COMP 273 Introduction to Computer Systems

INSTRUCTIONS

Tutorials B and C will be helpful for this assignment. The code to read and write from the MIPS keyboard and screen has been given in class. See the lecture slides.

Submit your answers in a single ASM text file called mini4.asm. The teaching assistants will hold office hours the week of February 13 to 17. Check myCourses for announcements.

You are expected to do all your work on your own. Plagiarism and cheating area serious offence. You may ask classmates, the TA s, and the professor clarification questions.

QUESTIONS

QUESTION ONE: MIPS Keyboard and Text Screen Drivers

Computers have many types of drivers: printer, keyboard, mouse, video, network, modem, hard disk, soundcard, etc.  Most of these drivers are created using assembler.

MARS only simulates two hardware devices: the keyboard and the text-screen.  These are both referred to as console I/O devices.

To input and display the characters from the console I/O devices you will need to access the MARS keyboard and MARS text screen from the following location:

Note:

In MIPS programming we have the following conventions: a MIPS subroutine can be implemented in two ways, as a driver or as a function. If a MIPS subroutine is implemented as a driver, then it does not use the run-time stack but uses only the register conventions for subroutine access.  In other words, A0-A4 for parameter passing and V0-V1 for returning values. It only uses the run-time stack for saving the S registers.  If a MIPS subroutine is implemented as a function, then it follows the C function calling convention: all parameters, local variables, and  saving registers are placed on the run-time stack, and returned values are placed in V0-V1.

For this assignment, write the code designated as “driver” using the register-passing

convention. Write the code designated as “function” using the C-passing convention using the run-time stack.

The assignment:

Write a MIPS program that prompts the user to enter their first name and then prompts them to enter their lastname.  The program then displays: “You entered: , ” .

Where and are the words the user input.  The program then stops. You will replace the syscall commands you normally use for I/O with your own drivers (as  described below).

Example :

First name: Bob

Last name: Smith

You entered: Smith, Bob.

Your program must adhere to the following instructions :

1.    Create a driver called GETCHAR, similar to the C library function char getchar(void) using MIPS, that interfaces with the MARS keyboard’s status and data registers. This driver, when called returns a single character from the keyboard buffer to register $V0. Do NOT use the OS syscall command.  Access the peripheral directly.  A slide in class was given to help you. Implement this program as a driver using the code presented in class.

2.    Create a driver called PUTCHAR, similar to the C library function void putchar(char c) using MIPS, that interfaces with the MIPS screen’s (console) status and data registers.  This driver, when called, assumes that register $A0 contains the single character that needs to be output.  Only one character is output. Do NOT use the OS syscall command.  Access the peripheral directly. A slide in class was given to help you.  Implement this program as a driver using the code presented in class.

3.   To test this, you will build the following functions in MIPS:

a.    int gets(char *buffer, int limit)

int puts(char *buffer)

The function GETS reads (and function PUTS writes) ASCII from the keyboard into a memory space pointed to by BUFFER (from BUFFER to the screen for PUTS).  It stops reading when the user either presses the enter key or when LIMIT characters is reached (stops printing when NULL is found for PUTS).  The string is terminated with a ‘\0’ (if there is space).  The function also returns the number of characters it read (outputted for PUTS) into $v0.

c.    Calculate the impact on running time in absolute real terms (ie. Time to wait for the file to be loaded)

d.    Where does the DMA lose all its time?

e.    How does DMA compare with polling and interrupts?

WHAT TO HAND IN

Hand in the following to MyCourses:

-     The MARS source code for question one, called mini4.asm

-      A PDF of the question two solutions, called mini4Q2.pdf

-      If you have trouble loading this on myCourses, you can ZIP all the files into mini4.zip.

HOW IT WILL BE GRADED

This assignment is worth 20 points.

Point Deductions:

-      For not following assignment instructions: -3 points.

-      Your program must execute to be graded (part marks will be given if it executes).

- Important that students did not usesyscalls for character and string I/O: -5 points. This is the case everywhere in their program for this assignment. They must use the drivers and functions instead.

Points Awarded:

Question 1