scroll-step

scroll-step is a variable defined in `C source code'.
Its value is

0

Documentation:
The number of lines to try scrolling a window by when point moves out.
If that fails to bring point back on frame, point is centered instead.
If this is zero, point is always centered after it moves off frame.
If you want scrolling to always be a line at a time, you should set
`scroll-conservatively' to a large value rather than set this to 1.

You can customize this variable.