looks like the start board is a gd32vf103cbt from the tiny print silk-screened on the chip. [[https://www.tme.com/us/en-us/details/gd32vf103c-start/development-kits-others/gigadevice/][TME]] says it's a gd32vf103d6t6

which means it has 128k flash and 32k sram and 37(!) gpio pins

| area        | start       | end         |
|-------------+-------------+-------------|
| sram region | ~0x2000_0000~ | ~0x2001_7fff~ |
| boot loader | ~0x1fff_b000~ | ~0x1fff_f7ff~ |
| main flash  | ~0x0800_0000~ | ~0x0801_ffff~ |

interrupt vector table is 4096 words starting from the start of flash (~mtvt~ register). ~_start~ should be at the end of the vector.

according to the user manual (pg 51), at least when waking up, the system boots from ~0x0000_0000~ (which is an alias to the start of the main flash, iirc)

usb is 2.0 full-speed (12Mbps)

* USART
On [[https://github.com/tyustli/rt-thread/tree/master/bsp/gd32vf103v-eval][github]] it says that ~uart0~ tx/rx is gpio 9/10 — but that's for the eval board.
See the [[https://github.com/riscv-mcu/GD32VF103_Firmware_Library/tree/master/Examples/USART/Printf][risc-v repository]] for a printf example.