Motorola DSP56305 User Manual download pdf (Page 66)

Instruction Cache and Memory Features

The Instruction Cache

MOTOROLA Optimizing DSP56300/DSP56600 Applications 5-5

and written over. Locking a sector is useful for time-critical

code sections, that should execute at maximum speed

whenever called. Locking them will prevent the need to

re-allocate a sector and re-load it by slow fetches.

2. Unlock a sector (PUNLOCK,PUNLOCKR)—Make the sector

available again for the LRU replacement algorithm. The

unlocked sector is considered “most-recently-used”, that is,

last in line for replacement.

3. Unlock all locked sectors (PFREE)

4. Flush the whole cache (PFLUSH), bringing all the sector tags

and LRU stack to their default values.

5. Flush only the unlocked sectors (PFLUSHUN).

The argument for the PLOCK and PUNLOCK instructions is an

address. The cache controller matches the tag field to the tag

registers, thus identifying the sector. This means that the 8 cache

sectors cannot be allocated by the user directly by using a sector

number or another designator. A sector can be accessed by

searching a match to its tag register. Selecting a sector for allocation

from one of the available (unlocked) sectors is always done by the

controller hardware using the LRU algorithm.

The PLOCK and PUNLOCK are given the address argument by

using one of the regular addressing modes (Absolute Address, or

Memory Indirect Using an Address Register). The PLOCKR and

PUNLOCKR use a PC relative displacement to calculate the address

argument.

Locking sectors enables the user to select actively what code

segments will be in the cache at any given moment. Unlocking

sectors enables changing the sector allocation map dynamically,

according to changing program needs. Software control over sector

allocation should normally be done by locking. It is possible to

allocate an available sector to a tag without locking it (using the

PUNLOCK/R commands on an unallocated address), but it has no

benefit over a regular sector miss during execution.

The multiple unlocking/flushing commands are useful for fast task

switching, in multi-tasking systems. For example, the routines from

the operating system's kernel could be in locked cache sectors, while

the unlocked sectors are for the use of the current task. The

PLFUSHUN instruction could be used at the beginning of a

context-switch.

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Motorola DSP56305 User Manual Page 66