An HDL editor for use with From Nand to Tetris HDL files.
From Nand to Tetris is a project by Noam Nisan and Shimon Schocken to enable "students, instructors, and self-learners" everything they need to know to build a "general-purpose" computer from scratch. The site I have linked above provides most of the materials needed for that endeavor, except for an HDL editor. That's where Hed comes in.
I decided to write Hed when I started a video series based on Nand2Tetris to help users edit these files.
The program is easy to use. Download the latest release, unzip it, and execute one of the install files to install the program. You may receive a warning about installing software from "unknown sources." Please ignore those warnings to complete the install.
When you first run Hed it will look like this:
The first thing you should do at this point is to open the "Settings" window by clicking on "Settings" and then "Resources."
You will see another window that looks like this:
At a minimum, select a "default save folder" in the "Save folder" field. You can use the "Select" button to its right to open the folder browser to find the folder you want to use. Normally you would choose one of the project folders from within the Nand2Tetris resource directories that you would get from Nisan's and Schocken's site above. I am referring to the fourteen numbered folders under the "projects" folder.
Completing the rest of the fields in the window is useful but not necessary. For example, the "Components folder" field specifies a directory where preexisting components might be found. I use this to point to the Nand2Tetris "builtInChips" folder.
Once you are satisfied with your changes, click on the "Save" button on the bottom right of the window. Otherwise, the settings will not be saved for the next time you execute Hed.
Now you can really start using Hed.
Back in the main window, open the "File" menu and click "Open." You should see an "Open an hdl file" window opened on the directory you selected as your default save folder in the "Settings" window. From this window, you can open any available HDL file.
I have opened the "Not.hdl" file in the following screenshot.
The "Not" gate is just about the simplest logic gate, so there isn't much to see in this window. But you should see a couple of changes. First, the name of the gate is now specified in the "Gate Name" field, and one input pin called "in" is specified in the grid below the seven tab headers right below the "Gate Name."
"Input Pins" is highlighted, meaning we are looking at input pins in the grid below. The "Not" gate has only one input pin. The gate also has only one output pin called "out." But on the "Parts" page, zero parts are specified.
That's because the file we loaded is only a stub file. It provides the inputs and outputs, and describes what the logic gate is supposed to do, but it is left for someone else to supply the details about its implementation.
I will do that now to show it. First, I click within the cell below "Name" and type the letter "N." (HDL is case sensitive, so it's important to consider capitalization here. Most preexisting gates in Nand2Tetris start with capital letters. Typing a lowercase "n" here would not produce the same results.)
A list of logic gates that Hed knows about starting with the letter "N" appears. At this point, the only defined gate in the list is the "Nand" gate, which we will use to create the "Not" gate.
We can either select the "Nand" gate from the list or type out "Nand" to select it. From the "Name" cell, we can tab over to the "Description" cell to give some detail about it. This is the only gate we need for "Not," so we say so.
We have to use the grid on the right to edit the part's connections. With the "Nand" part selected on the left, click on the row that says, "Click here to add a new row" in the "Selected Part Connections" grid.
Now we can start editing here.
Under "Part Pin," we will type "a" to specify one of the "Nand's" inputs and then press the tab key to move to the "External Pin" column. This column is where pins external to the selected part are specified. In this case, we want to specify the logic gate's "in" pin, so we type "in" and press tab key again.
The "Nand" gate has two inputs. We are going to tie the "Not's" input pin to both of those pins, so for the second row, we type "b," tab, "in," and tab once again.
You might notice a change under "Connections" now on the left. The "Connections" column on the left is non-editable; it reformats the data you enter in the second grid so it can be seen whether the row is selected or not.
We still have a little more work to finish the "Not" gate's connections, which is to hook up its out pin. If necessary, click on the bottom row under "Part Pin" in the second grid. Then type "out," tab, and then "out" again. Here the first "out" refers to the "Nand" gate's out pin, and the second "out" refers to the "Not" gate's out pin.
The "Header Text," "API Text," and "Description Text" pages contain whatever was in the original HDL stub file we loaded at first. For now, we won't make any changes there. But let's look at the "File View" tab.
There's our completed HDL file, including the changes we made under the "PARTS" section. All we need to do is save the file, and we will be ready to load the file up in the Nand2Tetris Hardware Simulator and run tests to see if it works as expected.
Saving is accomplished by clicking on the disk icon in the window's upper right. Hed will save the file to a file named "Not.hdl" (named after the logic gate's name) directly in the default save directory you specified in the "Settings" window. Since that's the directory from which we loaded the original "Not.hdl" file, that file will be overwritten with our changes. If that is not your intent, you should save a copy of the original to another location before saving the changes to the file.
Well, that's about it. I have included a Users Guide for Hed in the install directory but, honestly, I don't think you will need it. I hope you enjoy this tool, and that it helps in your study of logic gate design.