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Memory Management 7-5
7.3.2.3 Access Control Fields
Several access controls are available in the UTLB entries.
ZSEL (zone select, 4 bits)
Selects one of 16 zone fields (Z0—Z15) from the Zone Protection Register (ZPR). The ZPR field bits
can modify the access protection specified by the TLB_entry[V, EX, WR] bits of a TLB entry. Zone
protection is described in detail in “Zone Protection” on page 7-14.
EX (execute enable, 1 bit)
When set (TLBLO_entry[EX] = 1), enables instruction execution at addresses within a page. ZPR
settings can override TLBLO_entry[EX]; see “Zone Protection” on page 7-14, for more information.
WR (write-enable 1 bit)
When set (TLBLO_entry[WR] = 1), enables store operations to addresses in a page. ZPR settings
can override TLBLO_entry[WR]; see “Zone Protection” on page 7-14.
7.3.2.4 Storage Attribute Fields
TLB entries contain bits that control and provide information about the storage control attributes. Four
of the attributes (W, I, M, and G) are defined in the PowerPC Architecture. The E storage attribute is
defined in the IBM PowerPC Embedded Environment. The U0 attribute is implementation-specific.
W (write-through,1 bit)
When set (TLBLO_entry[W] = 1), stores are specified as write-through. If data in the referenced page
is in the data cache, a store updates the cached copy of the data and the external memory location.
Contrast this with a write-back strategy, which updates memory only when a cache line is flushed.
In real mode, the Data Cache Write-through Register (DCWR) controls the write strategy.
Note that the PowerPC Architecture does not support memory models in which write-through is
enabled and caching is inhibited. It is considered a programming error to use these memory models;
the results are undefined.
I (caching inhibited,1 bit)
When set (TLBLO_entry[I] = 1), a memory access is completed by using the location in main memory,
bypassing the cache arrays. During the access, the accessed location is not put into the cache arrays.
In real mode, the Instruction Cache Cachability Register (ICCR) and Data Cache Cachability Register
(DCCR) control cachability. In these registers, the setting of the bit is reversed; 1 indicates that a
storage control region is cachable, rather than caching inhibited.
Note that the PowerPC Architecture does not support memory models in which write-through is
enabled and caching is inhibited. It is considered a programming error to use these memory models;
the results are undefined.
It is considered a programming error if the target location of a load/store, dcbz, or fetch access to
caching inhibited storage is in the cache; the results are undefined. It is
not
considered a
programming error for the target locations of other cache control instructions to be in the cache when
caching is inhibited.