Assignment 2, Solutions

In a conventional linker, when memory location x is marked as a reference to label L, once the value of L is found during linking, the linker simply does M[x]=value(L) in order to link the instruciton to the label.

a) If the reference to label L is in the context of a PC-relative branch or call instruction, how does th linker link the instruction to the label? (0.5 points)

the linker does M[x]=value(L)-X.

b) Position independent code allows the program's code to be loaded anywhere in memory without relocation. Does it give similar freedom with respect to the program's stack segment? (0.5 points)

No, but not because there is no such freedom. Position independent code is irrelevant to the question of whether or not the stack can be relocated. The stack can be put anywhere in memory because the stack pointer register points to the stack. The program will use whatever memory is allocated to the stack, so long as it is started with the stack pointer register pointing to that memory.

c) Position independent code allows the program's code to be loaded anywhere in memory without relocation. Does it give similar freedom with respect to the program's static segment? (0.5 points)

No. If the program contains absolute addresses of global variables, it moving the static segment requires changing all of these addresses.

• book.tex -- the main file that links all the pieces below including intermediate files generated by the shell commands that make the book.
• ch1.tex -- chapter 1
• ch1.bib -- bibliography for chapter 1
• ch2.tex -- chapter 2
• ch1.bib -- bibliography for chapter 2

The following shell commands build (in the appropriate combination) book.pdf from all of the above, plus several intermediate files (simplified):

• bibtex book -- makes book.bbl by scanning book.tex and all files referenced by it.
• makeindex book -- makes book.ind by sorting book.idx
• pdflatex book -- makes book.idx, book.aux and book.pdf as it processes book.tex, book.aux and book.ind

In the above, some files are both read and written by the same shell command. In these cases, the original is read first, and then the replacement written.

A Problem: Write a makefile to generate book.pdf. (There is no perfect answer because of the cyclic dependencies between some of the files.) (1.0 points)

book.pdf: book.tex book.bbl book.ind book.aux
       pdflatex book
book.ind: book.idx
       makeindex book
book.bbl: book.tex
       bibtex book

The C standard library includes a number of string manipulation routines, notably strncpy() for copying a null-terminated string, strstr() searches for one null-terminated string within another, and strchr() searches for the first occurance of a character in a string.

A Problem: Write a brief chunk of code to enumerate the successive directory names in the current search path. (0.5 points)

char path = getenv("PATH");
char buffer[MAXPATH];
char * p; /* will point to a copy of the path. */

strncpy( buffer, path, MAXPATH-1 );
while ((p != NULL) && (*p != '\0')) {
        /* p points to one component of the path,
           where that component either ends with ':' or '\0' */
        char * component = p;

        /* find and fix the end of the component */
        p = strchr(p,':');
	if (p != NULL) {
                *p = '\0';
                p++';
	}

	/* component points to a null-terminated component of the path */
	enumerate( component )
}