// 步骤3：计算最终能看到的人数
With her hands steady at the controls, her voice calm as she spoke to mission control, Collins piloted the craft through a slow, graceful somersault. With the shuttle's underside now visible, the damage was quickly spotted - and a spacewalk was carried out to repair it.
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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.
Kevin Church,Science team