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Master the fundamental concepts of protected mode transition through this focused micro-challenge.
You have read the whole brief, and the concepts above stay free on every task. Writing and running the code needs a plan.
Three hints are available for this task, revealed one at a time inside the code workspace so you can struggle productively before seeing them.
Every task includes starter code, theory, and hidden tests so you can implement and verify locally in the browser.
How it worksRings 0-3 encode privilege in segment descriptors and gate targets. Enter ring 3 with a far return (RETF or IRET) that pops a lower RPL CS and sets CPL=3. For example, an interrupt return frame can load user CS selector 0x1B (index 3, RPL 3) and user EIP to start unprivileged code.
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Once in ring 3, CLI and HLT fault because they are privileged. Plan how user tasks will request kernel services later via interrupts or SYSENTER. The user stack must be mapped writable in page tables if paging is on. Verify the user code segment is marked non-conforming execute-only readable as required for code segments with DPL 3.
You will work the privilege arithmetic directly: build selectors with RPL bits, decode selectors back into RPL/TI/index, and simulate IRET validation, including the SS RPL vs CS RPL check the CPU performs to block privilege escalation. This exercise requires separate GDT entries with DPL 3 and a prepared IRET stack frame.
Simulate selector privilege arithmetic and the ring 0 to ring 3 IRET transition in C, driven by a command script on stdin.
Protocol (all numbers in hex):
Process every command on stdin until EOF, printing one line per command.