Saving kernel crash dumps

Hi, I'm trying to save a kernel crash dump on sparc64 so I can examine
it later with GDB.

db{0}> sync
Frame pointer is at 0x1331ccc351
[   3.6760039] Call traceback:
...
[   3.6760039] cpu0: rebooting

but later when I look at /var/crash there is no dump.

The system was booted from a USB flash drive having a single FFSv2 root
file system and no swap space.

How does NetBSD on sparc64 save crash dumps? Does it require swap
space? If yes, does it need to be bigger than the system RAM size? Also
how does it know to which device to save crash dumps and is there a way
to configure it during boot or via ddb prompt?

Thanks.