In theory, a smart compiler could make heavy use of the DP register (6809 feature) and then local variables within a compiled function could access these "fast" global variables instead. This was a common pattern when coding in assembly, and it's much faster than accessing variables off the stack. The function wouldn't be reentrant, but a compiler pragma could be used to enable/disable the DP mode. Declaring the local variables as static should be sufficient, however.