I’m writing regular logs on hackaday.io about the m68k-based retro computer I mentioned I was building a while back, but I thought I’d post a quick update here too. If you’re interested in the project, be sure to follow it on hackaday as that’s where the regular updates are going.
The main progress so far has been:
Get the CPU free-running
Decide on a memory layout, and design an address decoder to support that layout
Build the address decoder
Decide how I want to lay the buses out on the board for easier interconnection
Actually wire up the buses accordingly
I actually designed the address decoder a while ago, but gathering the parts and then wiring it up has taken some time – I finished it yesterday. Since then, I’ve been breaking out the CPU signals onto control, data and address buses, which I’m doing on an otherwise-bare breadboard in the middle of the main boards.
Right now, the rosco_m68k looks like this:
It’s still free-running at the moment (I’ve not wired the memory to the bus yet as you can see) but all the control pins I care about (basically everything except the legacy 6800 peripheral pins and the function code) are now on the control bus. The jumpers you can see there are configuring those control pins for free-run mode and will be replaced by connections to the RAM and ROM.
I did come across a couple of design errors while building the address decoder, some of which I fixed and others I’ve decided I can live with for now (e.g. an issue with the expansion select line I only discovered once I’d completely built the decoder and hooked up the LEDs at bottom left).
Anyway, that’s where it’s at right now. I expect that after another day’s wiring, it’ll be ready to run real code, and I can’t wait.
Somewhere around the time I got bored with Advent of Code 2018 (well, the challenges grew to become total time-sinks), I decided my project for the new year was going to be a homebrew retro computer.
I decided that, rather than something 8-bit based on the Z80 or 6502, I wanted to go up a generation and build something 16-bit.
Apart from gaming and a bit of BASIC programming on 8-bit machines like the Spectrum and C64, my first “real” computing was done on the Amiga, a machine which still has a special place in my heart today. For that reason, I decided I wanted to build something based on the Motorola 68000 series processor.
I chose the 68010 which (as far as I’m aware) was never used in the Amiga line, but which is slightly more capable than the 68000 (e.g. interrupt vectors can be moved around and it can support virtual memory when used with a 68451 MMU, though I’m not likely to do the latter in this build). Mine is rated for 10MHz, but I’m only running it at 4MHz at the moment).
The key differences between this and an 8-bit Z80 or 6502 build are:
Dynamic registers, so the clock cannot be stopped or single-stepped (this can be emulated by manipulating /DTACK though).
Asynchronous data bus and DMA support, with /BGR, /BGA, /BERR and /DTACK to take care of.
16-bit data bus, so more wires needed.
24-bit (with some caveats) address bus, so lots more wires needed, but 16MB of address space available!
Internally, the m68k family is 32-bit.
/UDS and /LDS lines for selecting upper and lower data bytes, so a more complicated address decoder and ROM/RAM in pairs.
No separate IO space, further complicating the address decoder.
More complicated requirements around /RESET and /HALT handling, especially at power-on.
I started gathering parts in January, and so far have the crystal clock circuit, the reset/halt circuit, and have gotten the CPU freerunning with /DTACK grounded and the various other bus management lines tied high or low as appropriate.
The address decoder is designed, and I already have the RAM and ROM chips ready to go. I’ve also built an EEPROM programmer because I’m too impatient/cheap to order one and wait for it to arrive.
All this means that, pretty soon, I should be able to get it running some actual code!
Fun times are ahead! If you’re interested, you can follow the project over on Hackaday.
Today is day 10 of Advent of Code 2018, the fun daily coding challenge I mentioned I was taking part in this year, and I thought it a good time to take a quick look back at my story so far.
I’ve still not managed to rank globally for any of the challenges (the closest I came was on day 5 when I completed part two 696th) but things are going reasonably well on our private BJSS leaderboard, and the challenges have mostly been fun. So far I’ve stuck with Ruby as my language of choice, although I almost broke out Unity 2D for today’s graphical challenge (Early-morning me obviously had grand ideas which were quickly set aside when I realised I could zoom out my terminal and stick with Ruby).
