz can cause data storage interrupts. dcbz can cause a data storage interrupt when executed in
the problem state and all access is denied (ZPR[Z
n
] = 00); dcbi cannot cause a data storage interrupt
because it is a privileged instruction.
The dcba instruction enables “speculative” line establishment in the cache arrays; the established
lines do not cause a line fill. Because the effects of dcba are speculative, interrupts that would
otherwise result when ZPR[Z
n
] = 00 or TLB_entry[WR] = 0 do not occur. In such cases, dcba is
treated as a no-op.
The dccci instruction can also be considered a “store” because it can change data by invalidating a
dirty line; however, dccci is not address-specific (it affects an entire congruence class regardless of
Table 7-2. Protection Applied to Cache Control Instructions
Instruction
Possible Data Storage interrupt
When ZPR[Zn] = 00 When TLB_entry[WR] = 0
dcba No (instruction no-ops) No (instruction no-ops)
dcbf Ye s N o
dcbi No Yes
dcbst Ye s N o
dcbt No (instruction no-ops) No
dcbtst No (instruction no-ops) No
dcbz Yes Ye s
dccci No Yes
dcread No No
icbi Ye s N o
icbt No (instruction no-ops) No
iccci No No
icread No No