CS2200 - Systems and Networks: Project 1 Solved

Requirements
•    Download the proper version of CircuitSim. A copy of CircuitSim is available under Files on Canvas. You may also download it from the CircuitSim website (https://ra4king.github.io/CircuitSim/). In order to run CircuitSim, Java must be installed. If you are a Mac user, you may need to right-click on the JAR file and select “Open” in the menu to bypass Gatekeeper restrictions.

•    CircuitSim is still under development and may have unknown bugs. Please back up your work using some form of version control, such as a local/private git repository or Dropbox. Do not use public git repositories; it is against the Georgia Tech Honor Code.

•    The LC-900 assembler is written in Python. If you do not have Python 2.6 or newer installed on your system, you will need to install it before you continue.

2         Project Overview and Description
Project 1 is designed to give you a good feel for exactly how a processor works. In Phase 1, you will design a datapath in CircuitSim to implement a supplied instruction set architecture. You will use the datapath as a tool to determine the control signals needed to execute each instruction. In Phases 2 and 3, you are required to build a simple finite state machine (the “control unit”) to control your computer and actually run programs on it.

Note: You will need to have a working knowledge of CircuitSim. Make sure that you know how to make basic circuits as well as subcircuits before proceeding. The TAs are always here if you need help.


3          Phase 1 - Implement the Datapath
 

Figure 1: Datapath for the LC-900 Processor
In this phase of the project, you must learn the Instruction Set Architecture (ISA) for the processor we will be implementing. Afterwards, we will implement a complete LC-900 datapath in CircuitSim using what you have just learned.

You must do the following:

1.    Learn and understand the LC-900 ISA. The ISA is fully specified and defined in Appendix A: LC-900

Instruction Set Architecture. Do not move on until you have fully read and understood the ISA specification. Every single detail will be relevant to implementing your datapath in the next step.

2.    Using CircuitSim, implement the LC-900 datapath. To do this, you will need to use the details of the LC-900 datapath defined in Appendix A: LC-900 Instruction Set Architecture. You should model your datapath on Figure 1.

3.1     Hints
3.1.1       Subcircuits
CircuitSim enables you to break create reusable components in the form of subcircuits. We highly recommend that you break parts of your design up into subcircuits. At a minimum, you will want to implement your ALU in a subcircuit. The control unit you implement in Phase 2 is another prime candidate for a subcircuit.

3.1.2       Debugging
As you build the datapath, you should consider adding functionality that will allow you to operate the whole datapath by hand. This will make testing individual operations quite simple. We suggest your datapath include devices that will allow you to put arbitrary values on the bus and to view the current value of the bus. Feel free to add any additional hardware that will help you understand what is going on.

3.1.3        Memory Addresses
Because of CircuitSim limitations, the RAM module is limited to no more than 16 address bits. Therefore, per our ISA, any 32-bit values used as memory addresses will be truncated to 16 bits (with the 16 most significant bits disregarded). We would like you to implement this (i.e. truncate the most significant bits) before feeding the address value from the MAR (Memory Address Register) to the RAM.

3.1.4        Comparison Logic
The “comparison logic” box in Figure 1 is responsible for performing the comparison logic associated with the SKP instructions. The comparison logic should read the current value on the bus plus the mode bits (CmpSel) from the IR. When executing the SKP instructions, you should compute A−B using the ALU. While this result of the ALU is being driven on the bus, the comparison logic should read the result A−B and output a single “true” or “false” bit for the selected condition (CmpSel).

Your comparison logic should be purely combinational. Feel free to use any CircuitSim components you wish to aid in your implementation. Note that the three outputs of a comparator are gt, eq, lt which correspond to bits 2, 1, and 0 in the SKP instruction. Also note that bits 0:2 can be obtained from the value chosen by RegSel.

3.1.5        Register File
You must implement your own register file. That is to say, you cannot use CircuitSim’s built-in RAM to create the register file. Consider what logic components you may want to use to implement addressing functionality (multiplexers, demultiplexers, decoders, etc). Your zero register must be implemented such that writes to it are ineffective, i.e., attempting to write a non-zero value to the zero register will do nothing. Do not forget to do this or you will lose points!

3.1.6        Register Select
From lecture and the textbook, you should be familiar with the “register select” (RegSel) multiplexer. The mux is responsible for feeding the register number from the correct field in the instruction into the register file. See Table 4 for a list of inputs your mux should have.

4          Phase 2 - Implement the Microcontrol Unit
In this phase of the project, you will use CircuitSim to implement the microcontrol unit for the LC-900 processor. This component is referred to as the “Control Logic” in the images and schematics. The microcontroller will contain all of the signal lines to the various parts of the datapath.

You must do the following:

1.    Read and understand the microcontroller logic:

•    Please refer to Appendix B: Microcontrol Unit for details.

•    Note: You will be required to generate the control signals for each state of the processor in the next phase, so make sure you understand the connections between the datapath and the microcontrol unit before moving on.

2.    Implement the Microcontrol Unit using CircuitSim. The appendix contains all of the necessary information. Take note that the input and output signals on the schematics directly match the signals marked in the LC-900 datapath schematic (see Figure 1).

5         Phase 3 - Microcode and Testing
In this final stage of the project, you will write the microcode control program that will be loaded into the microcontrol unit you implemented in Phase 2. Then, you will hook up the control unit you built in Phase 2 of the project to the datapath you implemented in Phase 1. Finally, you will test your completed computer using a simple test program and ensure that it properly executes.

You must do the following:

1.    Open and fill out microcode.xlsx file we’ve provided you (note: the formulas in the provided file will not work with Numbers, but work with Excel and Google Sheets). You will need to mark which control signal is high (that is 1) for each of the states.

2.    After you have completed all the microstates, convert the binary strings you just computed into hex and move them into the main ROM. You can just copy and paste the hex column (highlighted yellow) from the spreadsheet directly into the ROM component in Circuitsim. Do the same for the sequencer and condition ROMs.

3.    Connect the completed control unit to the datapath you implemented in Phase 1. Using Figures 1 and 2, connect the control signals to their appropriate spots.

4.    Finally, it is time to test your completed computer. Use the provided assembler (found in the “assembly” folder) to convert a test program from assembly to hex. For instructions on how to use the assembler and simulator, see README.txt in the “assembly” folder. Note: The simulator does not test your project, it simply provides a model. To test your design, you must load the assembled HEX into CircuitSim. We recommend using test programs that contain a single instruction since you are bound to have a few bugs at this stage of the project. Once you have built confidence, test your processor with the provided pow.s program as a more comprehensive test case.

6      Deliverables
Please submit all of the following files to Gradescope in a .tar.gz archive generated by one of the following:

•    On Linux/Mac: Use the provided Makefile. The Makefile will work on any Unix or Linux-based machine (on Ubuntu, you may need to sudo apt-get install build-essential if you have never installed the build tools). Run make submit to automatically package your project into the correct archive format.

•    On Windows: Use the provided submit.bat script. Submitting through this method will require 7zip (https://www.7-zip.org/) to be installed on your system. Run submit.bat to automatically package your project into the correct archive format.

The generated archive should contain at a minimum the following files:

•    CircuitSim datapath file (LC-900.sim)

•    Microcode file (microcode.mc)

Always re-download your assignment from Gradescope after submitting to ensure that all necessary files were properly uploaded. If what we download does not work, you will get a 0 regardless of what is on your machine.

This project will be demoed. In order to receive full credit, you must sign up for a demo slot and complete the demo. We will announce when demo times are released.