# Sylva OS — Agent Guide

x86_64 UEFI hobby OS. C17 boot → C++17 kernel, ELF64 → PE32+ via objcopy.

## Build & Run

| Command | What |
|---------|------|
| `make` | Build `build/BOOTX64.EFI` + `build/Kernel.elf` |
| `make run` | QEMU + OVMF, serial 1 → `serial.log`, serial 2 → stdio, paused for GDB (`:1234`) |
| `make vdir` | Stage both binaries into `vdir/EFI/BOOT/` for manual boot |
| `make clean` | **Always before commit** — untracked build artifacts leak outside `build/` |

## Architecture

- Two-stage boot: `boot.c` (UEFI app) loads `Kernel.elf` from FAT volume, parses ELF PHDRs, jumps to entry
- `kernel/entry.cpp` — `_start` saves `SystemTable`, calls `kernel_main()`
- `kernel/main.cpp` — `extern "C" kernel_main()`: init GOP → serial → PMM → heap → idle (`hlt`)
- `kernel/serial.cpp` — UEFI serial protocol wrapper (write/read/hex)
- `kernel/memory/pmm.cpp` — bitmap + free-list physical page allocator
- `kernel/memory/heap.cpp` — kmalloc/kfree/kcalloc/krealloc (first-fit w/ coalescing)
- `graphics/` — GOP framebuffer, `fonts/` — Hankaku 8×16 font via `pf_print()`
- `efi/` — bundled gnu-efi sources: `gnuefi/` (crt0, lds, reloc), `lib/`, `inc/`
- Kernel linked at `0x100000` (`kernel/kernel.ld`)

## Conventions

- Commits: `[type] message` with types `feat`, `fix`, `chore`, `docs` (git log)
- `#define ASM asm volatile` in `include/common.h`
- C17 (`boot.c`), C++17 (`kernel/`) — `-ffreestanding -fno-stack-protector -fshort-wchar -mno-red-zone`
- `uefi_call_wrapper` required for all UEFI protocol calls (omission → page-fault)
- `kernel_main` must be `extern "C"` (C→C++ linkage)
- QEMU starts paused: connect GDB to `:1234` to proceed
- No tests, no CI, no lint config