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 4610
        0333 4610 4709 4758
        4808 4857 4866 5264
        5277 5362 5410 5490

indicates that namei is defined on line 4610 and is mentioned on twelve lines
on sheets 03, 46, 47, 48, 52, 53, and 54.