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  $Header: /home/ff/cs61c/cvsroot/assignments/lab/05.asmptrs/index.rst,v 1.6 2005/11/30 23:42:29 cs61c-tc Exp $

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CS61C Lab 5
===========
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Assembly Pointers
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Background
==========

Goals
-----

This lab gives you more practice with using the assembly language 
equivalent of C arrays and pointers, in particular with the technique of 
writing table-driven code. (This is like data-directed programming in CS 
61A.) It also introduces you to characteristics of numerical 
representations at the machine level.

Reading
-------

* P&H Sections on MIPS and Procedures
* You should familiarize yourself with the code provided for this 
  assignment in order to make most productive use of lab time.

Exercises
=========

Setup
-----

Copy the contents of ~cs61c/labs/05 to a suitable location in
your home directory.

::

	$ mkdir ~/lab
	$ cp -R ~cs61c/labs/05/ ~/lab

Exercise 1
----------

Info
~~~~

An instruction on the IBM 360, which we'll call scan, worked as follows. 
Given the address of the start of a string and the address of a 256-byte 
table of values, it essentially implemented the following C code::

	char *sptr;
	char table[256];
	for (sptr = address of start of the string; table[*sptr] == 0; sptr++) {
	}

That is, it scans the string looking for the first character whose 
corresponding table entry is nonzero.

The file lab5.s contains the framework of a program to skip past white 
space in a string; for exercise 1, you complete this program. Test your 
solution code with the following input lines:

	* an empty input line;
	* a line containing only blanks and tabs;
	* lines that start with each of the following characters: / (slash),
	  0..9, : (colon), @ (at sign), A..Z, [ (left bracket), ` (backquote),
	  a..z, and { (left brace).

Part A
~~~~~~

At the location in the program labeled by table, provide a table of 
bytes with the following contents.

    * Entries corresponding to the blank, tab, carriage return, and line 
      feed characters should be 0.
    * The entry corresponding to the null character should be 1.
    * Entries corresponding to letters (either upper-case or 
      lower-case), digits, and the underscore character should be 2.
    * All other entries should be 3.

ASCII tables are available online (search for "ascii table" at Google).

A useful abbreviation allowed by spim in the .byte directive is the 
notation value:count, which initializes the next count bytes with the 
given value. For example, the directive .byte 0:39 initializes the next 
39 bytes with 0.

Part B
~~~~~~

Supply the code that implements the scan instruction. Your code should 
be given the string address in register $s1 and the table address in 
register $s2; it should branch to the label scan_done when it finds a 
character in the string whose corresponding table value is nonzero, 
leaving the address of that character in $s1 and the corresponding table 
value in $v0.

Exercise 2
----------

Verify that C ignores integer overflow in two ways:

    1. Add 1 to the largest possible positive signed integer, and print 
       the result.

    2. Using mips-gcc with no optimizations (see `lab 4`_, and use -O0), 
       generate the assembly language equivalent of your program for (a) 
       and examine the arithmetic instructions it uses.

.. _`lab 4`: ../04