First, PasRISCV not only emulates userspace (RV64GC usermode), but a complete, modern machine (SMP/MultiHART, RV64, full RVA23.1 including Hypervisor, Crypto, Vector stuff, etc.), which is capable of booting a normal Linux kernel (either using direct OpenSBI -> kernel boot, but currently everything has to be in the initrd then), or using U-Boot (OpenSBI -> U-Boot -> kernel).
The emulator uses a hybrid of interpreter with a tracing JIT. Currently, the JIT is limited to host x86-64. About 99% of all instructions and are supported by the JIT. For example, the host FPU is passed through to the guest. But PasRISCV also supports an optional stricter FPU mode, where with soft floats and interpreter only, because IEEE754-2008 has specific rules, that don't exist on the x86 architecture.
Disk images are provided either using VirtIO-Block or NVMe via PCIe. We can either use disks in normal read-write mode, or also in read-only mode. This allows us to have a root partition, that we can overwrite with game updates, and have a user partition, where custom player stuff is stored. We also can mount host dirs as external paths/mounts over VirtIO-9P or VirtIO-FS, which is especially useful for development because this allows us to compile guest tools directly on the host system.
To display graphics on our virtual game screen(s), we currently use a framebuffer device (via SimpleFB with a custom MMIO framebuffer). But PasRISCV also supports other graphics "adapters", such as VirtIO GPU (with EDID, 2D, and experimental 3D/virgl support), or using PCI emulation (bochs-drm and Cirrus-DRM). We have some custom shaders modifying the framebuffer output, so that we can add some retro FX like scanlines, phosphor glow, etc.
The emulator also has audio using emulated CMI8738 and FM801 cards (both with OPL2/3 support), as well as Intel HDA and VirtIO-Sound. Our little space game (Space Prowler) inside SEEDS has a title tune which is made using PCM+OPL3. I coded a small tracker, which runs in a plain Linux console, so I can even compose inside our game on its 80×50 (640×400px) framebuffer console, albeit it's a bit cumbersome. I'll release the tracker as FOSS when we have a bit more spare time, as I need to make it a bit more user-friendly.
We use vsock (Linux virtual sockets) to interface with the game outside of the emulated machine. We use a small serialization format (basically something similar to binary JSON) to communicate bi-directionally, like you'd expect over normal network. We have a small priority queue and message referencing for urgent and request-response communication. For example, the client to display a planet sphere on the in-game computer can ask for updated positions of e.g. terrain or all animals.
The emulator embeds its own disassembler and debugger, and also can act as gdb server, which means you can use your favourite debugging tools, as long as it speaks gdb's server protocol.
It has real network support through a Slirp-like usermode NAT (without using external libs) which supports IPv4 and IPv6. We can run APK's update and install directly on the emulated machine.
Our game uses a slightly modified Alpine Linux 3.23 and custom kernel build, but a recent stock Alpine image also runs without problems.
PasRISCV is licensed under the permissive zlib license.
If you have any questions, I'm sure BeRo is happy to answer questions with far more detail than I could. :)