16 x 16 lookup tables

[Ciciora Steve]

  What do you people think about look up tables?  I seem to remember years back
in a DFI catalog that they used 16 x 16 look up tables with 4 point linear
interpolation.  Without having any hands on experience with EFI systems, this
is what I plan to use in my system.  For the fuel map, I will have an 8 bit RPM
go in on one axis and an 8 bit "Density of Air" number (calculated from Air
Intake Temp and MAP pressure) go in on the other axis.  The output will be an 8
bit pulse width.  I will use the 4 most significant bits of each byte (hence
the 16 x 16 table part) to get the 4 nearest points to linearly interpolate
between.  I will then take the top two points and do a linear fit to them
(y=mx+b) and plug in the lower 4 bit number into x.  Dito for the bottom two
points.  Then I will take these two y values, do another linear fit (y=mx+b)
and this result will be my pulse width.  Hmm, reading the above isn't too clear
to me, so let me try to re-word it.  Assume these names for the 4 points:
A     B      X1 and X2 are the 4 bit lower nibbles to interpolate
             with.
C     D
Y1=(B-A)*X1 + A
Y2=(D-C)*X1 + C
Result=(Y2-Y1)*X2 + Y2
A, B, C, and D are 8 bit unsigned numbers.
X1 and X2 are 4 bit unsigned numbers.
(B-A) and (D-C) are 8 bit signed tempoary results.
Y1 and Y2 are (8 bits good enough?) signed numbers.

Although the 68332 (sp?) looks like an ideal processor to use because of the
timer unit, I have too much time, $$, and effort invested in the 6811.  I will
be implementing the above in assembly language.

Does anyone have any comments/suggestions to the above?  Am I on the right
track?
In thinking about how a carb works, I would need a 'Modifier' for the EFI
equivilent of a choke on a carb (works off coolent temp?) and a modifier for
the accelerator pump (works off the rate at which the gas petal is being
pressed down).  Besides for a ignition curve, what else would I need a lookup
table for?
Thanks for your help,
-Steven Ciciora
sciciora at aztec.al.bldrdoc.gov