MaplePad VS KallistiOS

https://github.com/mackieks/MaplePad/blob/main/src/maple.pio

The cycle accurate assembly language has enabled a number of timing sensitive FIFO data processes. What had to be done with FPGA's before has some limited support with PIO. I hope RPI increases the number of instructions and simultaneous running PIO machines in the future.

However if you're really sold on the idea you should use something like Maplepad instead of a stock VMU: https://github.com/mackieks/MaplePad

FTA:

"DreamShell

The Dreamshell is the operating system for the Sega Dreamcast based on the KallistiOS[0] kernel. It has a dynamic loadable modular system and interface for creating applications with XML UI and both C/C++ and Lua script on. You can see examples in ready-made applications and modules, drivers for various devices, formats and interfaces. Examples for audio and video decoding, compression, packaging, binding, network, emulation, scripts and more. From hardcore low-level assembler to high-level applications. There are also large subproject is the ISO Loader, which contains emulation of BIOS system calls, CDDA playback and VMU, also it can hooking interrupts for various SDKs and more."

"KallistiOS is an unofficial development kit for the SEGA Dreamcast game console with some support for the NAOMI and NAOMI 2 arcade boards.

KOS was developed from scratch over the internet by a group of free software developers and has no relation to the official Sega Katana or Microsoft Windows CE Dreamcast development kits. This has allowed it to fuel a thriving Dreamcast homebrew scene, powering many commercial releases for the platform over the years. It supports a significant portion of the Dreamcast's hardware capabilities and a wide variety of peripherals, accessories, and add-ons for the console, including custom hardware modifications that have been created by the scene.

Despite the console's age, KOS offers an extremely modern, programmer-friendly development environment. Using the latest GCC toolchain, it supports the entirety of C17 and C++20 including their standard libraries, along with support for portions of C23, C++23, Objective-C, and various POSIX APIs. Additionally, KOS-ports offers a rich set of add-on libraries such as SDL, OpenGL, OpenAL, and Lua for the platform."[0]

[0] https://github.com/KallistiOS/KallistiOS

You’re going to want to use KallistiOS to get started: https://github.com/KallistiOS/KallistiOS. It’s essentially a pseudo-OS for the Dreamcast that provides a lot of Linux-like functions.