The Simulator Translation Caches (STCs) are designed to increase execution performance. The D-STC caches data translations (logical to physical to real (host) address). The I-STC caches instruction translations of taken jumps. Finally, the IO-STC caches logical to physical address translation for device accesses.
By default the STCs are on.
When a memory hierarchy is connected (such as a cache module) it must have been designed to work along with the STCs, or it may not be called for all the memory transactions it is interested in. These commands can be used to detect if too many translations are kept in the STCs, causing the simulation to be incorrect.
Turning the STCs off means that current contents will be flushed and no more entries will be inserted into the STCs.