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A Gentle Guide to Using the LC-3 Simulator

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0. Conventions

Throughout this document, commands that you have to type or buttonsyou have to click will appear like so.

1. Getting Java

The LC-3 Simulator is written in Java, which means Java must beinstalled on the computer you plan to use. Java should already beavailable on all public SEAS machines. If you plan to work on yourpersonal machine, you may need to install Java yourself. You can download Java here. TheLC-3 simulator requires Java 1.4 or newer (which is available forWindows, Linux, and Mac OS X).

2. Getting the Simulator

Next, you need to download the simulator. It is distributed in a .jarfile (short for Java ARchive) which you can download here. In Windows or on a Mac, youshould be able to double-click the .jar file to launch thesimulator. You can also launch the simulator from the command line ofyour operating system (such as Linux) by using the command java -jar (jarfile). You should see the simulatorscreen, much like the screenshot below.

If you have any problems starting the simulator, please post yourproblems to the CSE240 General Discussion Forum. This will ensure thefastest response.

We will not be distributing the source to the simulator, becausea later assignment will build parts of the simulator in C (which issimilar to Java in many ways).

3. Assembling and Loading Software

Now the simulator is running, but to get it to do anythinginteresting, we need to load some software. The first piece ofsoftware we should load is, naturally, an operating system. The LC-3operating system is very basic: it handles simple I/O operations andis responsible for starting other programs, such as the ones you'llwrite for this homework. Download the LC-3 OShere.

So that you can understand what the operating system does, wedistribute it as an assembly language file. But the LC-3 machinedoesn't understand assembly directly; we first have to 'assemble' theassembly code into machine language (a .obj file containing binarydata). The LC-3 simulator has a built-in assembler, accessible (as isthe case for most of its functionality) via the Command Line text box(see screenshot above). To assemble the operating system, type as lc3os.asm at the command line and hitenter. Make sure that the OS file is in the same directory as the .jarfile; the as command also understandsrelative and absolute paths if the OS is in a different directory. Output from the assembly process is displayed in the CommandLine Output Pane. Afterassembling the OS, you should notice that 2 new files, lc3os.obj andlc3os.sym, have been created. The .obj file is the machine languageencoding of the assembly language file, and the .sym file is a textfile that holds symbol information so the simulator can display yoursymbols. Recall that symbols are really just a convenience for sillyhumans; the machine language encoding knows only about offsets.

Now we can load the lc3os.obj file into the simulator, either via thecommand load lc3os.obj or by going to theFile menu and selecting Open .obj file. Notice that the contents of thememory change when the OS is loaded.

Now assemble and load the solution file forProblem 0 into the simulator. The memory has changed again, but youmay not notice since the relevant memory addresses (starting at x3000)aren't visible unless you've scrolled the screen. User-level programs(i.e., non-OS code) start, by convention, at x3000. If you type thecommand list x3000 the memory view willjump to x3000 and you can see the 1-instruction solution to thisproblem.

4. Running Code

To actually run code, you can use the 4 control buttons at the top ofthe simulator, or type commands into the command line interface (thecommand names are the same as the buttons). Note that the PC registeris set to x0200, which is the entry point to the operating system byconvention. Recall that the solution code for Problem0 increments the value in R2 and puts the result in R5. Set thevalue in R2 to something you fancy, either by double-clicking it inthe Registers section, or via the command set R2(value). Now, actually run the code by hitting the continue button. The value magically getsincremented and moved to R5. Neat-o! Also observe that the PC nowpoints to x0263, an apparently random value somewhere in the operatingsystem, and that the Status Display tells us the machine is halted.

4a. Running Code...slowly

Clearly, some things are going on here. But to determine what they areexactly, we need to slow the execution down. You can hit the stop button to pause the machine while it'sexecuting, but this doesn't give you very fine-grained control. Thecontinue command will start runninginstructions as fast as possible, but often we want to go just oneinstruction at a time. This is what the next and stepcommands allow us to do. They both go one instruction at a time, but thestep command will 'follow' function callsand traps, while next just goes from lineto line, 'over' function calls and traps and stopping only at the instructionimmediately after them. Both next and step will 'follow' branches. The LC-3 Manual has a more involved discussion,with an example, of the difference between next and step.

Let's try running the program again, but just one instruction at atime. Notice that from the halted state, the PC points to aninstruction that will branch us right back to the start of theoperating system. So we can hit next onceand start the cycle over again. Note that registers are as we leftthem. You can put a new value into R2 if you want, and the old valuein R5 will get overwritten. Sometimes, having old values lying aroundeverywhere can be problematic, and it's good to do a real 'reboot' viathe reset command. This clears allof memory and resets registers to default values, so you have toreload the OS and your program.

