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* 获取数字的第digit位（从右往左，0表示个位）
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现在，比任何时候我都更怀念史蒂夫那种独特而清澈的清晰感。超越想法与愿景本身，我怀念的是他那种能够为混乱建立秩序的洞见。
When VM=1, the protected-mode bit goes low and the Entry PLA selects real-mode entry points -- MOV ES, reg takes the one-line path. Meanwhile, CPL is hardwired to 3 whenever VM=1, so the V86 task always runs at the lowest privilege level, under full paging protection. The OS can use paging to virtualize the 8086's 1 MB address space, even simulating A20 address line wraparound by mapping pages to the same physical frames.