Control (in conjunction with the .nolist directive) whether or not assembly listings are generated. These two directives maintain an internal counter (which is zero initially). .list increments the counter, and .nolist decrements it. Assembly listings are generated whenever the counter is greater than zero.
By default, listings are disabled. When you enable them (with the -a command line option; Chapter 3 Command-Line Options), the initial value of the listing counter is one.