What Does an Operating System Do? Kernel Mode, User Mode and System Calls
An operating system virtualizes the CPU and memory, shares and protects resources between processes and makes data persistent; when your application needs something from it, it makes a system call that switches from user mode to kernel mode. We trace the seven syscalls behind File.ReadAllText with strace, see .NET's System.Native layer, and measure that writing a log by reopening the file for every line is 38 times slower than buffered writing.
14 Nov 2026
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14 min
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