Instruction Cache Programming Model
MOTOROLA
Instruction Cache
5-7
Table 5-5 Cache Control Bit Descriptions
BIT NAME
DESCRIPTION
CENB
The Cache Enable bit generally provides longword references used for sequential fetches. If the processor
branches to an odd word address, a word-sized fetch is generated. The memory array of the instruction cache
is enabled only if CENB is asserted.
0 = Cache disabled
1 = Cache enabled
CPDI
When the disable CPUSHL Invalidation instruction is executed, the cache entry defined by bits [8:4] of the
address is invalidated if CPDI = 0. If CPDI = 1, no operation is performed.
0 = Enable invalidation
1 = Disable invalidation
CFRZ
The Cache Freeze bit allows users to freeze the contents of the cache. When CFRZ is asserted line fetches
can be initiated and loaded into the line-fill buffer, but a valid cache entry can not be overwritten. If a given
cache location is invalid, the contents of the line-fill buffer can be written into the memory array while CFRZ is
asserted.
0 = Normal Operation
1 = Freeze valid cache lines
CINV
The Cache Invalidate bit forces the cache to invalidate each tag array entry. The invalidation process requires
32 machine cycles, with a single cache entry cleared per machine cycle. The state of this bit is always read as
a zero. After a hardware reset, the cache must be invalidated before it is enabled.
0 = No operation
1 = Invalidate all cache locations
CEIB
The Cache Enable Noncacheable Instruction Bursting bit enables the line-fill buffer to be loaded with burst
transfers under control of CLINF[1:0] for non-cacheable accesses. Noncacheable accesses are never written
into the memory array.
0 = Disable burst fetches on noncacheable accesses
1 = Enable burst fetches on noncacheable accesses
DCM
The Default Cache Mode bit defines the default cache mode: 0 is cacheable, 1 is noncacheable.
0 = Default cacheable
1 = Default noncacheable
DBWE
The Default Buffered Write Enable bit defines the default value for enabling buffered writes. If DBWE = 0, the
termination of an operand write cycle on the processor's local bus is delayed until the external bus cycle is
completed. If DBWE = 1, the write cycle on the local bus is terminated immediately and the operation buffered
in the bus controller. In this mode, operand write cycles are effectively decoupled between the processor's
local bus and the external bus.
Generally, enabled buffered writes provide higher system performance but recovery from access errors can be
more difficult. For the ColdFire CPU, reporting access errors on operand writes is always imprecise and
enabling buffered writes simply further decouples the write instruction from the signaling of the fault.
0 = Disable buffered writes
1 = Enable buffered writes
DWP
Default Write Protection
0 = Read and write accesses permitted
1 = Only read accesses permitted
CLNF[1:0]
The Cache Line Fill bits control the size of the memory request the cache issues to the bus controller for
different initial line access offsets. The following table shows the fetch size.
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Freescale Semiconductor, Inc.
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