The <tt>i386/isa</tt> directory holds the routines that deal with the ISA
I/O bus in the PC architecture.
The 4.3BSD Internals book by Leffler, McKusick, Karels and Quarterman
doesn't describe this anywhere at all, because this is machine-specific.
You can however glean some information from the following chapter:

<dl>
<dt> <b>Chapter 8</b>, pp 225-256 <i>Device Drivers</i>.
</dl>
<p>