The 80286 introduced "Protected Mode" in 1982. It was not popular. The mode was difficult to use, lacked paging, and offered no way to return to real mode without a hardware reset. The 80386, arriving three years later, made protection usable -- adding paging, a flat 32-bit address space, per-page User/Supervisor control, and Virtual 8086 mode so that DOS programs could run inside a protected multitasking system. These features made possible Windows 3.0, OS/2, and early Linux.
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，详情可参考搜狗输入法2026
The answer is essentially hardware-level dependency injection. Before calling LD_DESCRIPTOR, the caller saves its desired test constant into a hardware latch using a micro-op called PTSAV (Protection Save). Within LD_DESCRIPTOR, another micro-op called PTOVRR (Protection Override) retrieves and fires the saved test.
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