==========================
Spring 06 CS61C Homework 4
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MIPS Procedures
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Updated (2/13/06)
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1. Fixed typo at questino 5.  The resulting vector should be stored in the ``struct vector`` pointed by ``vp``.
2. Added clarification on ``struct vector`` memory layout.

Corresponding Reading
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* P&H 2.6, 2.7, 2.9 page 95,96

Problems
--------

Question 1
~~~~~~~~~~
P&H Exercise 2.14.  Reproduced here for ease of reference:

The MIPS translation of the C segment::

  while (save[i] == k)
      i = i += 1;

on page 72 uses both a conditional branch and an unconditional jump
each time through the loop. Only poor compilers would produce code
with this loop over-head. Rewrite the assembly code so that it uses at
most one branch or jump each time through the loop. How many
instructions are executed before and after the optimization if the
number of iterations of the loop is 10 (i.e., ``save[i +10*j]`` do not
equal ``k`` and ``save[i]`` ,..., ``save[i + 9*j]`` equal ``k`` )? 

Save your answer to a file named ``hw4q1.s``.

Question 2
~~~~~~~~~~
P&H Exercise 2.15.

Save your answer to a file named ``hw4q2.s``.

Question 3
~~~~~~~~~~
P&H Exercise 2.16.

Save your answer to a file named ``hw4q3.s``.

Question 4
~~~~~~~~~~
P&H Exercise 2.21 of P&H.  Reproduced here for ease of reference:

Write a program in MIPS assembly language to convert an ASCII decimal
string to an integer. Your program should expect register ``$a0`` to hold
the address of a null-terminated string containing some combination of
the digits 0 through 9. Your program should compute the integer value
equivalent to this string of digits, then place the number in register
``$v0``. Your program need not handle negative numbers. If a nondigit
character appears anywhere in the string, your program should stop
with the value -1 in register ``$v0``. For example, if register ``$a0``
points to a sequence of three bytes 50\ :sub:`ten` , 52\ :sub:`ten` , 0\
:sub:`ten` (the null-terminated string "24"), then when the program
stops, register ``$v0`` should contain the value 24\ :sub:`ten` . (The
subscript ten means base 10.)

Save your answer to a file named ``hw4q4.s``.

Question 5
~~~~~~~~~~
Below is a C code that prints the sum of an array of vectors: ::

 /*
  * Compute sum of an array of vectors
  */
 #include <stdio.h>
 #include <stdlib.h>
 #define NUMVECTORS 4

 struct vector {
     int x;
     int y;
 };

 struct vector *vectors[NUMVECTORS];

 void VectorSum(struct vector* vp, struct vector** vectors, int len) {
	...
 }

 int main()
 {
     int i;
     struct vector retv;

     for (i = 0; i < NUMVECTORS; i++) {
 	 vectors[i] = (struct vector*) malloc(sizeof(struct vector));
 	 if (!vectors[i]) {
	     printf("Out of memory\n"); return 1;
	 }
     }

     FillVectorsFromFile(vectors, NUMVECTORS);

     VectorSum(&retv, vectors, NUMVECTORS);

     printf("Sum of vector = (%d,%d)\n", retv.x, retv.y);
     return 0;
 }

The function FillVectorsFromFile() reads values from a file to initialize the vector array passed.  You task is to complete the VectorSum procedure in MIPS assembly code.  You should follow the calling convention described in the book.

The function VectorSum takes an array of pointers to ``struct vector``
and calculates the sum of all vectors.  The sum of two vectors
*(x1,y1)* and *(x2,y2)* is simply *(x1+x2, y1+y2)*.  The resulting
vector should be stored in the ``struct vector`` pointed to by
``vp``.

You can assume the integer ``x`` is stored at lower memory address than ``y``.  Therefore, if the address of a ``struct vector`` is at 0x50000, than the member ``x`` is located at 0x50000, and the member ``y`` is located at 0x50004.

Save your answer to a file named ``hw4q5.s``.

Submission
----------
Save all your files (``hw4q1.s``, ``hw4q2.s``, ``hw4q3.s``,
``hw4q4.s``, ``hw4q5.s``) in a directory named
``hw4``.  From the directory ``hw4`` run: ::

     submit hw4