Kernel/User Boundary & Syscall Flow

What this topic will cover

• Privilege levels — user mode vs. kernel mode (ARM EL0/EL1, x86 ring3/ring0).
• Address space split — why the kernel is mapped high in every process, and why user code can't touch it.
• Syscall flow, step by step — trap instruction, mode switch, syscall number, syscall table, handler, return.
• Syscall vs. library call — why printf isn't a syscall, and what the difference costs.
• Process vs. interrupt context — what counts as each, and what's forbidden in interrupt context.
• Kernel preemption — the CONFIG_PREEMPT one-liner.