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 4760
        0333 4760 4859 4908
        4958 5007 5016 5414
        5427 5512 5560 5640

indicates that namei is defined on line 4760 and is mentioned on twelve lines
on sheets 03, 47, 48, 49, 50, 54, 55, and 56.