The x86 protection model is notoriously complex, with four privilege rings, segmentation, paging, call gates, task switches, and virtual 8086 mode. What's interesting from a hardware perspective is how the 386 manages this complexity on a 275,000-transistor budget. The 386 employs a variety of techniques to implement protection: a dedicated PLA for protection checking, a hardware state machine for page table walks, segment and paging caches, and microcode for everything else.
JIO_OK ("jump if I/O OK") tests whether CPL ≤ IOPL. The same check gates PUSHF, POPF, INT n, and IRET. The monitor then emulates each instruction as appropriate: maintaining a virtual interrupt flag per V86 task, reflecting software interrupts through the real-mode interrupt vector table, virtualizing I/O accesses, and so on.
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