Assignment 11

Due at 12 noon, on Monday, December 5, 1994

Grader: Sriram Narasimhan

Purpose

The purpose of this assignment is to modify a version of a memory mapped kernel that handles I/O functions with spin-wait loops.

Problem

The user program for this assignment reads in a user-entered string, and then prints the string back out twice, once forward and once backward.

The difficult part of this assignment is to add code to the incomplete kernel (in ~cs354-1/public/kernel.11) to implement getc, putc, and puts.

Several sample runs of the program might look like:

	Enter a string     : abcde
	The string forward : abcde
	The string backward: edcba

	Enter a string     : 9 3 zzz
	The string forward : 9 3 zzz
	The string backward: zzz 3 9

About I/O

Suggested Plan of Attack

To focus your effort, I suggest the following steps:

Write the user program that has a user enter a string, and then prints the string back out forwards and backwards. Test this code using sim.
Modify the user program completed in step 1 to use syscall instead of getc, putc, and puts. Again, test this code using sim. Keep a separate copy of this program. You will want to use it again for assignment 12.
This is the difficult part of the program. Copy kernel.11 and concatenate it with your program from step 2. The kernel goes first in the resulting source code. Add code to appropriate places in order to implement getc, putc, and puts syscall operations. Use isim to test this code.

How to turn in your assignment

You are to turn in the source code for your program by using the handin11 program. When you have completed the assignment, hand in your source code by typing:


      ~cs354-1/bin/handin11 filename

where filename is the file containing the source code.

Details

The virtual instructions getc, putc, and puts all use the MAL instruction syscall.

Consider the following MAL program:


    .data

prompt: .asciiz "enter a letter: "

    .text
     la   $9, prompt
     puts $9
     getc $10
     add  $10, $10, 1
     putc $10
     .....

After converting the MAL code using syscall instructions, the MAL program will look like:


    .data

prompt: .asciiz "enter a letter: "

    .text
     la   $9, prompt

     li   $2, 4
     move $4, $9
     syscall             # puts $9

     li   $2, 12
     syscall
     move $10, $2        # getc $10

     add  $10, $10, 1 

     li   $2, 11
     move $4, $10
     syscall             # putc $10

     .......

The syscall instruction is similar to a procedure call. Like a procedure call, arguments and results are passed in registers; and, after the syscall has finished, execution begins at the next instruction following the syscall.

A syscall is a request for service from the operating system. The section of the operating system which handles a syscall is known as the kernel.

A syscall differs from jal in a number of ways. There is no address specified in a syscall. The address of the subroutine called by a syscall is fixed at 0x80000080. Unlike a jal instruction, the return address of a syscall is stored in the EPC (exception program counter) rather than a general purpose register. It is also important to remember that registers $k0 and $k1 ($26 and $27) are reserved for the kernel and may be changed by a syscall.

Each syscall service is assigned an operation code. The operation codes for the syscall instructions which you will be implementing during the course of this assignment are:

syscall 			Operation code.
instructions			(value in $2)
puts				4
putc				11
getc				12
done				10

The syscall instruction makes use of the general purpose registers to pass information to and return information from the kernel. The value of the operation code of the instruction to be executed by syscall is placed in register 2. Depending upong the instruction, a value is returned in $2 (as in the case of getc), or an additional value is passed in $4 (as in the cases of putc and puts).

For example,

For the getc instruction, the character that is read in from the keyboard is returned in the least significant byte of register 2.
For the putc instruction, the character to be printed is placed in register 4.
For the puts instruction, the address of the first character of the string is placed in register 4.

To execute the syscall instructions on this simulator, you will need to add the file ~cs354-1/public/kernel.11 in the file with your MAL code.

This can be done using the UNIX cat command.

e.g.	
	cat ~cs354-1/public/kernel.11 MAL_code > final_code.

where MAL_code is the file with your MAL code, and final_code will be the resulting file.

The kernel.11 contains the code that will be run whenever a syscall is executed. However, the will not run as expected on the simulator isim because the kernel.11 doesn't have the code needed to service the putc, puts and the getc syscalls. You must add code to the kernel in to ensure that it handles putc, puts and getc.