Assignment 4, due Sep 19

On every assignment, write your name legibly as it appears on your University ID card! Homework is due on paper in discussion section on Thursday. Some parts of assignments may be submitted on-line and completed in discussion section. Exceptions will be made only by advance arrangement (excepting "acts of God"). Late work must be turned in to the TA's mailbox (ask the CS receptionist in 14 MLH for help). Never push homework under someone's door!

1. Background: Chapter 5, exercise 4 suggests experimenting with the Hello World program in both C and Hawk vesions to see what some of the control characters do when output as part of a string using the C Library puts and Hawk Monitor PUTS routines.

   Assignment: The backspace, tab, vertical tab, linefeed and carriage return characters are the most important format effectors among the ASCII control characters. What do they do when you output them to the terminal window under C? What do they do when you output them from a Hawk program? Give your answer as a short sentence for each of these telling what it does and if the behavior is different or the same in the Hawk and C environments. (1.0 points, 0.2 per character)

   Suggestion: You can design a single string that replaces "Hello world" and allows you to see the effect of each of these with a single experiment. You do not need to show your work, but if you show us your test strings for C and the Hawk environments, we can diagnose your errors.

2. Background: PC relative addressing can be confusing, so it's worth working a few examples. Here is a small one, 3 consecutive words of Hawk memory:

   000000: 000230F0
   000004: 30F00200
   000008: FA00FFFA
   

   A Problem: Give equivalent Hawk code, with labels such as A, B and C used instead of numeric memory addresses. First, try it by hand, then use the emulator to check your work, and figure out what your errors were. (0.6 points, 0.1 point per instruction or operand error)

3. Background: One of the routines defined in stdlib.h but not mentioned in Chapter 5 is malloc (the C version) or MALLOC (the Hawk version). Either version takes a single parameter, the number of bytes to allocate, and returns a pointer to a memory region big enough to hold that many bytes. Here is a little C program for playing with malloc:

   #include <stdio.h>
   #include <stdlib.h>
   #include <stdint.h>
   int main() {
       int i;
       for (i = 0; i < 100; i++) {
   	void * p = malloc(i);
   	printf( "%lX\n", (intptr_t)p );
       }
       return 0;
   }
   

   This program just prints out the addresses of 100 blocks of memory. The %lX in the format string asks it to print a long integer in hexadecimal, and the (intptr_t) cast on the pointer forces the pointer to be passed to printf as an integer, using an integer representation large enough to hold one pointer.

   a) When you call malloc on the divisional Linux machines, what alignment guarantee does it give you about the pointer it returns? That is, does it return an arbitrary address, an address divisible by 2, divisible by 4, or what? (0.7 points)

   b) When you call MALLOC on the Hawk, what alignment guarantee does it give you about the pointer it returns? (0.7 points)

   Suggestion: Instead of printing out 100 addresses and then trying to find a pattern, consider oring all of the addresses into a single value, then look at how many zero bits are on the bottom of the result after 100 allocations of various sizes of blocks. In Python, Java, C and C++, the a|b operator will or two values of any integer type; on the Hawk, the OR machine instruction does this for any two 32-bit registers.

   If you use this suggestion, you can show your work by showing the value you got and your interpretation of that value.