Here’s a quick rundown (with possible mild spoilers) of the challenges to date. The links take you to my solutions on GitLab, which obviously do contain spoilers, so don’t click them if you haven’t already done the puzzle for that day.
Day 1 was a very simple problem involving summing a list of numbers, and finding the first duplicated number. Nothing much to say here, nice and quick.
Day 2 involved finding characters duplicated two- and three-times in a string, with part two focusing on finding strings that differed by one character. I went with hash-based solutions for this one.
Day 3 was about finding overlapping rectangles. I made a simulation for this as the maths escaped me. This was the first solution that was properly object-oriented Ruby, and I got stuck for a while because I accidentally transposed my X and Y coordinates in one of the methods!
Day 4 involved finding the guard who slept the most, and (in part two) the minute in which they were most often asleep. Another simulation (I quite enjoyed this one).
Day 5 was about reducing a string by removing pairs until none remained. Regex and recursion to the rescue here – a really quick solution.
Day 6 was, frankly, a nightmare. I couldn’t really visualise the problem at first, and it took me a lot longer than it should have to really get to grips with it. Solved in the end, but after many hours on-and-off. I did learn something new though 🙂
Day 7 involved a dependency graph and another simulation. I had a lot of fun with this, and even made an ANSI-based visual mode so you could watch the simulation unfold in the terminal. Great fun.
Day 8 was a simple tree problem involving both breadth-first and depth-first traversal. The depth-first part had a little twist that meant it took a little thinking about.
Day 9 caused me a few problems – partly due to some vague wording in the puzzle, and partly due to my not understanding the problem properly for a while. It was made worse by the fact that I initially decided to write it with the wrong data-structure because I felt it would be quicker. After a few false starts I gave up on that and finally made it work using circular linked lists.
Day 10 was another fun one involving points and velocities to work out when the points (stars) align to display a message. Initially I planned to switch to something with proper graphical support (in a fit of complete overkill, I fired up Unity) but settled in the end on a Ruby solution after I realised I could make the output fit in a terminal.
And that’s it so far! The most fun I’ve had so far was probably on day 7, while my least favourite has been day 6.
Taking part? Let me know in the comments what your favourite puzzle has been thus far.
This year I’m taking part in the awesome Advent of Code, and because I haven’t done a lot of Ruby over the past couple of years I thought this would be an excellent opportunity to refresh my skills. In case you’re unfamiliar, AoC is a series of twenty-five coding challenges. Each day leading up to Christmas a new challenge is posted on the site, and there’s a global leaderboard. You can also have local leaderboards (We have one at BJSS for example).
I did originally consider using the opportunity to learn a new language, but the desire to get back to Ruby proved too strong. In truth, I’m glad it did – every time I come back to Ruby after a break, I’m pleasantly surprised by both the bits I’d forgotten and the bits that have been added.
This post is a little rough, and is intended both as a memory aid for myself and as an example for anyone else who needs to know this stuff. I’ve recently decided to push toward x86_64 support in my toy/research OS project, and needed to build a GCC cross compiler for that target. In my case I already had an x86 cross compiler built, but I’ve tried to make the steps general for those that don’t.
These steps are tested with binutils 2.26.1 and GCC 6.1.0. I’m using those versions as they match the version of my x86 cross compiler – These steps might work with the more recent versions, but I can’t guarantee it. I will at some point update my cross compilers, at which point I’ll update these steps.
1. Go to your $HOME/src. Create first if needed.
# mkdir ~/src
# cd ~/src
2. Set up prefixes. PATH is only needed if not already set (e.g. this is your first cross compiler on this system).
Java’s default methods (introduced back in Java 8) are one of those features that solve the intended problem reasonably well, while at the same time allowing all kinds of nasty code and weird inheritance stuff when used in a general-purpose kind of way. Indeed, they’ve been the subject of a tonofposts and the way they (fail to) work still surprises people. They’ve been around for a while now, and there’s a lot of good advice out there on how they should and shouldn’t be used, so that’s not what I’m going to talk about in this post.
