x <- runif(10)
y <- 1+x+rnorm(10,sd=0.1)
d <- data.frame(x,y)

library(bbmle)
m1 <- mle2(y~dnorm(a+b*x,sd=exp(s)),start=list(a=1,b=1,s=log(0.1)),data=d)

m2 <- mle2(y~dnorm(a+b*x,sd=exp(s)),start=list(a=1,b=1,s=log(0.1)),
  method="L-BFGS-B",lower=c(a=0,b=0,s=-Inf),data=d)

m2F <- mle2(y~dnorm(a+b*x,sd=exp(s)),start=list(a=1,b=1,s=log(0.1)),
  method="L-BFGS-B",lower=c(a=0,b=0,s=-Inf),
  fixed=list(a=1),data=d)