Write an assembly language program to perform a rot13¹ conversion on characters in a text file.  The program should read charatcers from the input file, perform the rot13 conversion, and write the characters to the output file.  The program should read the buffer size (large or small), name of an input file, and the name of an output file from the command line.  For example:

ed-vm% ./rot13  -b l  -i file.txt  -o tmp.txt

The provided main program calls the following routines:

? A getOptions() procedure that reads and checks the command line arguments. The command line arguments must be in the format of:  /b= /i= /o= in that order.  The buffer size should be "l" or "s".  To check the file name, attempt to open the file.  If the files open correctly, the routine should return the file descriptor (for each file).  If there is an error, an appropriate error message should be displayed (see examples).

? A readBuff() procedure to read the input file and return the characters in a buffer.  The procedure will use the buffer address, file descriptor, and buffer size flag to read the file.  The procedure should return the characters in the buffer and the count of characters.  The procedure should also return a flag (true/false) for the last read.

? A rotate13() procedure to perform the rot13 function on the characters in the buffer.  The rot13 function is as follows:

NewCharacter = (Character+13) mod 26

The procedure will use the buffer address and character count.

? A writeBuff() procedure to write the modified buffer to the output file.  The procedure will use the buffer address, file descriptor, and count of characters.

To ensure efficient I/O operations, the program must buffer the data read and written.  For this assignment, the buffer size should be either 2 for the 'small' or 60,000 for the 'large' buffer size.

Note, a utility for testing will be made available on the class web page.

Note, your procedures must be placed into a separate file and linked with the provided main.  Refer to the handout for directions how to link multiple files.  Only the procedures file, not the provided main, will be submitted on-line.  As such, you must not change the provided main!

Testing

A batch file to execute the program on a series of pre-defined inputs will be provided.  The test script compares the program output to pre-defined expected output (based on the example I/O).