Add BIOS and tiny kernel roadmap phases

2026-04-28 13:23:03 +02:00
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+# Phase 9 - GCC-Built BIOS / Serial Monitor
+
+## Context
+
+This phase turns the GCC bring-up work into a persistent interactive firmware. The
+CPU boots into a small monitor instead of a one-off test program.
+
+## Goals
+
+- Build a freestanding C/assembly BIOS with its own linker script and startup code.
+- Provide a UART command prompt.
+- Add simple commands for memory inspection, loading, and jumping to test payloads.
+
+## New Concepts
+
+- Monitor: small firmware that lets you inspect and control a machine interactively.
+- Command parser: text interface that maps typed commands to firmware functions.
+- Firmware ABI: the calling convention and data contract between loaded code and BIOS.
+- Executable RAM window: memory that software can write and the fetch path can execute.
+
+## How To Think About It
+
+The BIOS is both a milestone and a tool. Keep it boring and reliable: UART in, UART out,
+explicit commands, clear error messages, and no hidden dependencies on host tooling.
+
+## Learning Tasks
+
+- Decide where UART-loaded code can live and how the I-bus fetches it.
+- Define a tiny BIOS call table for console I/O and returning to the monitor.
+- Write down the exact register state expected by `run <addr>`.
+- Add commands one at a time and test each on hardware.
+
+## Pitfalls
+
+- Loading code into data memory that the instruction fetch path cannot see.
+- Letting a failed command corrupt the monitor's own stack or globals.
+- Building a clever shell before the load/run/debug basics work.
+
+## Tooling And Testing
+
+- Test the monitor in simulation with scripted UART input where practical.
+- Use a terminal program that can send raw files without changing line endings.
+- Keep a known-good tiny payload that prints one line and returns to the monitor.
+
+## References
+
+- RISC-V ELF psABI: https://github.com/riscv-non-isa/riscv-elf-psabi-doc
+- GNU linker scripts: https://sourceware.org/binutils/docs/ld/Scripts.html
+- OSDev bare bones background: https://wiki.osdev.org/Bare_Bones