Description
1 Objective
The objective of this lab is to make you comfortable with MIPS control instructions by
writing, running, and debugging several complex programs.
2 Pre-requisite
Before starting with this lab, you should be familiar with MIPS control instructions as well
as MARS.
3 Control Instructions
As you know, the PC points to the current instruction; however, for efficiency reasons, the
PC is actually updated to the next instruction before the current instruction has completed
1
Computer Architecture and Design, Lab 3 2
its execution. This means that the offset used in computation of PC-Relative addressing
is actually relative to the next instruction instead of the current one. The assembler in
MARS hides this complexity from the user when it computes offsets in instructions, but
you can see this if you examine the instructions it creates.
1. Enter the following code into a file named lab3-1.s and run it using MARS. Use it to
answer the questions below:
1 . text
2 . g lob l main
3
4 main : l i $a1 , 10
5 add $t0 , $ze r o , $ z e r o
6 l o o p : beq $a1 , $ze r o , f i n i s h
7 add $t0 , $t0 , $a0
8 sub $a1 , $a1 , 1
9 j l o o p
10 f i n i s h : addi $t0 , $t0 , 100
11 add $v0 , $t0 , $ z e r o
(a) Step through the program and find the branch instruction. Locate the value of
the 16-bit branch address and fill in Table 1.
Tab. 1: Branch Instruction Address (when delayed branches is disabled)
Instruction Address field in instruction
(16 bits)
Target Address
(b) Step further through the program and find the jump instruction. Locate the
value of the 26-bit jump address and fill in Table 2.
Tab. 2: Jump Instruction Address (when delayed branches is disabled)
Instruction Address field in instruction (26 bits)
Address field shifted
left by 2 bits (28 bits)
Target Address
Computer Architecture and Design, Lab 3 3
2. (a) Write a MIPS program with the following specifications:
• Reserve space in memory for a variable called UIN of size word. The initial
value of UIN will be the sum of the digits in your UIN.
• The program should implement the following piece of C-code:
for ( i =0; i <10; i++)
UIN = UIN−1;
• Save your file as “lab3-2.s”, and make sure you comment the code.
(b) What is the value of UIN before and after executing the code:
Variable Value before the run value after the run
UIN
(c) Demonstrate your program to the TA; .
3. Create a program with the following specifications:
• Reserve space in memory for an array of words of size 10. Use the ‘.space’
directive. The array is called my array.
• The program should implement the following piece of C-code. The value of
initial value is the first digit of your UIN.
int j = i n i t i a l v a l u e ;
for ( i =0; i <10; i++)
my array [ i ] = j ;
j ++;
}
• Save your file as “lab3-3.s”, and make sure you comment the code.
• Demonstrate your program to the TA; .
Hint: A common mistake here is to forget that when addressing words in memory,
any word will be located 4 bytes after the previous word, not 1.
4 Deliverables
• Submit completed copy of this lab manual.
• Include the following in your eCampus submission:
– The source code for all .s files.
