As a mentioned earlier, I've been looking at pushing to the next round of project improvements, and that meant a better testing process. I tried using a "control" CPU, which would be compared to the output of the CPU under test, however that assumes that the number of clock cycles required for each operation wouldn't change. While useful in a few cases, a lot of the changes I'm interested in involve timing, and so that wouldn't work.
I decided instead to make two ROM modes. The default one runs the monitor code, which allows for basic memory interaction as well as parsing of ELF binaries on the microSD to bootstrap other programs. The new ROM module is a set of POST routines written to progressively test the CPU as well as IO functions to check for functional regressions. This method has already paid for itself, since I found a small bug in a couple of the floating point opcodes.
The method of test is fairly simple. I need to assume that some basic operations work, otherwise it won't even run the POST, which means immediate load of a register, immediate add, integer compare, and branch if not equal. The first tests evaluate register operations, the ALU and FPU. Then we test stack operations, branch tests, and all of the load and store operations. For the math and branch operations, we can compute the expected result and store them in the code, and generate an error when the result isn't as expected.
In addition to the basic CPU tests, I'm also implementing a set of memory tests. This will allow me to better test the cache module, which I'll describe in the Doom project update.