Portability notes for contributors

The tree builds on GCC 14.x locally and on newer toolchains on CI. Three classes of difference have already bitten - keep them in mind:

1. Stricter warning classes on newer GCC

-Waddress-of-packed-member (part of -Wall since GCC 9, enforced harder on newer releases): casting a packed-struct pointer to a naturally-aligned type is an error under -Werror. The multiboot2 tag types are ALL declared __attribute__((packed)) for exactly this reason - including the mmap tag (4x u32, no layout change) and the BASIC_MEMINFO reader which goes through a packed tag type instead of a bare u32* cast.

Rule: when you cast a pointer that came from packed memory, target a packed type.

2. Stricter assemblers (binutils/GAS)

setcc/setc accept BYTE registers only. The rdrand wrapper declared its flag as u32 with an "=q" constraint - older GAS silently truncated to the byte subregister, newer GAS rejects the operand size. It is u8 now.

Rule: setcc-style flags are u8, constraint "=q".

3. Toolchain layout assumptions

OC_TOOLS may point at a relocatable tree (<root>/usr/{bin,lib}) OR a system root (/usr with {bin,lib} directly under it). The embed_grub generator probes both layouts - do not assume either one.

Compile-flag contract

The kernel flags are fixed in the Makefile: -ffreestanding -fno-stack-protector -fno-pie -fno-pic -mno-red-zone -mno-sse -mno-mmx -mno-3dnow -mcmodel=kernel -fno-asynchronous-unwind-tables -Wall -Wextra -Werror -O2 -g -std=gnu11. New flags belong in the Makefile (one place), not scattered in files.

Cross builds

make ARCH=arm64 / ARCH=riscv use the aarch64/riscv64 cross toolchains and their own linker rules (--hash-style=sysv --build-id=none; -n and PHDR layouts do not mix). The bring-up shells are real (PL011/NS16550A divisors, timer counters, heartbeat) but deliberately minimal - x86_64 is the feature-complete architecture.