5.2. Caching SPR and Memory Access

When stalled, SPR and memory values cannot change. It therefore makes sense to cache these values to eliminate JTAG activity on duplicate accesses when stalled. The classes to provide this functionality (SprCache and MemCache) were described in Section 4.3.

Caching can be disabled when building the model by defining NOCACHE on the command line or in DebugUnit.h.

	Caution
	The Next Program Counter NPC is always cached for the reasons described above in Section 4.6.1.

The effect of caching on server performance and client load times can be seen in Table 5.2. This compares the performance of an unoptimized server with and without caches.

Run Description	Cycles	Time	Perf	Load Rate
No optimization, no cache	12,593,861	374.02 s	34 kHz	161 bytes/s
No optimization, with cache	8,097,241	257.34 s	31 kHz	164 bytes/s

Table 5.2. Effect of SPR and memory caches on GDB server and client performance.

The extra cycles due to the debugging load have been reduced by 4.5 million, just over 40%. The time taken to load a program is unchanged. The memory cache cannot help, since this is an initial load.