# Phase 10 - Minimal ELF Loader

## Context

This phase lets the BIOS receive GCC-built programs as ELF files and run them without
rebuilding the FPGA bitstream.

## Goals

- Parse enough ELF32 to recognize RISC-V executable files.
- Load `PT_LOAD` segments into memory and zero BSS.
- Jump to `e_entry` with a fixed tiny-program ABI.

## New Concepts

- ELF header: file-level metadata describing architecture, entry point, and tables.
- Program header: runtime load description used by loaders.
- `PT_LOAD`: segment that must be copied into memory before execution.
- BSS: zero-initialized memory represented by `memsz > filesz`.

## How To Think About It

Do not build a general-purpose dynamic loader. Build a strict loader for the exact
firmware shape you produce: static, little-endian, non-relocatable RV32 ELF files with
fixed physical addresses.

## Learning Tasks

- Read the ELF header fields needed to reject unsupported files.
- Map `p_paddr` or `p_vaddr` to your executable memory window.
- Define the stack pointer and argument registers at program entry.
- Decide how a loaded program returns a status code to the BIOS.

## Pitfalls

- Trusting malformed ELF offsets or sizes.
- Forgetting to zero the BSS tail of a loadable segment.
- Accidentally accepting relocatable or dynamically linked binaries.
- Loading a segment over the BIOS itself.

## Tooling And Testing

- Use `readelf -h -l` and `objdump -d` on every early payload.
- Start with one fixed link address and one loadable segment.
- Add loud rejection messages for unsupported ELF class, endianness, machine, or type.

## References

- ELF specification overview: https://refspecs.linuxfoundation.org/elf/elf.pdf
- RISC-V ELF psABI: https://github.com/riscv-non-isa/riscv-elf-psabi-doc
- GNU binutils `readelf` documentation: https://sourceware.org/binutils/docs/binutils/readelf.html