Project 7: Qsort in Assembly, UMBC CMSC313 Fall 2012

CMSC313, Computer Organization & Assembly Language Programming, Fall 2012

Project 7: Qsort in Assembly

Due: Thursday November 8, 2012, 11:59pm

Objective

The objective of this programming exercise is to practice writing assembly language programs that use the C function call conventions.

Assignment

Your assignment is to write an assembly language program that asks the user to enter a sequence of numbers in stdin then outputs the same sequence sorted in increasing order to stdout. The numbers must be input and output in base 10 and stored as signed 32-bit numbers. You must use the C functions scanf() and printf() for I/O. You must use the qsort() function from the C standard library to sort the data. Using these C functions makes the program much shorter.

The qsort() function has the following function prototype:

void qsort(void *base, size_t nmemb, size_t size, int (*compar)(const void *, const void *)); The first parameter to qsort() is the address of the "array" to be sorted. The second parameter is the number of items in the array. The third parameter specifies the size of each element of the array in bytes (in our case, 32 bits equals 4 bytes). The last parameter is the entry address of a comparison function. You must write this comparison function in assembly language. The comparison function takes two pointers to 32-bit signed integers and returns a negative number, zero or a positive number if the first integer is respectively less than, equal to or greater than the second integer.

A sample run of your program might look like:

linux3% ./a.out
Enter some integers (no more than 100), end with ctrl-D.
? 9
? 12
? 7
? 14
? 3
? 21
? 1
? 
Here are the numbers in sorted order
1
3
7
9
12
14
21
linux3%

Implementation Notes:

Your programming experience will go a lot more smoothly, if you follow these steps:
1. Review the notes on C function call conventions here. This was discussed extensively in class on Thursday, October 25. You will not be able to do the project if you do not understand these conventions.
2. Write a program that uses printf() to print out a message.
3. Add some code to read in a single number using scanf().
4. Use gdb to check that the number was read in correctly.
5. Use a loop to keep reading numbers using scanf() until the user enters ctrl-D.
6. Use gdb to check that the numbers were read in correctly.
7. Use a loop to print out the numbers using printf().
8. Add a call to qsort() between the two loops.
9. Make sure the user doesn't enter more than 100 numbers.
Your assembly language program should implement a main() function. Then you can compile and link your program with:
```
   linux3% nasm -g -f elf proj7.asm
   linux3% gcc -g proj7.o
```
Notes on using gcc to link assembly language programs that make calls to standard C library functions are available here.
You may assume that the user will not enter more than 100 numbers. If the user exceeds the capacity of your array, your program should generate an error.
You should reserve memory in the .bss section to hold the numbers entered by the user. This is your "array".
When the user enters ctrl-D, scanf() will return a value less than or equal to 0. That's how you know when to stop. The return value is stored in the EAX register. (See notes on C function call conventions here.)
Even the main() function has to preserve the EBX, ESI and EDI registers.
When you call printf(), scanf() and qsort(), the EAX, ECX and EDX registers will be clobbered. Your two choices are to avoid these registers or save them.
Remember that your comparison function will be given pointers to int values. You have to dereference.
Yes, you can use immediate operands with the PUSH instruction.
Remember that scanf() and printf() expect null-terminated strings. NASM does not add a null character at the end of its string constants. You must add the 0 yourself.
In your assembly language program, you must declare scanf(), printf() and qsort() to be extern.
You have to call your assembly language implementation of the main function main. You also have to make main global.
You can use I/O redirection to test your program on larger input.

Turning in your program

Use the UNIX submit command on the GL system to turn in your project. When you are done, use the script Unix command to record yourself running your program a few times. You do not have record yourself using gdb for this project.

You should submit 2 files: proj7.asm and typescript. The UNIX command to submit should look like:

submit cs313 proj7 proj7.asm typescript

Last Modified: 31 Oct 2012 19:48:27 EDT by Richard Chang

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