Lab 5 Basic Assembly Programming

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Due 7 October before lab.

Exercise 1. Name that Loop (20 points)

1. Describe in your own words the function performed by the following code fragment when it reaches the label end. Consider the following hints. The code fragment operates on the contents of register a0, and the loop will always complete.
```        addi  \$t0,\$0,32
loop:	andi  \$t1,\$a0,1
sub   \$t0,\$t0,\$t1
srl   \$a0,\$a0,1
bne   \$a0,\$0,loop
```
2. Describe how the operation of the above code fragment changes if the srl instruction is replaced with an sra instruction with the same arguments.
3. Describe the function of the following code fragment in your own words:
```      	la    \$t1,a
loop: lw    \$t0,4(\$t1)
sw    \$t0,(\$t1)
bne   \$t0,\$0,loop
```
4. Describe the function of the following code fragment in your own words:
```      	la    \$t1,a
loop: lw    \$t0,(\$t1)
bne   \$t0,\$0,loop
la    \$t0,a
sub   \$t0,\$t1,\$t0
sra   \$t0,\$t0,2
```

Exercise 2. A Loop of Your Own in C (30 points)

You are given an array called Pixels, each element of which is a 32-bit word representing a color value. The lowest significant 8 bits of each color value denote an unsigned integer (from 0 to 255) representing the color’s “blue” value, the next 8 bits are the “green” value, the next 8 bits are the “red” value, and the most significant 8 bits are all zeros. The size of the array is not known, but it is known that the last element of the array is a negative number (e.g., “-1”); this negative value should itself not be considered as a valid color value, of course. For example, the following array has three color values:

```int Pixels[] = { 0x010101, 0x5, 0x1111, -1 };
```

Your C program should access the Pixels array, one element at a time, until the end is reached, and for each element, it should print the following line to the console:

```Pixel n = r g b
```

Where n is the number of the array element read (first one is 0th), and r, g and b are integers (from 0 to 255) representing the red, green and blue values for that pixel. You need not worry too much about formatting and white space etc., but print only one element per line.

So, if you used the Pixels array given above, your output should be:

```Pixel 0 = 1 1 1
Pixel 1 = 0 0 5
Pixel 2 = 0 17 17
```

A template C program pixel.c is provided for you. Download and edit it to include your name and onyen in the comments at the top and edit your code in at the place indicated.

Grading: Your program will be graded as correct (full-credit), partially correct (half-credit), or incorrect (no credit).

Exercise 3. A Loop of Your Own in Assembly (50 points)

Download and install MARS, the MIPS instruction set simulator (see class website for download link). Then, write an assembly program to do the same thing as in exercise 2.

Your code fragment will begin execution at the label “main”. You will use the syscall instruction (Appendix B. 43-45) to print the above output, and also to terminate your program.

A starter assembly file pixel.asm is provided for you. Download the file and edit it to include your name and onyen in the comments at the top and edit your code in at the place indicated.

Submit your code on blackboard as usual.

Grading: Your program will be graded as correct (full-credit), partially correct (half-credit), or incorrect (no credit).

Collaboration:You are free to take as much help from other students as you need to write and debug your code. You must, however, understand everything you submit, and be able to explain if asked.