I finally figured out how to send commands to DCLOAD! It requires netcat/ncat and a raw payload file.
Use this on Windows (requires downloading ncat, which is part of the nmap package here
): ncat -4 -u [Dreamcast's IP] < [path to payload]
Using a tool like Wireshark to view the response packets, the 64-bit data is stored in the last 8 bytes of the response when a read command is sent. It's in little endian, as I didn't byteorder-flip it for transmission across the network, and my reasoning is so it could be directly fed into a program on x86 without any extra conversion steps.
So I took some measurements of performance counter 1 and found that it appears to run at about 11.96988 x 200MHz. It would be great if someone double-checked my calculations here. I'm not sure how there's even any circuitry capable of running at 12x per clock on the Dreamcast, but here it is.
...And then I came across this, in the Renesas SH7750 hardware manual, page 67:
So now I'm off to check whether result_high and result_low need to be flipped. -- Edit: Nope, doesn't look like I got it wrong. If I unmask the upper bits they're just 0, and checking the byte order in the packet for small numbers (like <100 secs) has zeros in the expected places. The elapsed time counters are just incrementing way faster than the clock speed, so the max lease time that can be supported is actually 117576 seconds, or about 1 day, 8 hours, and 40 minutes. I think that's fine, especially since the most I've ever seen in practice is one day, anyways.
I am still curious to know how the designers pulled that ~12x multiplier off, though. 2393976245 is just a weird number. I didn't see any obvious patterns in the bits comparing it to 200000000, either.
EDIT: Regarding remaining thing #1, well, wow, what a wild goose chase. The exception-test example program doesn't even work with stock dcload, so I'm just not going to bother with it. And I think #2 is caused by go.s clearing all the registers and resetting the stack. GCC probably doesn't like that!
12/3/2019: Removed CDI image. It's way old now. The other attachments are still applicable.