What must be saved and restored during a manual context switch?

A strong answer

A strong candidate frames What must be saved and restored during a manual context switch? around the exact memory layout, compiler semantics, and hardware execution model covered in Manual Assembly Context Switching: Callee-Saved Registers and Stack Pivots. Explain saving callee-saved registers, stack pointer, instruction pointer, and restoring them to resume execution.

In Lesson 3, we measured POSIX swapcontext() and discovered that it takes 620 nanoseconds per context switch. The culprit was the rt_sigprocmask system call, which crosses the operating system privilege boundary on every switch to save and restore signal masks.

Now consider the performance requirements of a high-throughput runtime:

• An event-driven network server handling 500,000 active client connections.
• An in-memory financial engine coordinating 10,000,000 cooperative transactions per second.

In a production interview, always connect the specification rule to its concrete runtime consequence: state what lives on the stack versus the heap or static storage segment, how the compiler or operating system validates alignments, lifetimes, and error codes, and what happens under adversarial or boundary inputs (such as zero-length buffers, integer overflow, concurrent mutation, or partial I/O). Backing your explanation with a small trace of register/memory state transitions immediately separates senior systems engineers from candidates who only memorized syntax.

What the interviewer is listening for

• Mastery of core invariant: Author an x86-64 assembly context switch routine adhering to the System V AMD64 ABI.
• Mastery of core invariant: Preserve and restore callee-saved registers (%rbx, %rbp, %r12-%r15) without kernel traps.
• Mastery of core invariant: Execute an atomic stack pivot between outgoing and incoming fiber stack pointers.
• Hardware and ABI awareness: Explaining how sizes, alignments, calling conventions, or kernel boundaries behave on real 64-bit targets rather than relying on implementation-defined assumptions.
• Defensive engineering: Naming the exact compiler flags (-Wall -Wextra -Werror), static analyzers, or runtime sanitizers (-fsanitize=address,undefined,thread) that verify correctness in CI.

Common mistakes

• Explaining only surface syntax without tracing the underlying memory addresses, ownership lifetime, or kernel/user-space state transitions.
• Assuming undefined behavior (such as signed overflow, uninitialized reads, strict-aliasing violations, or dangling pointers) will "just work" because it passed an unoptimized -O0 debug build.
• Ignoring return codes, partial reads/writes, alignment padding, or cleanup paths when an allocation or syscall fails midway through a function.