The source listing is preceded by a cross-reference that lists every defined 
constant, struct, global variable, and function in xv6.  Each entry gives,
on the same line as the name, the line number (or, in a few cases, numbers)
where the name is defined.  Successive lines in an entry list the line
numbers where the name is used.  For example, this entry:

    namei 4660
        0333 4660 4759 4808
        4858 4907 4916 5314
        5327 5412 5460 5540

indicates that namei is defined on line 4660 and is mentioned on twelve lines
on sheets 03, 46, 47, 48, 49, 53, 54, and 55.