Sample Homework Assignment #3

Overview

This is a “get your feet wet with assembly programming” assignment.

Solutions

Solutions are available in this archive

Exercise 1: Warm Up

Write an assembly program called hw3_ex1, with source in file hw3_ex1.asm, that prompts the user for one lower-case letter and one integer. Then the program prints out the upper-case version of the letter and the opposite of the integer.

Below are program invocation examples (‘%’ is the command-line prompt, and user input is shown as well), and your program should match that output format:

% ./hw3_ex1
Enter a lower-case letter: a
Enter an integer: 12
The upper-case letter is 'A'
The opposite integer is '-12'
% ./hw3_ex1
Enter a lower-case letter: z
Enter an integer: -43
The upper-case letter is 'Z'
The opposite integer is '43'
% ./hw3_ex1
Enter a lower-case letter: u
Enter an integer: 0
The upper-case letter is 'U'
The opposite integer is '0'

You can assume that the user only enters valid input!

Hints:

• Reading a character from the keyboard is done via the read_char macro (see page 17 of the textbook for a description). So you’ll have to call this macro twice: once to get the lower case letter and a second time to consume the linefeed.
• Remember that when you print a string it should be null-terminated, meaning that it should have an extra byte with value zero at the end.

Exercise #2: String transformation

Write an assembly program called hw3_ex2, with source in file hw3_ex2.asm, that prompts the user for a 3-character string of lower-case letters, s1, and then for a 4-character string, s2. The program then prints, as part of a message, the following “encoded” string: s2[3] S1[0] S1[0] s2[2] S1[1] S1[1] s2[1] S1[2] S1[2] s2[0] (where “S1[i]” means the upper-case version of “s1[i]”).

For instance, if s1=”xyz” and s2=”aBcT”, the output should be “TXXcYYBZZa”.

Below are program invocation examples (‘%’ is the command-line prompt, and user input is shown as well). Your program should match that output format exactly:

./hw3_ex2
Enter a 3-character lower-case string: abc
Enter a 4-character string: #$%^
The encoded string is: ^AA%BB$CC#
% ./hw3_ex2
Enter a 3-character lower-case string: xyz
Enter a 4-character string: qwer
The encoded string is: rXXeYYwZZq
% ./hw3_ex2
Enter a 3-character lower-case string: yup
Enter a 4-character string: 1234
The encoded string is: 4YY3UU2PP1

You may assume that the user will always type exactly 3 lower-case characters for the first string followed by a carriage return (linefeed) and 4 characters for the second string followed by a carriage return (linefeed).

Even with our very limited knowledge of assembly, there are quite a few ways to write this program. And yes, because we can’t do loops yet, sections of code will have to be cut-and-pasted (which is very bad practice but unavoidable for now).

Hints:

• Reading a character from the keyboard is done via the read_char macro (see page 17 of the textbook for a description). You’ll have to call this macro 4 times (three times to get the 3 characters in s1 and a fourth time to consume the linefeed) and then 4 times (4 times to get the 4 characters in s2 and a 5th time to consume the linefeed).
• Remember that when you print a string it should be null-terminated, meaning that it should have an extra byte with value zero at the end.

BOBUS CHALLENGE:

• Your program uses no more than 3 calls to print_string and no call to print_char