What I am going to talk about, however, is the way I’m using them in Moxy, as a way to do a lot less work in runtime-generated bytecode.
A bit of background
Moxy is a mock framework, and it does it’s job by generating mock classes at runtime. These mock classes naturally use a fair bit of bytecode generation, replacing real method implementations with new methods that implement the mock behaviour.
These mock methods do the standard mock thing of recording their invocation, and then they implement behaviour that has been set-up on the mock beforehand.
In order to implement this behaviour, mock objects need access to all kinds of state that’s specific to the instance – stubbed return values, exceptions they should throw, and more. They also need to have easy access to the mock engine that created them (so they can record invocations, for example).
This is often implemented via static methods on some class somewhere whose job it is to store state. The generated code will typically be peppered with a ton of invokestatic instructions that calls out to methods on those classes in order to get things done.
This works, but it has a few problems:
Some kind of mapping between instances and their state must be maintained.
It can make it difficult to swap out mock strategies in a clean fashion.
It can make testing (of the framework) difficult.
The Moxy approach
Instead of storing state statically, Moxy stores it right there in the mock, and all interaction with the state is done with instance methods. These methods can’t be inherited from a superclass (since mocks usually subclass the mocked class), and I really don’t want to generate them (I like to keep generated code to a minimum, as it’s harder to maintain and test that regular Java), so instead Moxy abuses default methods.
It works like this:
All mocks implement an interface, ASMMockSupport.
This interface defines one abstract method, __moxy_asm_ivars(). This method is implemented in generated code (since interfaces don’t have instance variables, the mocks hold the variables and expose them via this method)*.
The support interfaces defines a ton of default methods that do various kinds of work based on those instance variables.
The actual generated code for mocked methods uses invokeinterface calls on this to get stuff done as needed.
Other parts of the framework that interact with the mocks (e.g. stubbers) simple cast the mock instance to ASMMockSupport and call the methods they need.
* Yes, that’s a strange name for a method. Moxy does this to lessen the chances of its built-in methods clashing with methods being mocked.
The default methods on the support interface handle things like finding the engine that created the mock, adding stubbing, finding appropriate stubbing for a given invocation, running delegate methods and actions, and determining whether the current thread is currently in what Moxy calls a monitored invocation (used for recording purposes only, when mock behaviour is disabled).
This has the benefit that a whole lot of fairly-complex code that would otherwise be either generated, or stuffed into static methods, is now implemented, in Java, as instance methods. They’re easy to unit-test, and they’re easy to maintain. The added bonus is the generated bytecode is also easier to maintain, since it’s as short as possible.
Is this the intended use of default methods? Not at all. Is it correct, or appropriate? I’ll leave that to you to decide. I firmly believe, however, that in this very limited use-case it’s a good solution to a tricky problem, all things considered.
If you’re interested in looking at the code, you’ll find it here.
We all love to use mocks in our Java tests, right? Add the usual mock framework to your test dependencies, sprinkle a few mock, when and thenWhatever calls in to your code, and happy days. You’re able to test things in isolation, and as an added bonus ensure your code is actually calling the stuff it’s supposed to be calling by verifying the mock afterwards. The sun is shining, life is good, and your code is tested to perfection.
But then the clouds roll in, and you have to test a final class. Or you need to mock out a static method in some legacy code. Or, horror, some code that directly calls new on a class you want to mock.
You could hit up StackOverflow, where you’ll find that all these things are possible with existing frameworks, assuming you’re prepared to add in another dependency, use a ton of boilerplate, and refactor your (production) code a bit. Depending on which framework you’re currently using there are a ton of different ways you can kind of achieve what you need.
By now you’ve sunk another hour or two, and it’s still not quite working.
What if there was another way?
The other way
What if I told you there was a framework out there that let you take a final class, and just do this?
I believe I mentioned that I’m back at work on the Curb project. In case you missed that, Curb is a ruby extension that provides bindings to the libcurl library. I started it way back, left to concentrate on other things (having kids, making money, etc) and now I’m involved again, working with Todd Fisher who took over maintaining the project when I bowed out. I’ll be fixing bugs, answering questions and generally helping out.
