Forge

ForgeOS64 — a 16-to-64-bit boot chain in pure Forge

A real-mode → long-mode boot chain written entirely in Forge — no inline assembly anywhere. It is the reference implementation of the x86_64 raw kernel target and lives in examples/os64/.

Three stages

Stage	Source	Output	Sectors (LBA)	Loaded at
1 — boot sector	src/boot/boot.dev	build/boot.bin	0	0x7C00 (BIOS)
2 — loader	src/boot/loader.dev	build/loader.raw	1–2	0x9000
3 — kernel	src/kernel/kernel.dev	build/kernel.raw	3–10	0x100000 (physical)

• The boot sector reads sectors 1–2 with _dev_bios_disk_read (INT 13h CHS) and far-jumps to the loader with _dev_jump.
• The loader reads the kernel into a staging buffer at 0x8000 via INT 13h AH=42h (_dev_bios_disk_read_lba) — the same low-1 MiB limitation as ForgeOS32.
• _dev_enter_long_mode(lo, hi) runs the entire 16→32→64 switch as a single codegen16 builtin:
  1. A20 enable
  2. 16→32 GDT + CR0.PE, far jump to a 32-bit trampoline
  3. Trampoline relocates the 4096-byte staging buffer from 0x8000 to 0x100000 and stashes the entry at 0x8FF8
  4. 4-level identity page tables covering 0..1 GiB (PDPT[0] = 0x83, which also satisfies the PAE 3-level transition window)
  5. CR3 / CR4.PAE / EFER.LME via wrmsr, 64-bit GDT + lgdt, CR0.PG, far jmp to a 64-bit code segment
  6. Long-mode trampoline: mov rax,0x90000; mov rax,[0x8FF8]; jmp rax — landing at the kernel’s _start (linked at 0x100000)

Before switching, the builtin probes CPUID for long-mode support (leaf 0x80000001’s LM bit, guarded by the max-extended-leaf check) and, on a 32-bit-only CPU, prints No 64-bit CPU over COM1 and halts instead of triple-faulting.

Build and run

cargo build --release
./examples/os64/build.sh             # assemble build/os64.img
./examples/os64/build.sh run         # ...and boot it in QEMU

qemu-system-x86_64 -accel tcg,thread=multi \
    -drive file=examples/os64/build/os64.img,format=raw,if=ide -nographic

Expected output:

ForgeOS64: loading kernel: ok
ForgeOS64: hello from 64-bit long mode

The kernel then halts.

Linker descriptors

• os64-loader.fld — ARCH x86_16 / FORMAT raw / LOAD 0x9000
• os64-kernel.fld — ARCH x86_64 / FORMAT raw / ENTRY _start / LOAD 0x100000 (the kernel link base)

The kernel alone can be built with:

./target/release/forgec kernel.dev -o kernel.raw --linker examples/os64/os64-kernel.fld

The kernel uses std.hal for its serial and VGA setup — port I/O through typed wrappers, no extern declarations.

Testing

QEMU integration tests cover the full chain: os64_boot_chain_enters_long_mode boots the three-stage image and asserts the 64-bit banner; os64_32bit_cpu_guard_halts verifies the CPUID guard falls back gracefully on a 32-bit-only CPU. They need qemu-system-x86_64 on PATH.

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