Port positioning (setting and getting) started on the alist, and then helper procedures were added.

beep, x-size and y-size operations were added for console ports.

For ports used for communicating with Emacs, operations for reading and setting the default directory, for prompting for various types of input (so that Emacs would know what text was the prompt), and for noting the start and end of garbage collection (so that Emacs would be able to display GC indicators) were added.

For various types of textual ports, there are all kinds of operations for setting the standard line ending, getting and setting the encoding, and inquiring about what encodings are supported.

The alist acts like a set of mixins.  The ability to extend ports beyond the standard I/O operations is used all over the place in MIT Scheme, and seems like a nice, general facility.