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G2 PowerPC Core Reference Manual
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MOTOROLA
Implementation-Specific Information
software selectable. In addition, the core uses an interim 52-bit virtual address and hashed
page tables for generating 32-bit physical addresses. The MMUs in the G2 core rely on the
exception processing mechanism for the implementation of the paged virtual memory
environment and for enforcing protection of designated memory areas.
Instruction and data TLBs provide address translation in parallel with the on-chip cache
access, incurring no additional time penalty in the event of a TLB hit. A TLB is a cache of
the most recently used page table entries. Software is responsible for maintaining the
consistency of the TLB with memory. The core TLBs are 64-entry, two-way set-associative
caches that contain instruction and data address translations. The core provides hardware
assist for software table search operations through the hashed page table on TLB misses.
Supervisor software can invalidate TLB entries selectively.
For instructions and data that maintain address translations for blocks of memory, the G2
core and the G2_LE core provide independent four- and eight-entry BAT arrays,
respectively. These entries define blocks that can vary from 128 Kbytes to 256 Mbytes. The
BAT arrays are maintained by system software. HID2[HBE] is added to the G2_LE for
enabling or disabling the four additional pairs of BAT registers. However, regardless of the
setting of HID2[HBE], these BATs are accessible by
mfspr
and
mtspr
.
As specified by the PowerPC architecture, the hashed page table is a variable-sized data
structure that defines the mapping between virtual page numbers and physical page
numbers. The page table size is a power of two, and its starting address is a multiple of its
size.
Also as specified by the PowerPC architecture, the page table contains a number of PTEGs.
A PTEG contains 8 PTEs of 8 bytes each; therefore, each PTEG is 64 bytes long. PTEG
addresses are entry points for table search operations.
1.3.6
Instruction Timing
The G2 core is a pipelined superscalar processor core. Because instruction processing is
reduced into a series of stages, an instruction does not require all of the resources of an
execution unit at the same time. For example, after an instruction completes the decode
stage, it can pass on to the next stage, while the subsequent instruction can advance into the
decode stage. This improves the throughput of the instruction flow. For example, it may
take three cycles for a single floating-point instruction to execute, but if there are no stalls
in the floating-point pipeline, a series of floating-point instructions can have a throughput
of one instruction per cycle.
The core instruction pipeline has four major pipeline stages, described as follows:
The fetch pipeline stage primarily involves retrieving instructions from the memory
system and determining the location of the next instruction fetch. Additionally, if
possible, the BPU decodes branches during the fetch stage and folds out branch
instructions before the dispatch stage.
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n
.