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Chapter 30 Flash Memory
MPC5606BK Microcontroller Reference Manual, Rev. 2
Freescale Semiconductor
883
Selection of a buffer to be loaded on a miss is based on the following replacement algorithm:
1. First, the buffers are examined to determine if there are any invalid buffers. If there are multiple
invalid buffers, the one to be used is selected using a simple numeric priority, where buffer 0 is
selected first, then buffer 1, etc.
2. If there are no invalid buffers, the least recently used buffer is selected for replacement.
Once the candidate page buffer has been selected, the flash memory array is accessed and read data loaded
into the buffer. If the buffer load was in response to a miss, the just-loaded buffer is immediately marked
as most recently used. If the buffer load was in response to a speculative fetch to the next-sequential line
address after a buffer hit, the recently used status is not changed. Rather, it is marked as most recently used
only after a subsequent buffer hit.
This policy maximizes performance based on reference patterns of flash memory accesses and allows for
prefetched data to remain valid when non-prefetch enabled bus masters are granted flash memory access.
Several algorithms are available for prefetch control that trade off performance versus power. They are
defined by the B
x
_P
y
_PFLM (prefetch limit) register field. More aggressive prefetching increases power
slightly due to the number of wasted (discarded) prefetches, but may increase performance by lowering
average read latency.
In order for prefetching to occur, a number of control bits must be enabled. Specifically, the global buffer
enable (PFCR
n
[B
x
_P
y
_BFE]) must be set, the prefetch limit (PFCR
n
[B
x
_P
y
_PFLM]) must be non-zero,
either instruction prefetching (PFCR
n
[B
x
_P
y
_IPFE]) or data prefetching (PFCR
n
[B
x
_P
y
_DPFE])
enabled, and Master Access must be enabled (PFAPR[MxPFD]). See
, for a description of these control fields.
30.8.8.1
Instruction/Data prefetch triggering
Prefetch triggering may be enabled for instruction reads via the B
x
_P
y
_IPFE control field, while
prefetching for data reads is enabled via the B
x
_P
y
_DPFE control field. Additionally, the B
x
_P
y
_PFLIM
field must be set to enable prefetching. Prefetches are never triggered by write cycles.
30.8.8.2
Per-master prefetch triggering
Prefetch triggering may be also controlled for individual bus masters. See
Flash Access Protection Register (PFAPR)
, for details on these controls.
30.8.8.3
Buffer allocation
Allocation of the line read buffers is controlled via page buffer configuration (B
x
_P
y
_BCFG) field. This
field defines the operating organization of the four page buffers. The buffers can be organized as a pool of
available resources (with all four buffers in the pool) or with a fixed partition between buffers allocated to
instruction or data accesses. For the fixed partition, two configurations are supported. In one configuration,
buffers 0 and 1 are allocated for instruction fetches and buffers 2 and 3 for data accesses. In the second
configuration, buffers 0, 1, and 2 are allocated for instruction fetches and buffer 3 reserved for data
accesses.
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