Sunday, 16 February 2020

Regression tests

In a previous post I mentioned the importance of creating a proper framework for testing.

In order to properly debug more complex programs like some functions in the libc library, I created a simulator. This simulator allows you to set breakpoints and single step through a program but also contains features that come in handy for regression tests, like dumping the contents of registers and memory locations after running a program. This information can be checked against known values in a test.

The simulator is great for more complex programs and to verify that elements of the toolchain like the assembler keep working as designed and as such these tests can even be automated as a GitHub push triggered action. These tests do not test the actual hardware though.

Our monitor program is scriptable however, so I designed a couple of tests that execute all instructions and alu operations and verify the results against known values. These tests still don't cover all edge cases but are sufficient to verify proper performance once I start refactoring the cpu and alu. Also, the hardware tests are mirrored in a test for the simulator (in the makefile) , so can be used to test its behavior as well.

Refactoring the cpu and alu

There are many things that can be improved in the design, especially when considering resource usage, instruction set design and performance.

I already started rewriting the very complex state machine into something simpler but without changing the instruction set (apart from removing the obsolete loadw and storw instructions). This already saves more than a hundred LUTs but I want to do more.

Things I have in mind:

  • Removing carry related functionality
  • Moving pop/push from 'special' sub-opcodes to their own instruction opcodes
  • Merging the setxxx and branch instructions into a combined instruction

Now that we have a somewhat decent test framework in place these changes can be more easily tested against regressions. The results of these refactorings will be reported on in future articles.

Saturday, 8 February 2020

A simulator for the Robin cpu

When I started playing around with a SoC design that I wanted to implement on the iCEbreaker, I quickly realized that without proper testing tools even a moderately ambiteous design would quickly become too complex to change and improve.

There exist of course tools to simulate verilog designs and even perform formal verification but my skill level is not quite up to that yet. On top of that I am convinced that many changes that I want to try out in the cpu would benefit from regression tests that are based on real code, i.e. code generated by a compiler instead of artificial tiny bits of code: code that you do not directly implement yourself tends to expose issues in the instruction set or bugs in it hardware implementation quicker than when you deliberately try to construct tiny test cases.

For these more realistic tests an assembler and a C compiler were created and they were used  to implement small string and floating point libraries mimicking some of the functions in the C standard library. And they proved their worth as they uncovered among other things bugs in the handling of conditional branches for example.

However, as we will use the assembler and compiler to perform regression tests on the cpu it is important that these tools themselves are as bug free as possible, even when we add new functionality or change implementation details. Ideally some contineous integration would be implemented using GitHub actions that would be triggered on every push.

There is one catch here though: we cannot perform the final test in our chain of dependencies simply because the GitHub machines do not have an iCEbreaker board attached ☺️

We can deal with this challenge by creating a program the will simulate the cpu we have implemented on our fpga. This way we should be able to perform the tests for the compiler/assembler toolchain against this simulator with the added benefit of having more debugging options available (because they are much easier to implement in a bit of Python that in our resource constrained hardware.

The first version of this simulator is now commited and i hope to create some contineous integration actions in the near future.

Saturday, 1 February 2020

The Robin SoC on the iCEbreaker: current status

It is perhaps a bit weird, i started first with the iCEstick and then with the iCEbreaker to play around with FPGAs but actually i spend more time tweaking the assembler and C compiler than working on the hardware.

This is a good thing really, for it means that the hardware is working quite well. The reason for writing an assembler and a compiler (besides being fun) is to be able to test the SoC in a more thorough manner than just poking some bytes to see what it does.

You can of course simulate and verify correct behaviour of individual components but i find it infeasible to simulate a complete CPU (at least not with my limited experience). This means that at least in my opinion simulations are the hardware equivalent of unit tests, important in their own right but insufficient to test a complete design in a way proper programs put a CPU through its motions. Another goal is to find out whether the Instruction Set Architecture (ISA) i have thrown together is workable from a compiler point of view.

Over the last couple of weeks i slowly expanded the functionality of the C compiler (although it is a far cry from being standards compliant and it likely will stay that way) and while this compiler currently just supports char and int types I started writing a soft float library. And indeed while doing this I encountered a serious bug in my hardware design: conditional branches were not always taken correctly.

This was exactly why i started writing actual programs that exercise the CPU in more realistic ways. However, most of the things i encountered where bugs in the compiler rather than in the hardware but all the work until now resulted in this provisional list of observations:

  • Single stepping

It would be nice to have the option to single step through a program on the hardware level. The CPU has a halt instruction and in its ROM i have implemented a small program that dumps all registers but there is no easy way to restart again. The need for single stepping is somewhat lessened now that i implemented print and itoa functions but it still would be nice to have.

  • Interrupts and the UART

I didn't design the UART myself because i wanted to implement a monitor first and my verilog skills weren't up to it at that point. Later I added FIFOs to the UART to prevent overruns and made it available to the CPU via memory mapping. However, currently the UART is a bottleneck: it doesn't perform reliably at speeds over 115200 baud and even then it is fairly easy to confuse it. Obviously this would be a prime candidate for a proper redesign and part of this would be dealing with interrupts so that the CPU would waste its time polling for an available character or making sure enough time has passed to send another one.

  • Complexity

The current CPU implementation is quite complex i think: instead of calculating many control signals with a single meaning most of the logic is implemented in a rather long and deep state machine. The SoC as a whole (monitor, CPU and supporting components) currently eats up all but one (!) of the LUTs of the up5k on the iCEbreaker. On the other hand i noticed that even in the current RISC like design, some instructions are never used: the 16 bit move, load and store instructions for example. This of course because my compiler doesn't bother with anything that isn't a byte or a 4byte entity but it nevertheless shows that depending on the area of application we might reconsider the design.

Next steps

The next couple of weeks i will stay focused on improving the C compiler and the test suite until i am confident that changes in the hardware design can be properly checked for regressions. And because compiler writing and especially code generation is fun I'll probably write about some interesting finds along the way.

Then I'll probably focus on simplifying the CPU design to free up hardware resources. At that point i also hope to start on properly documenting the design as currently it is a bit of a mess (like this rambling blog ☺️). After that I'll probably start on the difficult stuff, i.e improving the UART, but we'll see.

CPU design

The CPU design as currently implemented largely follows the diagram shown below. It features a 16 x 32bit register file and 16 bit instructi...