No-Std Rust Program

When std Assumes an Operating System

#![no_std] drops the standard library and keeps core: Option, Result, iterators, but no Vec, files, threads, or networking unless you add alloc yourself. Embassy, Oreboot, and Rust-for-Linux all live here.

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• #[no_main]: you supply _start or a board-specific entry
• Panic handler: mandatory; otherwise the linker errors
• #[global_allocator]: only if you pull in alloc for Box/Vec

core vs std

core works on bare metal; std layers OS syscalls, I/O, and synchronization. Cross-compile with --target x86_64-unknown-none or a board JSON when libc is absent.

For this exercise, you will build a minimal no_std binary with a custom entry and panic handler. This task asks you to explain what disappeared compared to cargo new, because every embedded bug that "works in std" traces back to missing runtime support.

Keep the relevant man page, ABI doc, or Rust reference chapter open while you work. When your output disagrees with the reference implementation on the same machine, the mismatch is usually an alignment rule, an off-by-one terminator, or a register slot you misread in GDB. Skim the official documentation for the tool or ABI named in the exercise; the prose changes, but register roles, syscall numbers, and ownership rules stay stable across releases. Treat each failure as a contract test: the CPU, kernel, and borrow checker enforce rules whether or not the tutorial mentioned them explicitly.