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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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.
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