You want to build your own CPU? That’s great fun, but you might find it isn’t as hard as you think. I’ve done several CPUs over the years, and there’s no shortage of other custom CPUs out there ranging from pretty serious attempts to computers made out of discrete chips to computers made with relays. Not to trivialize the attempt, but the real problem isn’t the CPU. It is the infrastructure.
What Kind of Infrastructure?
I suppose the holy grail would be to bootstrap your custom CPU into a full-blown Linux system. That’s a big enough job that I haven’t done it. Although you might be more productive than I am, you probably need a certain amount of sleep, and so you may want to consider if you can really get it all done in a reasonable time. Many custom CPUs, for example, don’t run interactive operating systems (or any operating system, for that matter). In extreme cases, custom CPUs don’t have any infrastructure and you program them in straight machine code.
Machine code is error prone so, you really need an assembler. If you are working on a big machine, you might even want a linker. Assembly language coding gets tedious after a while, so maybe you want a C compiler (or some other language). A debugger? What about an operating system?
Each one of those things is a pretty serious project all by itself (on top of the project of making a fairly capable CPU). Unless you have a lot of free time on your hands or a big team, you are going to have to consider how to hack some shortcuts.
The easiest way to get infrastructure is to steal it. But that means your CPU has to be compatible with some other available CPU (like OpenSparc or OpenRisc) and what fun is that? Still, the Internet is full of clone CPUs that work this way. What good is a clone CPU? Presumably, the designer wants to use that particular processor, but wants to integrate it with other items to produce a system on a chip. Of course, sometimes, people just want to emulate an old machine, and that can be fun too.
In general, though, the appeal to developing your own CPU is to make it your own. Maybe you want to experiment with strange instruction set architectures. Perhaps you have an idea about how to minimize processor stalls. Or you could be like me and just want a computer that models the way you think better than any commercial alternative. If so, what do you do? You could try porting infrastructure. This is about midway between stealing and building from scratch.
There are quite a few options for portable assemblers. Assuming your processor doesn’t look too strange and you don’t mind conventional assembler conventions about labels and symbols, you might consider TDASM or TASM. I have my own variation on this, AXASM, and I’ll talk about it more in the near future.
Assembly language is fine, but you really want a high level language. Of course, your first thought will be to port gcc, which is a great C and C++ compiler (among other things). There’s good news, bad news, and worse news. The good news is that gcc is made to be portable as long as your architecture fits some predefined notions (for example, at least 32 bit integers and a flat address space). The bad news is that it is fairly difficult to do a port. The worst news is there is only a limited amount of documentation and a lot of it is very out of date.
Still, it is possible. There are only three things you have to create to produce a cross compiler:
- A machine description
- A machine header
- Some machine-specific functions
However, building these is fairly complex and uses a Lisp-like notation that isn’t always intuitive. If you want to tackle it, there are several documents of interest. There’s a very good slide show overview, very out of date official documentation, and some guy’s master’s thesis. However, be prepared to read a lot of source code and experiment, too. Then you’ll probably also want to port gdb, which is also non-trivial (see the video below).
There are other C compilers. The llvm project has clang which you might find slightly easier to port, although it is still not what I would consider trivial. The lcc compiler started out as a book in 1995. It uses iburg to do code generation, and that tool might be useful with some other retargeting projects, as well. Although the vbcc compiler isn’t frequently updated, the documentation of its backend looks very good and it appears to be one of the easier compilers to port. There is a portable C compiler, PCC, that is quite venerable. I’ve seen people port some of the “small C” variants to a different CPU, although since they aren’t standard C, that is only of limited use.
Keep in mind, there’s more to doing a gcc port than just the C compiler. You’ll need to define your ABI (Application Binary Interface; basically how memory is organized and arguments passed). You’ll also need to provide at least some bootstrap C library, although you may be able to repurpose a lot of the standard library after you get the compiler working.
So maybe the C compiler is a bit much. There are other ways to get a high level language going. Producing a workable JVM (or other virtual machine) would allow you to cross compile Java and is probably less work overall. Still not easy, though, and the performance of your JVM will probably not be even close to a compiled program. I have found that versions of Forth are easy to get going. Jones on Forth is a good place to start if you can find a backup copy of it.
If you do bite the bullet and build a C compiler, the operating system is the next hurdle. Most Linux builds assume you have advanced features like memory management. There is a version, uClinux, that might be slightly easier to port. You might be better off looking at something like Contiki or FreeRTOS.
A Shot of Realism
Building a new CPU isn’t for the fainthearted and probably not your best bet for a first FPGA project. Sometimes, just getting a decent front panel can be a challenge (see video below), never mind compilers and operating systems.
Bootstrapping a new system to a full Linux-running monster would be a lot of work for one hacker. It might be more appropriate for a team of hackers. Maybe you can host your project on Hackaday.io.
Still, just because you can’t whip up the next 128-bit superscalar CPU on a weekend, doesn’t mean you shouldn’t try your hand at building a CPU. You’ll learn a lot and–who knows–you might even invent something new.