created 06/27/2003

Chapter 18 Programming Exercises

For these programming exercises, use only those instructions that have been discussed so far in these notes:

 add div mflo slt, slti addi divu mult sltu, sltiu addiu j multu sra addu lb nor srl and lbu or sub andi lh ori subu beq lhu sb sw bgez lui sh xor bltz lw sll xori bne mfhi

In the Settings menu of SPIM set Bare Machine ON, Allow Pseudo Instructions OFF, Load Trap File OFF, Delayed Branches ON, Delayed Loads ON, Mapped IO OFF, Quiet OFF.

Run the programs by setting the value of the PC to 0x400000 and then single stepping (pushing F10) or by multiple stepping (push F11 and enter a number of steps). Observing the results in the SPIM window.

*Exercise 1 — Sum of Evens; Sum of Odds

Write a program that computes three sums:

1 + 2 + 3 + 4 + ... + 99 + 100

1 + 3 + 5 + 7 + ... + 97 + 99

2 + 4 + 6 + 8 + ... + 98 + 100

Use a register (say \$8) for the sum of evens, a register (say \$9) for the sum of odds, and another (say \$10) for the sum of all numbers.

Do this with one counting loop. The loop body contains logic to add the count to the proper sums.

*Exercise 2 — Significant Bits

With an ori instruction, initialize \$8 to a bit pattern that represents a positive integer. Now the program determines how many significant bits are in the pattern. The significant bits are the leftmost one bit and all bits to its right. So the bit pattern:

0000 0000 0010 1001 1000 1101 0111 1101

... has 22 significant bits. (To load register \$8 with the above pattern, 0x00298D7D, use an ori followed by a left shift followed by another ori.)

**Exercise 3 — Allowed Ranges

A temperature in \$8 is allowed to be within either of two ranges: 20 <= temp <= 40 and 60 <= temp <= 80. Write a program that sets a flag (register \$3) to 1 if the temperature is in an allowed range and to 0 if the temperature is not in an allowed range.

It would be helpful to draw a flowchart before you start programming.

****Exercise 4 — Median of Three

Write a program that computes the median of three values in memory. After it has been found, store the median in memory.

.data
A:    .word 23
B:    .word 98
C:    .word 17

The median of three integers is greater than or equal to one integer and less than or equal to the other. With the above three integers the median is "23". Assume that the data changes from run to run. Here is some more possible data:

.data
A:    .word 9
B:    .word 98
C:    .word 9

With the new data, the median is "9".