Thanks for the pointers !
Mecrisp-Quintus 0.17 is released now, containing full support for RV32M1 Vega and an additional precompiled binary including disassembler, multitasker and s31.32 fixpoint math library. As my Forth core is written in pure hand-crafted assembler, everyone who wishes to conquer this interesting chip in a “bare metal” way will hopefully find a good starting point in its sources.
While searching for the correct clock configuration in GDB, I found special, undocumented read-only values in the SCG IO range. For example, 0x4002C20C is read-only (raises exception when written to) and reads as 0x013d0141. Neither 0x4002c310 nor 0x4002c314 nor 0x4002c510 are documented, too.
0x4002c200: 0x01000005 0x00010101 0x00000001 0x013d0141 0x4002c310: 0x000d1e36 0x000f1c41 0x00001e36 0x00000000 0x4002c510: 0x00000064 0x00000064
See yourself in GDB with x/326xw 0x4002C000