CMSC313, Computer Organization & Assembly Language Programming, Spring 2013

Project 7: String Functions in Assembly

Due: Tuesday April 16, 2013, 11:59pm

Objective

Assignment

The two functions that you must implement in assembly language have the following prototypes:

The strrev() function reverses the characters in the memory location addressed by the pointer str. For example, if strrev() is given the string "Hello World", then the string is changed to "dlroW olleH" after calling strrev(). In the call strrev(ptr), the pointer ptr must point to a memory location that can be modified. The call strrev("abcd") will result in a segmentation fault because "abcd" is a constant string.

The strrepl() function replaces each occurrence of a subset of characters in string addressed by str with the character c. The subset is specified by the third parameter, a function pointer, isinsubset(). The return value of strrepl() is the number of characters that were replaced.

For example, the following call to strrepl()

will change the string addressed by str1 to "ABC ### ### Hello World" and will return the value 6. Here isdigit() is a standard C Library function that returns non-zero if given a digit character ('0' – '9') and returns zero otherwise.

Similarly, the call

The function pointers passed as the third parameter to strrepl() must have the function signature:

Even though the intention is to pass a character to these functions, the parameter is nevertheless an int. This is because the end-of-file character (EOF) is often represented as -1, and C has traditionally allowed EOF to be passed to functions such as isdigit. (See man pages for ctype.)

A test program, main7.c, has been provided for you along with its output: main7.txt. As usual, testing is your responsibility and you should exercise your functions with your own test programs.

Implementation Notes:

• Place your assembly language implementation of the two functions in separate files: strrev.asm and strrepl.asm.

• Please name your test program test7.c.

• You must make labels strrev and strrepl global in their respective .asm files (and you must have labels called strrev and strrepl).

• Examples of assembly language programs using C function pointers are available here: Lecture 18.

• Remember that C strings are null-terminated and that the only way to determine the length of the string is to find the null character.

• When a char is passed to an int parameter, it is extended to 4 bytes in little endian. (Actually, whenever a char is passed, 4 bytes are pushed on the stack.)

• In our previous assembly language programs, we used the CALL instruction only with immediate operands: call subroutine1 However, CALL also allows r/m32 operands, so the address of the target subroutine can also be in a 32-bit register or a memory location.

• Both strrev() and strrepl() should do nothing when called with a NULL pointer. In particular, strrev() and strrepl() should not crash in these situations. Additionally, strrepl() should return 0 for the number of characters replaced.

• Review the C Function Call Conventions regarding which registers must be saved and restored and which may be altered by a function.

• Remember that when you call a C function from your assembly code, the C function is allowed to change some of the registers. So, choose your registers carefully.

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. You do not have record yourself using gdb for this project.

You should submit 4 files: strrev.asm, strrepl.asm, test7.c and typescript. The UNIX command to submit should look like:

submit cs313 proj7 strrev.asm strrepl.asm test7.c typescript