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User’s Manual
IBM PowerPC 750GX and 750GL RISC Microprocessor
Instruction-Cache and Data-Cache Operation
Page 126 of 377
gx_03.fm.(1.2)
March 27, 2006
These bits allow both uniprocessor and multiprocessor system designs to exploit numerous system-level
performance optimizations.
The WIMG attributes are programmed by the operating system for each page and block. The write-through
(W) and caching-inhibited (I) attributes control how the processor performing an access uses its own cache.
The memory coherency (M) attribute ensures that coherency is maintained for all copies of the addressed
memory location. The guarded memory (G) attribute prevents out-of-order loading and prefetching from the
addressed memory location.
The WIMG attributes occupy four bits in the BAT registers for block-address translation and in the PTEs for
page-address translation. The WIMG bits are programmed as follows.
• The operating system uses the Move-to Special Purpose Register (mtspr) instruction to program the
WIMG bits in the BAT registers for block-address translation. The instruction BAT (IBAT) register pairs do
not have a G bit, and all accesses that use the IBAT register pairs are considered not guarded.
• The operating system writes the WIMG bits for each page into the PTEs in system memory as it sets up
the page tables.
When an access requires coherency, the processor performing the access must inform the coherency mech-
anisms throughout the system that the access requires memory coherency. The M attribute determines the
kind of access performed on the bus (global or local).
Software must exercise care with respect to the use of these bits if coherent memory support is desired.
Careless specification of these bits might create situations that present coherency paradoxes to the
processor. In particular, this can happen when the state of these bits is changed without appropriate precau-
tions (such as flushing the pages that correspond to the changed bits from the caches of all processors in the
system) or when the address translations of aliased real addresses specify different values for any of the
WIMG bits. These coherency paradoxes can occur within a single processor or across several processors. It
is important to note that in the presence of a paradox, the operating system software is responsible for
correctness.
For data accesses in real-addressing mode (MSR[DR] = 0), the WIMG bits default to '0011' (write back,
caching enabled, memory coherent, guarded). For instruction accesses in real-addressing mode
(MSR[IR] = 0), the WIMG bits default to '0001' (write back, caching enabled, memory not coherent, guarded).
3.3.2 MEI Protocol
The 750GX data-cache-coherency protocol is a coherent subset of the standard MESI 4-state cache protocol
that omits the shared state. The 750GX’s data cache characterizes each 32-byte block it contains as being in
one of three MEI states. Addresses presented to the cache are indexed into the cache directory with bits
A[20–26], and the upper-order 20 bits from the physical address translation (PA[0–19]) are compared against
the indexed cache directory tags. If neither of the indexed tags matches, the result is a cache miss. If a tag
matches, a cache hit occurred, and the directory indicates the state of the cache block through two state bits
kept with the tag. The three possible states for a cache block in the cache are the modified state (M), the
exclusive state (E), and the invalid state (I). The three MEI states are defined in Table 3-1 on page 127.