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Added 'Computer From Scratch' article 

Signed-off-by: Collin J. Doering <collin.doering@rekahsoft.ca>
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Collin J. Doering 2015-07-19 03:19:26 -04:00
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@ -1,7 +1,7 @@
---
title: Computer From Scratch
author: Collin J. Doering
date: Jun 19, 2015
date: Jul 19, 2015
description: Building a computer from scratch using VHDL
tags: general, programming, hardware, nand-to-tetris
---
@ -67,22 +67,23 @@ $ vcd2fst wave/vcd/computer-fib.vcd wave/vcd/computer-fib.fst
First, clone the sources, then import and build them with `ghdl`. Finally run the simulation
using the default input program and output a vcd file to `src/wave/vcd/computer-fib.vcd` which
is then converted to a `fst` file using `vcd2fst`. Now once the simulation has complete the
*vcd* or *fst* file can be opened with a wave form viewer. I recommend and have only tested
with [GtkWave][]. To open the dump file open [GtkWave][] and go to `File -> Open New Tab` (or
`C-t`) and select the output file (in our case `src/wave/vcd/computer-fib.fst`). I have
provided pre-saved "views" for `computer_tb` (as well as the other entities within the project)
so that the signals of interest for the entity in question don't need to be repopulated into
the signals panel each time [GtkWave][] is opened. To open the [GtkWave][] save file go to
`File -> Read Save File` (or `C-o`) and select the appropriate save file. In our case we will
open `src/wave/gtkw/computer-fib.gtkw`.
is then converted to a `fst` file using `vcd2fst` (`vcd2fst` is included with [GtkWave][]). Now
once the simulation has complete the *vcd* or *fst* file can be opened with a wave form viewer.
I recommend and have only tested with [GtkWave][]. To open the dump file open [GtkWave][] and
go to `File -> Open New Tab` (or `C-t`) and select the output file (in our case
`src/wave/vcd/computer-fib.fst`). I have provided pre-saved "views" for `computer_tb` (as well
as the other entities within the project) so that the signals of interest for the entity in
question don't need to be repopulated into the signals panel each time [GtkWave][] is opened.
To open the [GtkWave][] save file go to `File -> Read Save File` (or `C-o`) and select the
appropriate save file. In our case we will open `src/wave/gtkw/computer-fib.gtkw`.
Note running this simulation will take some time (likely longer then an hour) and will consume
lots of RAM and CPU and generate a large vcd dump (> 20 GB). By changing the length of time the
simulation runs for one can limit the size of the vcd dump, but the usage of the CPU and RAM
will remain the same. The large vcd dump file is unavoidable (as far as I know) because *GHDL*
records data for all signals in the design and there's no way to filter its output until after
the simulation has run (using external tools).
will remain the same. The large vcd dump file is unavoidable because *GHDL* records data for
all signals in the design and there's no way to filter its output until after the simulation
has run (using external tools). Hopefully at some point the developers of *GHDL* can fix this
oversight.
![[GtkWave][] displaying a simulation dump from *computer_tb* running
*src/asm/Fib.hack*](files/images/gtkwave.png)
@ -106,8 +107,35 @@ want to run in the simulation. See the [documentation][hack-docs] for details.
![[GtkWave][] zoomed in to view approximately *100 ns* of signal
output](files/images/gtkwave-closeup.png)
So now that we've seen a quick rundown of how to use the simulator to run *Hack* programs, lets
take a moment to review some of the deficiencies of the implementation. The most noticeable
issue is that the memory maps for the *SCREEN* and *KEYBOARD* are currently not implemented.
This is mainly because it is difficult to simulate keyboard input and video output in real
time. Note that programs that access the *SCREEN* and *KEYBOARD* memory maps will successfully
be executed during simulation, but any data sent to the screen will never be remembered and the
keyboard will always be inactive.
TODO: talk about current state of project (deficiencies in regards to screen and keyboard memory maps)
One method to implemented the *KEYBOARD* memory map is using a text file similar to the
technique employed in the ROM entity. This would work fine but wouldn't provide an interactive
user experience as I would prefer. Similarly for video output, the raw VGA signals or even
simpler yet, the values of the screen memory map could be dumped to file during the simulation
and then be processed after the fact by an external program to create a video or set of images
of the output. This again however, isn't a interactive experience.
An option I haven't mentioned is to have the simulation and an external program communicate
through a pipe or socket while the simulation is running. This would allow the external program
to send key presses to the simulation while also simultaneously decoding the output of the
screen output file in real time and displaying it as a video to the user; essentially creating
a VM for the *Hack* machine. This approach seems the most promising but is quite challenging to
implement, and likely will be quite slow. So I determined for the time being, my time would be
better spent focusing on completing the implementing on a *FPGA*, once I'm able to obtain one.
Though at some point I may explore this as it would allow an easy way for students to further
explore after completing the *Nand to Tetris* course.
Anyways, hope you enjoyed reading this post. I will post a few follow up articles once I'm able
to obtain a *FPGA* and complete the implementation (will a VGA screen and a PS2 or USB
keyboard). Also just to reintegrate, for those of those interested in the inner workings of a
computer, I highly recommend checking out the *Nand to Tetris* course.
[hack-git]: http://git.rekahsoft.ca/hack/
[hack-docs]: http://git.rekahsoft.ca/hack/about