Under the hood basics: core components, system execution modes, hardware abstractions, and system call lifecycles.

Core Roles & Components

System Modes & Hardware Access

System Calls & Interrupts

Amazon SDE-1 Follow-up Scenario

Step-by-Step Kernel Execution Lifecycle

1. Python Runtime → 2. Trap System Call → 3. Permission & Inode Lookup → 4. Block Request / Context Switch → 5. Disk Controller I/O → 6. Hardware Interrupt → 7. Copy to User Space & Return

Translation: The Python runtime translates the `open()` call to the low-level library function (e.g., POSIX `open(2)`).
Mode Transition (Trap): The library places the system call identifier for open in a CPU register and executes a software interrupt/trap instruction (e.g., `syscall` or `int 0x80`). The CPU switches the mode bit from `1` (User) to `0` (Kernel) and jumps to the kernel's system call interrupt handler.
Descriptor Allocation: The kernel's VFS (Virtual File System) checks process permissions, resolves the path `"file.txt"` to an Inode, allocates an entry in the system-wide open file table, and returns a unique integer File Descriptor (FD) inside the process's file table.
I/O Scheduling: The kernel initiates a read request to the device driver for the disk sectors containing `"file.txt"`.
Blocked State: Because disk I/O is slow (milliseconds), the kernel moves the calling process from the Running state to the Blocked/Waiting queue. The CPU executes a Context Switch to run another active process, keeping CPU utilization high.
Interrupt Trigger: The disk controller retrieves the block bytes, stores them in its buffer, and signals a physical line on the interrupt controller. The controller raises a hardware interrupt.
Interrupt Handler: The CPU halts its current instruction, jumps to the kernel's interrupt service routine (ISR). The ISR reads bytes from the controller buffer into the kernel's Page Cache.
Wake Up: The kernel updates the waiting Python process state from Blocked to Ready in the scheduler queue.
Reschedule & Return: The CPU schedules the Python process. The kernel copies the file buffer data from kernel memory space to the process's user memory space, transitions back to User Mode (mode bit = 1), and return execution control back to Python.