CS 3423 Assignment 2 solution


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1. [40 points] Problem Set
From Chapter 2 of Sieberchatz 9th edition book. “OS Structures”
1. [20 points] 2.7: What are the two models of interprocess communication? What are
the strengths and weaknesses of the two approaches?
2. [20 points] 2.10: What is the main advantage of the microkernel approach to system
design? How do user programs and system services interact in a microkernel
architecture? What are the disadvantages of using the microkernel approach?
2. [40 points] Nachos Exercise
Before writing a program to run on Nachos, you need to know where things are and where to
make the change. We will use the following shorthand:
● {code} is for {Nachos}/code
● {test} is for {code}/test
● {userprog} is for {code}/userprog
Before you begin, type the command into your Ubuntu setup for Nachos:
$ sudo apt-get update
$ sudo apt-get install gcc-multilib g++-multilib
2.1 How halt.c Works
To recall, halt.c in the {test} directory from the last assignment is a user program that is
loaded and and run by Nachos. It is compiled by a special version of GCC (in
{Nachos}/usr/local/nachos/bin/decstation-ultrix/bin/, not the regular GCC
for your host system) to an executable file named halt in the MIPS instruction set, so it can
be executed by the nachos command nachos -e halt
Examine the source code for halt.c. It contains the line
#include “syscall.h”
but there is no file named syscall.h in the same directory. Since the Makefile specifies
the rule
INCDIR =-I../userprog -I../lib
GCC will look in two other directories for include files. In this case, syscall.h is found in
{userprog} directory. It defines the trap numbers for the system calls:
#define SC_Halt 0
#define SC_Exit 1
#define SC_Exec 2
#define SC_Join 3

Since Halt() is a system call, it means it is implemented as a trap to the Nachos kernel in
terms of a syscall instruction in MIPS with the code 0 in register $v0 (which is another
name for register $2).
2.2 Trap Handling by Nachos Kernel
In Nachos, traps are handled by the function
{userprog}/exception.cc:ExceptionHandler(). It is called by the (simulated) MIPS
processor when executing a trap. It contains a switch statement
switch(type) {
case SC_Halt:
2.3 [5 points] System Call Implementation
Note that Halt() is called a stub function, because it just a way for C programs to make the
call, but it redirects it to the kernel. You will not find it in any of the .c or .cc files; instead, it
is written in assembly language, because it involves the use of machine registers and
instructions. Assembly language files have .S in their names.
[5 points] On line {test}/start.S:48, you have the MIPS assembly instruction
addiu $2, $0, SC_Halt
which is another way of saying
register2 = 0 + SC_Halt;
Explain the purpose of this assignment statement.
2.4 [35 point] Hello World in Nachos
Suppose you want to write a Hello World program to run in Nachos by making a copy of
halt.c as hw2.c and calling Write():
#include “syscall.h”
int main() {
Write(“hello world\n”, 12, 1);
However, the Write() system call has not been implemented in the given
{userprog}/exception.cc source file, so it will crash. Nachos kernel can be modified to
handle the Write() system call.
2.4.1 Answer the following questions for the Write() system call. Hint: the answers can
be found in the source code of exception.cc, including the comments!
● [5 points] How does the kernel get the pointer to the array of bytes to write? Give the
description and C code.
● [5 points] How does the kernel get the number of bytes to write? Give the description
and C code.
● [5 points] How does the kernel return the value to the caller? (for the number of
bytes written?) Give the description and C code.
2.4.2 [10 points] Modify exception.cc so that it
● displays that Write is being called
● displays the pointer value (not the content) in hex using a DEBUG statement,
● displays the number of bytes to write using a DEBUG statement,
● copy the size parameter value as its return value.
2.4.3 [5 points] Modify {test}/Makefile to build hw2 as an executable. To do so, search
for halt in the Makefile and copy it for hw2. You should be able to run make in
the {test} directory to build hw2 as an executable.
2.4.4 [5 points] rebuild Nachos in the {code}/build.linux directory. If successful, you
should be able to run (from {test} directory) ../build.linux/nachos -d u -e
For parts 2.4.3 and 2.4.4, submit a typescript showing compilation and running your code.
To force recompiling, you and do a make clean on both hw2.c (in {test}) and Nachos itself (in
{build.linux}). In the following, $ is the prompt character:
$ script
Script started, file is typescript
$ cd {build.linux}
$ make clean
$ make
$ cd {test}
$ make clean
$ make
$ {build.linux}/nachos -d u -e hw2
$ ^D
Script done, file is typescript
You will find a file named typescript in the directory where you started. Submit the unedited
typescript. However, if your code fails to run correctly for any reason, then you must
make an appointment with a TA to get graded in person.
3. [20 points] Python Programming
This week, you are to write the YieldBST(T) function, which is a slight modification to the
PrintBST(T) function you wrote for Assignment 1. The difference is that you use the
yield keyword instead of print, so that it can be used as an iterator that feeds a for
loop. One thing you do have to do is that if your function calls another function that does a
yield (including recursive calls), then you would have to say yield from before calling that
You should be able to do this in your test case:
if __name__ == ‘__main__’:
L = []
T = … # from last week
for v in YieldBST(T):
print(L) # print list
when you run it as a top-level file
$ python hw2.py
and then it should print out
[6, 12, 14, 17, 32, 35, 40]
The use of yield keyword in a function makes it a generator. You can think of it as a
coroutine, i.e., a function that can continue execution after it produces return values. It is not
quite a thread yet, but has some flavor of concurrency. The difference is generators are