initial state argument to filter

[bantone]

Hi,

How exactly is the argument SI (initial state) for
the "filter" function handled?

I try to use the filter function as a simple harmonic oscillator.
y(n) = C * y(n-1) - y(n-2)
the frequency depends on C and the initial phase on the initial state.
I would expect that I get a cosine wave if I type something like
this:

y = filter([1,0,0], [1, -1.996, 1], zeros(1, 512), [1, 0.99]);

because 

y(1)= b(1)*x(1) -a(2)*y(1-1) -a(3)*y(1-2) = -(-1.996)*0.99 -1=0.97604
y(2)= b(1)*x(2) -a(2)*y(2-1) -a(3)*y(2-2) = -(-1.996)*0.99 -1.006=0.95818
...

instead I get

ans =

  1.0000  2.9860  4.9601  6.9143  ...

I expected y(1-2) and y(1-1) to be SI(1) and SI(2).
That would make sense to me.  Instead the initial state seems to
be just prepended to the input vector.  In other words the initial
state gets modified before the first output y(1) is calculated.
Is that the correct behaviour?

thanks,
anton