Histval or initval? - confused

Hi!

I’ve just begun learning dynare, and i’dont exactly understand the difference between histval and initval in the case of deterministic modells.
For example, i’d like to simulate the basic acceletar-multiplicator modell of Samuelson, here is the code:

[code]var y c i;

varexo g;

parameters c_a gamma psi;
c_a=1;
gamma=0.5;
psi=1;

model;
c=c_a+gammay(-1);
i=gamma
psi*(c-c(-1));
y=i+c+g;
end;

check;

histval;
y(0)=1;
c(0)=0.8;
end;

shocks;
var g;
periods 4:9;
values 5.1;
end;

simul(periods=100);
[/code]

But it wont work, and i think it needs histval-s too. Please help!

Thanks!