In the meantime, I’ve also started work on porting (if that’s the right word) Curb to FFI, with a view to moving away from the existing C code. The motivations are manifold:
As it stands, Curb is pretty much tied to MRI. In the modern Ruby world, where you’ve got JRuby and Rubinius and who-knows-what-next, this is recognised as a bad thing.
It’s a nightmare to get it working on Windows. This is because, and I can speak with some authority here as someone who develops on Windows every day, Windows sucks for development. Unless you’re using all-Microsoft tooling, in which case it’s pretty awesome. But for interoperability with portable code, and libraries targeted primarily at other platforms, it sucks.
FFI is probably the right way to do these things these days. 10+ years ago, when Curb was first hacked together in about six hours, C extensions were the shizz. Now, not so much. Unless you really need the level of hardware hackery and performance you can get with C, things are better off in Ruby code.
So to sum up, this port is about future-proofing Curb, making it easier to develop, easier to use cross-platform, and (in the long run) safer, probably more performant, and ensuring it can run on all Rubies, including whatever whizz-bang next-gen thing comes out next week (my bet is it’ll be written in Rust. Or Go. Or something…).
As I mentioned in my previous post, I’m now working on Curb again, and currently on Windows. Back in 2006 when I wrote the original Curb, I wouldn’t go near a Windows box for religious reasons, and the project has never officially supported the platform. There are plenty of bugs and posts around about people having mixed success with getting it to work.
Since then, my views have mellowed, partly I think because I’ve come to appreciate that there’s no room for religion in software engineering – you use the right tool for the job. Partly that, and partly by my desire to, you know, get paid and own stuff and whatnot. These days I do as much work on Windows as I do on *nix, and my bank balance is all the better for it. So anyway, back to the point of this post.
Building Curb on Windows is a bit of a nightmare. It took me a fair while to figure it out, and I wrote the damn thing. If it got me feeling like the guy in the picture above, then how must the everyday user trying to get the gem going feel? If they’re just trying to install it and write some code, I’m guessing they immediately give up and go with another library. But what if they are trying to use something else that depends on the Curb gem? I’m guessing that’s when they get so frustrated they file shouty bug reports with the project. And I don’t blame them. It would have been nice to have someone to shout at while I was trying to get it working.
In the end, I did get it going, but it required me to build libcurl from source. I posted an answer to the bug report I mentioned above with the how-to of it, and the poor person who filed it reported back that it worked. You could click through, but I’ll reproduce the steps below to save your finger.
Most likely, your compiled libcurl is incompatible with the ruby devkit compiler. You can verify this by running (this is on my machine, obviously change your paths):
> c:\Ruby200Devkit\mingw\bin\gcc.exe -L. -lcurl
# c:/[...]/ld.exe: skipping incompatible ./libcurl.dll when searching for -lcurl
# c:/[...]/ld.exe: skipping incompatible ./libcurl.dll when searching for -lcurl
# c:/[...]/bin/ld.exe: cannot find -lcurl
# collect2.exe: error: ld returned 1 exit status
Notice the lines about “Skipping incompatible” before the final “cannot find lcurl”. Try grabbing the curl source from https://curl.haxx.se/download/curl-7.54.0.tar.gz , extract it somewhere, and then open up a terminal and try the following:
> which gcc
# important - verify here that the gcc is the one from your ruby devkit!
> sh configure --with-winssl # and whatever other options you require
> make install
It’s very important that you don’t have any other mingw/msys/cygwin compilers in your path, and that you do this in a windows terminal, not msys/cygwin sh. Otherwise, you’ll probably just build another incompatible library.
You should now have libcurl-4.dll in c:\path-to-your-ruby-devkit\local\bin. Now it gets a bit messy, so hang in there. This part is probably a bug in the build, but easy to work around.
> cd c:\path-to-your-ruby-devkit\local\bin
> copy libcurl-4.dll libcurl.dll
The build needs libcurl.dll (it won’t compile with libcurl-4.dll, but the extension won’t run without the -4 suffix, so we’ll have to manually copy the dll later on).