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G2 PowerPC Core Reference Manual
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MOTOROLA
Implementation-Specific Information
The G2 core cache blocks are loaded in four beats of 64 bits each when the core is
configured with a 64-bit 60x data bus. When the core is configured with a 32-bit bus, cache
block loads are performed with eight beats of 32 bits each. The burst load is performed as
critical-double-word-first. The data cache is blocked to internal accesses until the load
completes; the instruction cache allows sequential fetching during a cache block load. In
the core, the critical-double-word is simultaneously written to the cache and forwarded to
the requesting unit, thus minimizing stalls due to load delays.
To ensure coherency among caches in a multiprocessor (or multiple caching-device)
implementation, the core implements the MEI protocol. The following three states indicate
the state of the cache block:
Modified—The cache block is modified with respect to system memory; that is, data
for this address is valid only in the cache and not in system memory.
Exclusive—This cache block holds valid data that is identical to the data at this
address in system memory. No other cache has this data.
Invalid—This cache block does not hold valid data.
Cache coherency is enforced by on-chip bus snooping logic. Because the G2 core data
cache tags are single-ported, a simultaneous load or store and snoop access represents a
resource contention. The snoop access is given first access to the tags. The load or store
then occurs on the clock following the snoop.
1.3.3.3
Instruction and Data Cache Way-Locking
The G2 core implements instruction and data cache way-locking, which guarantees that
certain memory accesses will hit in the cache. This provides deterministic access times for
those accesses. See Chapter 4, “Instruction and Data Cache Operation,” for more
information.
1.3.4
Exception Model
This section describes the PowerPC exception model and the G2 core implementation,
specifically. G2_LE core-specific information is noted where applicable.
1.3.4.1
PowerPC Exception Model
The PowerPC exception mechanism allows the core to change to supervisor state as a result
of external signals, errors, or unusual conditions arising in the execution of instructions, and
differs from the arithmetic exceptions defined by the IEEE for floating-point operations.
When exceptions occur, information about the state of the core is saved to certain registers
and the core begins execution at an address (exception vector) predetermined for each
exception type. Processing of exceptions occurs in supervisor mode.
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