11 Dec 2017 -
One of the difficult things about starting my emulator was trying to test it without having anything to test against. During the early stages there wasn’t any software I could boot, and I didn’t even have any form of visual feedback from the emulator until I had been working on it for six months. By my count there are 236 opcodes in the SuperH-4 instructin set, and most of them get used during the boot process. If one of those opcodes is wrong, the emulated software might loop forever (best-case scenario), or it might call the wrong function, or it might write a garbage-value somewhere and corrupt its own state. In the latter two cases it’s very difficult to figure out what went wrong because the root-cause of a bug could be very far removed from its symptons. Worse still, without any form of feedback from the emulated software I might not even know that something did go wrong.
The obvious solution is to write test programs that can detect when something is wrong, but in order for that to work I’d need a way to load them into the emulator and a way to get feedback from them. Loading them can be accomplished by writing a test program that replaces the firmware image, but then I still need a way to get feedback from them; there’s no way to get feedback from emulated software until the emulator is functional enough to support the mechanisms that software would use to send feedback, and I needed test programs to help get the emulator to that state.
My solution to this was to make a couple of unit-test programs which would link against WashingtonDC’s .o files, initialize some of the hardware, artificially inject sh4 code into the memory via an sh4 assembler library I wrote, execute that code, and then inspect the CPU’s registers to see if they matched the intended final values. This resulted in three separate tests: sh4mem_test, sh4inst_test, sh4div_test, sh4tmu_test, and sh4asm_test (which tested the assembler library). These four tests helped root out several bugs so I could move WashingtonDC along.
Since they all needed to link against WashingtonDC as a library, they had to be refactored every time WashingtonDC’s internal APIs changes. This was an especially arduous task in the case of sh4inst_test, which had swollen to be more than 10000 lines of code as I kept adding more and more testcases. Sometime after I got the firmware booting, I decided the work needed to keep them running was greater than the benefit I got from them so I let them die of bitrot, and eventually I deleted them from git.
Recently, I’ve been getting ready to make some major changes to the sh4 to support a JIT recompiler, and also to make the existing interpreter faster. This motivated me to bring back the unit_tests, only this time I have the infrastructure needed to build real test roms for the Dreamcast. My current approach is to have the roms do complicated operations and print the results to the serial port. The tests are designed so that if anything goes wrong the output will change. I have a couple perl scripts which launch WashingtonDC with the given test roms and compare the output to captures taken from the same roms running on a real Dreamcast. If there are any discrepancies then the perl script knows something is wrong and it raises an error.
This scheme sounds primitive but it works much better than the old unit_tests scheme did because it’s actually comparing WashingtonDC against a real Dreamcast instead of comparing it against my estimations of what a real Dreamcast would have done. I’m just getting started but I’ve already found and fixed two bugs which the old sh4inst_test was missing. I was worried initially that this would turn out to be a pedantic waste of time since WashingtonDC can already run a few games and I didn’t think there would be any basic instruction bugs which wouldn’t prevent something complicated like Crazy Taxi from booting; this assumption could not have been more incorrect.
I think the best part of the new test roms is that since they’re real Dreamcast programs, they’ll never die of bitrot like the old unit_tests did. I can keep making whatever major architectural changes I want to WashingtonDC and that will never require a major refactor of the test roms. I don’t really even need to keep the source code around (not that I would ever get rid of it).