I've spent a bit of time over the past few months trying to get my head around the firmware for the TD5 ECU. It's been a long tedious process but things are slowly falling into place.
I'd been puzzling over how the fuel map table were looked up and finally found the routine used to look up the tables. The key point is there is an index of tables addresses near the end of the fuel map. The variant map looks up the address of the required table using the index and then calls the table lookup with the address of the table and the values for the x and y variables. The subroutine returns a value based on this information which is then processed as required.
By referring to "known" tables listed in the Td5MapEditor application, I've been able to make some inroads into the way the tables are used, but it is still far from straight forward. The descriptors of the variables in Td5MapEditor don't really capture the complexity of the process.
As an example the fuel maps are described as a function of the RPM and throttle position. I've traced the "throttle position" input back through three levels of subroutines and still haven't hit a direct reference to the throttle position. The subroutine preceding the fuel map lookup modifies the "throttle position" variable based on two maps, one of which is applied if the revs are below 1000rpm, the other if the revs are above 4200rpm. If the revs are between 1000 and 4200 the values from both maps are extrapolated from the 1000rpm and 4200rpm values.