0x019: Maximum spark (2 bytes). This value is the highest
total advance that can be programmed into the
distributor. Any Calculated Total SA above this
value gets clipped to this value.

--- It's purpose is due to the physical limitations of
the distributor. You can only advance the timing
so far until you are sparking on the previous
| cylinder. With eight spark firings per distributor
| revolution, there are only 45 degrees between each
| set of plug wire terminals on the cap. GM uses the
| value of 42 degrees as the maximum advance. This
| always a safety margin to direct the spark to the
| proper terminal.
|
| However, this is not the limit that can be run at
| the crank. If you set the static timing to 10 degrees
| BTDC, a maximum of 52 degrees can be had at the crank.
|
| The 42 degree limit is only the distributor limit.
| Crank timing is the total of the static setting, and
| the amount programmed by the ECM into the distributor.
| It just becomes a balance of how much total crank advance
| is required, versa, how much static (inital), timing can
| be used (and still hot-start).

(if it becomes a limitation, you need a DIS system).

0x01B: Maximum Retard (2 bytes). Maximum total amount of retard
allowed. For the same reasons as max SA above.


OK, so what do I do with the above, and how does it all fit
together?

Note: These calc's are completed 80 times a second, asynchronous
to the actual timing signal from the distributor.


First, a quick run down of what the code does. Remember, just
because a value is calc'd, doesn't mean that it has to be
greater than zero in value.


1) A look up is done on the Main SA table, the value retrieved
is based on the current rpm and map values. Call this Total
Spark Advance, or TSA for now.

2) The Slope, if any, is calculated and added to the TSA.

3) A look up is done on the Coolant Compensation table using
the current engine temp and manifold vacuum. Add this to TSA.

|4) (adjust for any egr spark, SA is added if egr is active).
| (a lag filter calculation is done here also, but appears
| to be egr related only. The lag filter allows for the smooth
| change of SA whenever the EGR is enabled/disabled).

5) Add in any PE spark to TSA. Calc'd in a separate routine.

6) Add in any aldl/closed loop spark, to TSA.

7) Add in any Start-up Spark, to TSA.

8) Add in any Hi-way Spark, to TSA.

9) Subtract the Warm Bias spark value, from TSA.

10) Subtract the Cold Bias spark value, from TSA.

11) Subtract the Initial spark value, from TSA.

12) At this point, the TSA value is tested against
the Max Allowed Spark Value. If it is greater, it
will be set to the maximum allowed SA value.

13) Subtract any Burst Knock, from TSA.

13) Subtract any Knock Retard, from TSA.

14) The TSA value is tested against the Minimum
Allowed Spark, and adjusted to this minimum value
if required.

15) A Spark Latency correction is made and the end
value is sent to counters in the ecm. An output from
the counters control when the distributor will spark,
relative to the incoming distributor pulse.