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Assignment 1 – EECS 211

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Assignment 1 – EECS 211
Basics of Context Switching

Problem 1
Currently, the LC3 VM implements the “Trap” and “Interrupt” handling di↵erently than how
the hardware actually works. However, since the OS context switching depends mainly on the
interrupts and traps, it is important to re-implement this part in the LC3 VM to match what
happens in an actual hardware.
A trap occurs whenever the assembly instruction “TRAP (op code = 0xF)” is used. The assembly
instruction also contains a number called the trap vector (trapvect8). When the CPU executes the
trap, it performs the following two steps:
• The value in the program counter (PC) register will be copied to R7
• The PC will point to the address saved in Memory[trapvect8].
Let’s consider the example below:
Listing 1: Assembly Code # 1
1 .ORIG 0x0025
2 .FILL TRAP_HALT
3
4 .ORIG 0x0100
5 TRAP_HALT
6 …. ; put your Trap Service Routine code here
7
8
9
10 .ORIG 0x3000
11 …. ; the software code starts from here
12 TRAP 0x25 ; this is exactly equivalent to .HALT
Recall that the HALT trap has a trapvect8 equal to 0x25. In the example above, the value of
0x0100 is saved inside the memory location 0x25. Therefore, executing the HALT trap assembly
instruction will force the PC will point to the address 0x0100.
In this problem, make the following changes to the LC3 VM:
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1. Change the LC3 VM such to implement the right sequence of actions for the traps. Recall
this is the trap vector for the LC3 VM:
• TRAP GETC = 0x20, /* get character from keyboard, not echoed onto the terminal */
• TRAP OUT = 0x21, /* output a character */
• TRAP PUTS = 0x22, /* output a word string */
• TRAP IN = 0x23, /* get character from keyboard, echoed onto the terminal */
• TRAP PUTSP = 0x24, /* output a byte string */
• TRAP HALT = 0x25 /* halt the program */
2. If the software uses the TRAP instruction with any other value for trapvect8 (other than the
ones above), your VM should dump the memory/registers to a file (using the same template
used in the previous assignment).
3. Now you can write code for each trap service routines (which is considered the first part of
your OS). The code for the trap service routines will be located at 0x0100. For now, will
have very simple trap service routines that simply uses TRAP 0xFF to dump the memory
followed by an infinite loop.
To test your code, use the simple code below:
Listing 2: Assembly Code # 2
1 .ORIG 0x3000
2 TRAP 0x25 ; this is exactly equivalent to .HALT
Deliverable
You need to upload one file named “memory dump p1 1” that contains the memory/registers for
the assembly code #2 above.
Problem 2
In this problem, we are going to edit the HALT trap to implement a simple OS scheduler. This OS
scheduler will go through the available processes and start executing them in order.
To simplify the implementation, we will make the following assumptions. First, the maximum
number of processes that can be loaded in the memory is equal to 5. The OS will keep track of the
state of each process using the following data structure:
• Status of the first process (unsigned integer 8)
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• Status of the second process (unsigned integer 8)
• Status of the third process (unsigned integer 8)
• Status of the fourth process (unsigned integer 8)
• Status of the fifth process (unsigned integer 8)
A value of 1 means that the corresponding process is ready to run, a value of 2 means that this
process is the one that is currently running, and a value of 0 means that the corresponding process
has terminated. This data structure should be saved in the location 0x0200 which marks the
beginning of the OS stack section.
The scheduler should go through the state variables above (in order), and once it finds a process
that is ready to execute, it will jump to the code of the corresponding process. When the process
halts (by using the HALT trap), the OS will change the status of this process from 1 to 0 and
then search for another process to run. To simplify the implementation, we will assume the code of
the first process will be located in 0x4000, the second process will be located in 0x5000, the third
process will be located in 0x6000, and so on. If no process needs to run, then the scheduler will
enter an infinite loop.
To debug the scheduler and make sure it is implemented correctly, the scheduler should force a
dump of the memory at the very beginning of the HALT trap service routine (by using TRAP
0xFF instruction) and at the very end (just before it switches to the next process).
The main body of the OS is located at 0x3000 (where the CPU starts executing). For this simple
OS, it just needs to initialize the values in the state variables followed by forcing the scheduler to
operate by using a trap instruction.
Deliverable
Write the code of 5 simple (dummy) processes that will be located in 0x4000, 0x5000, 0x6000,
0x7000, and 0x8000, respectively. These processes should perform the following instructions:
1. Load a certain value in the register R1. For example, the first process should load the value
of “1“ into the register R1, the second process should load the value of “2” into the register
R1, and so on.
2. Halt.
Implement the scheduler described above to switch between these five processes. You will upload
one file called “os.asm”, one zip file that contains your VM, a read-me file explaining how to
use your VM, and 12 memory dump files called “memory dump p2 1”, “memory dump p2 2”,
“memory dump p2 3”, …, etc, that corresponds to the memory dumps issued during the switches
between these processes.
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Problem 3
In this problem, we are going to increase the set of allowed Traps. In particular, we would like to
add a trap for “YIELD”. Such trap can be used by the software to inform the OS that it would
like to yield the processor for now so the OS can run another process/thread. The main di↵erence
between YIELD and HALT is that HALT means that the software finished all its instructions and
does not need the processor anymore, while YIELD means that the process still needs the processor
in the future.
• Edit the LC3 VM to add another trap called “TRAP YIELD”. This new trap should be
assigned a trap vector of 0x26.
• Write a trap service routine for the TRAP YIELD. This trap service routine should change
the status of the process back to 1 (which means ready) followed by calling the scheduler
function to pick a new process to execute.
Deliverable
Write the code of 5 simple (dummy) processes that will be located in 0x4000, 0x5000, 0x6000,
0x7000, and 0x8000, respectively. These processes should perform the following instructions:
1. Load a certain value in the register R1. For example, the first process should load the value
of “1“ into the register R1, the second process should load the value of “2” into the register
R1, and so on.
2. Yield.
3. Increment the value of R1 by 2.
4. Halt.
Augment the scheduler to implement the Yield functionality. You will upload one file called
“os.asm”, one zip file that contains your VM, a read-me file explaining how to use your VM,
and the first 12 memory dump files called “memory dump p3 1”, “memory dump p3 2”, “memory dump p3 3”, …, etc, that corresponds to the memory dumps issued during the switches between
these processes.
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