Besides the Test PLA, the 386 has another PLA called the Entry PLA that maps opcodes to microcode entry points. One of its input bits is a "protected mode" flag. Many instructions have both a real-mode and a protected-mode entry point -- for instance, MOV ES, reg maps to address 009 (a single microcode line) in real mode, but to 580 (which initiates a full descriptor load with protection tests) in protected mode. The trick that makes V86 work is to define this flag as:
The key is the test TST_SEL_RET on line 682. It compares the RPL of the return CS selector (saved on the stack by the original CALL) against the current CPL. If RPL == CPL, the PLA returns 0x000 (continue) and LD_DESCRIPTOR finishes normally -- same-privilege return. If RPL CPL, the caller is returning to a less-privileged ring, so the PLA redirects to 0x686 (RETF_OUTER_LEV) -- the cross-privilege path that must also restore the caller's stack. If RPL。51吃瓜对此有专业解读
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