Preliminary C8051F120/1/2/3 C8051F124/5/6/7 - KEMT FEI TUKE

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C8051F120/1/2/3 C8051F124/5/6/7 Preliminary beginning. When CHALGM is set to ‘1’, the cache will use the pseudo-random algorithm to replace cache locations. The pseudo-random algorithm uses a pseudo-random number to determine which cache location to replace. The cache can be manually emptied by writing a ‘1’ to the CHFLUSH bit (CCH0CN.4). Figure 16.2. Branch Target Cache Organiztion Valid Bit Address Data Prefetch Data VL LINEAR TAG LINEAR SLOT V0 V1 V2 TAG 0 SLOT 0 TAG 1 SLOT 1 TAG 2 SLOT 2 Cache Data V58 V59 V60 V61 V62 TAG 58 SLOT 58 TAG 59 SLOT 59 TAG 60 SLOT 60 TAG 61 SLOT 61 TAG 62 SLOT 62 A16 A2 A1 A0 0 0 0 1 TAG = 15 MSBs of Absolute FLASH Address 1 0 1 1 Byte 0 Byte 1 Byte 2 Byte 3 SLOT = 4 Instruction Data Bytes 16.2. Cache and Prefetch Optimization By default, the branch target cache is configured to provide code speed improvements for a broad range of circumstances. In most applications, the cache control registers should be left in their reset states. Sometimes it is desirable to optimize the execution time of a specific routine or critical timing loop. The branch target cache includes options to exclude caching of certain types of data, as well as the ability to pre-load and lock time-critical branch locations to optimize execution speed. The most basic level of cache control is implemented with the Cache Miss Penalty Threshold bits, CHMSTH (CCH0TN.1-0). If the processor is stalled during a prefetch operation for more clock cycles than the number stored in CHMSTH, the requested data will be cached when it becomes available. The CHMSTH bits are set to zero by default, meaning that any time the processor is stalled, the new data will be cached. If, for example, CHMSTH is equal to 2, any cache miss causing a delay of 3 or 4 clock cycles will be cached, while a cache miss causing a delay of 1-2 clock cycles will not be cached. Certain types of instruction data or certain blocks of code can also be excluded from caching. The destinations of RETIinstructions are, by default, excluded from caching. To enable caching of RETIdestinations, the CHRETIbit (CCH0CN.3) can be set to ‘1’. It is generally not beneficial to cache RETI destinations unless the same instruction is Page 182 DS008-0.8-AUG02 © 2002 Cygnal Integrated Products, Inc.
Preliminary C8051F120/1/2/3 C8051F124/5/6/7 likely to be interrupted repeatedly (such as a code loop that is waiting for an interrupt to happen). Instructions that are part of an interrupt service routine (ISR) can also be excluded from caching. By default, ISR instructions are cached, but this can be disabled by clearing the CHISR bit (CCH0CN.2) to ‘0’. The other information that can be explicitly excluded from caching are the data returned by MOVC instructions. Clearing the CHMOV bit (CCH0CN.1) to ‘0’ will disable caching of MOVC data. If MOVC caching is allowed, it can be restricted to only use slot 0 for the MOVC information (excluding cache push operations). The CHFIXM bit (CCH0TN.2) controls this behavior. Further cache control can be implemented by disabling all cache writes. Cache writes can be disabled by clearing the CHWREN bit (CCH0CN.7) to ‘0’. Although normal cache writes (such as those after a cache miss) are disabled, data can still be written to the cache with a cache push operation. Disabling cache writes can be used to prevent a non-critical section of code from changing the cache contents. Note that regardless of the value of CHWREN, a FLASH write or erase operation automatically removes the affected bytes from the cache. Cache reads and the prefetch engine can also be individually disabled. Disabling cache reads forces all instructions data to execute from FLASH memory or from the prefetch engine. To disable cache reads, the CHRDEN bit (CCH0CN.6) can be cleared to ‘0’. Note that when cache reads are disabled, cache writes will still occur (if CHWREN is set to ‘1’). Disabling the prefetch engine is accomplished using the CHPFEN bit (CCH0CN.5). When this bit is cleared to ‘0’, the prefetch engine will be disabled. If both CHPFEN and CHRDEN are ‘0’, code will execute at a fixed rate, as instructions become available from the FLASH memory. Cache locations can also be pre-loaded and locked with time-critical branch destinations. For example, in a system with an ISR that must respond as fast as possible, the entry point for the ISR can be locked into a cache location to minimize the response latency of the ISR. Up to 61 locations can be locked into the cache at one time. Instructions are locked into cache by enabling cache push operations with the CHPUSH bit (CCH0LC.7). When CHPUSH is set to ‘1’, a MOVC instruction will cause the four-byte segment containing the data byte to be written to the cache slot location indicated by CHSLOT (CCH0LC.5-0). CHSLOT is them decremented to point to the next lockable cache location. This process is called a cache push operation. Cache locations that are above CHSLOT are “locked”, and cannot be changed by the processor core, as shown in Figure 16.3. Cache locations can be unlocked by using a cache pop operation. A cache pop is performed by writing a ‘1’ to the CHPOP bit (CCH0LC.6). When a cache pop is initiated, the value of CHSLOT is incremented. This unlocks the most recently locked cache location, but does not remove the information from the cache. Note that a cache pop should not be initiated if CHSLOT is equal to 111110b. Doing so may have an adverse effect on cache performance. Important: Although locking cache location 1 is not explicitly disabled by hardware, the entire cache will be unlocked when CHSLOT is equal to 000000b. Therefore, cache locations 1 and 0 must remain unlocked at all times. Figure 16.3. Cache Lock Operation Cache Push Operations Decrement CHSLOT TAG 0 SLOT 0 TAG 1 SLOT 1 TAG 2 SLOT 2 Lock Status UNLOCKED UNLOCKED UNLOCKED UNLOCKED CHSLOT = 58 Cache Pop Operations Increment CHSLOT TAG 57 SLOT 57 UNLOCKED TAG 58 SLOT 58 UNLOCKED TAG 59 SLOT 59 LOCKED TAG 60 SLOT 60 LOCKED TAG 61 SLOT 61 LOCKED TAG 62 SLOT 62 LOCKED © 2002 Cygnal Integrated Products, Inc. DS008-0.8-AUG02 Page 183
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