You can keep next-ing over the OS code;eventually you will hit the RTT instruction at location x0205 thatjumps to the start of our program at x3000. Now you can see the 2instructions that constitute our program. You can see the ADD beingperformed, and then the machine gets halted again.

Continue running the increment-R2-into-R5 code until, if ever, you getbored. Then move on to Problem 1.

4b. Running Code...for a little while

Going one instruction at a time is great, but somewhat tedious. Weneed a happy medium between not knowing what's going on at all, andhaving to go through every single instruction, whether we care aboutit or not. Breakpoints are this happy medium.

A breakpoint is set at a particular memory location, and tells thesimulator to stop execution upon reaching that point. Memory locationswith breakpoints set on them show up in the simulator with a redsquare in the 'BP' column. It is left as an exercise to the reader todetermine what 'BP' stands for. You can set a breakpoint at a memorylocation with the command break set (memorylocation), or by checking the checkbox in the 'BP' column. Youcan get rid of a previously-set breakpoint with the command break clear (memory location), or byun-checking a previously checked box. You can also set and clearbreakpoints at labels, instead of specifying a hex memory location.

When you tell the simulator to continue,it will only run until it hits a breakpoint (or the system halts orhas an error). When you are writing and testing your answer forProblem 1, you can use the command breakpointset START to set a breakpoint at the beginning of yourcode. Then, you can use continue to skipall the OS code and get to the instructions you care about. Then youcan next over your code to make sure it'sdoing what you want it to do.

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5. Running Scripts

Now try running some of the test scripts that we've provided forProblem 1. You can do this with the command script (scriptfile). LC-3 script files are .lcsfiles, but are really just plain text, and the file extension doesn'treally matter. If you open the script file with a text editor, you cansee that the script commands are the same as those you type at thecommand line. For repetitive tasks, using a script file can save youtime, and perhaps the lifelong agony of RepetitiveStrain Injury.

Scripting is also a great way of testing your code. You can write afew test cases and check your code easily, especially for the problemsin this homework which are pretty easy to test. Use the check command to verify that a value is whatyou think it should be. We've distributed a few test scripts for theproblems in this homework, but they don't cover all the importantcases, so you should augment our scripts with some of your own.

6. General Help and Advice

This document doesn't cover all of the simulator's functionality; foran extended discussion of usage see the LC-3Simulator Manual. For quick help within the simulator itself,you can use the help command to see alist of all of the LC-3's commands. Use help(command) to get help on a specific command.

Many of the LC-3 commands have shortcuts - b is short for break, n fornext, and so forth. Use the help (or h)command to see what shortcuts exist.

The LC-3 Command Line has a history feature - use the up and downarrow keys to go backwards and forwards through the commands you'vepreviously entered.

If you resize the simulator window to make it bigger, the CommandlineOutput Pane will grow. If you have a small screen and the CommandlineOutput Pane still isn't big enough, you can open an external, resizableCommand Output Window by selecting the OpenCommand Output Window option from the File menu. This lets you see a lot more outputfrom the commands you run, and is particularly useful for viewing theerrors the assembler generates.

If you have trouble running the simulator, try checking the classforum in case someone else has had the same problem as you. Postingyour questions to the forum is a good idea in general, because thenother people can learn from your experience. The forum is checkedregularly by Professor Lewis and the TAs. Of course, you can alsoemail cse240@seas, or drop by office hours.

If you think you've found a bug in the simulator (which is,theoretically speaking, a possibility:), check the Simulator BugsPage to see if you've found a known bug with a workaround. Ifyou've found a new bug, post to the forums (if you want to embarrass us)or email cse240@seas (if you're feeling kinder). Be sure to include thefollowing:

1. A description of the bug.
2. What you were doing that caused the bug to occur.
3. What version of the simulator you were using. Find this via the simulator'sAbout menu.
4. What version of Java you were using. Find this by running java -version from the commandline of your OS.
5. What operating system you were using - Windows, Linux, Mac.

Also look in the directory where you placed the simulator and checkfor a file called 'lc3sim_errorlog.txt', and email it tocse240@seas. This file is a stack trace generated on some errors,which may help us figure out how to fix the bug. It contains nosensitive system information - it's in plain text, so you can see whatit is that you're sending us.