But different callers need different validation rules. A MOV DS, AX needs to reject call gates but accept data segments. A CALL FAR needs to accept call gates and code segments. How can one shared subroutine perform different validation?
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Nearly every protection-related instruction -- far CALL, far JMP, far RET, INT, IRET, MOV to segment register, task switch -- needs to load a segment descriptor from the GDT or LDT. The 386 microcode centralizes this into a shared subroutine called LD_DESCRIPTOR, which reads the 8-byte descriptor from memory and feeds the high DWORD (containing Type, DPL, S, and P bits) to the Test PLA for validation.