Program size

The Compiler can never do this, since the compiler only sees a single source file at a time - therefore it can never know the full size of your complete project.

Only the Linker knows that.

So the question should be: why didn't the Linker warn you?

It should have warned you; the fact that it didn't must mean that you have misconfigured your project somehow - so that the Linker thinks that you have >4K available...

The Compiler can never do this, since the compiler only sees a single source file at a time - therefore it can never know the full size of your complete project.

Only the Linker knows that.

So the question should be: why didn't the Linker warn you?

It should have warned you; the fact that it didn't must mean that you have misconfigured your project somehow - so that the Linker thinks that you have >4K available...

First, ensure that your source is optimal: http://www.keil.com/support/man/docs/c51/c51_xc.htm

Then see: http://www.keil.com/support/man/docs/c51/c51_optimize.htm

Thanks Andy I'll go through it, You are always a great help.

This is a problem sometimes not often in Keil IDE. sometimes it throws error at compilation time. We need to optimise the compiler in the begining after we select the controller.

We need to optimise the compiler in the begining after we select the controller.
who is "we"? are you working for Keil? I have to say, it does not sound like that...

Please explain what you mean by that.

What you wrote doesn't make sense - but I guess that could well just be a matter of English usage?