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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 worksv8086 lets protected-mode kernels run real-mode BIOS calls safely by trapping sensitive instructions. Set the VM bit in EFLAGS via a task switch or IRET trick, and the CPU re-enters a sandboxed real-mode environment. For example, a monitor OS can invoke INT 0x10 without dropping global protection.
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The TSS can point to an I/O bitmap that traps IN/OUT from v8086 tasks. If the bitmap offset equals the TSS limit, all port I/O traps to the monitor. Many teaching kernels instead copy a 16-bit BIOS thunk below 1 MB and call it with a deliberate real-mode transition because full v8086 emulation is tedious. Document which approach your design uses so reviewers can follow the control flow.
You will do the arithmetic the monitor does: decode EFLAGS (VM, IOPL, IF), compute real-mode segment:offset linear addresses that still flow through paging, and decide which instructions execute versus trap to the monitor. This exercise requires setting the VM flag and handling traps if the CPU denies privileged instructions.
Simulate Virtual 8086 mode mechanics 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.