Hi Ethan, thanks for suggesting MAME - did some research and somehow I do not think it emulates UltraSparc but only 32bit Sarc. But saw, that QEMU has a UltraSparc emulation and they...
https://www.qemu.org/docs/master/system/target-sparc64.html ...explicitly claim that a NVRAM is emulated, although they are doing a M48T59 there is at least some chance that the memory address can be found in the QEMU sources. Will take a look in the next weeks ;-)
