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80386 Programmer's Reference Manual -- Opcode LAR

LAR -- Load Access Rights Byte

Opcode	Instruction	Clocks	Description	Example
0F 02 /r	larw r/m16,r16	pm=15/16	r16 := r/m16 masked by FF00	`larw %cx,%bx larw (%ebx,1),%bx larw (%ebx,2),%bx larw (%ebx,%ebp,1),%bx`
0F 02 /r	larl r/m32,r32	pm=15/16	r32 := r/m32 masked by 00FxFF00	`larl %ecx,%ebx larl (%ebx,2),%ebx larl (%ebx,4),%ebx larl (%ebx,%ebp,1),%ebx`

Description

The LAR instruction stores a marked form of the second doubleword of the descriptor for the source selector if the selector is visible at the CPL (modified by the selector's RPL) and is a valid descriptor type. The destination register is loaded with the high-order doubleword of the descriptor masked by 00FxFF00, and ZF is set to 1. The x indicates that the four bits corresponding to the upper four bits of the limit are undefined in the value loaded by LAR. If the selector is invisible or of the wrong type, ZF is cleared.

If the 32-bit operand size is specified, the entire 32-bit value is loaded into the 32-bit destination register. If the 16-bit operand size is specified, the lower 16-bits of this value are stored in the 16-bit destination register.

All code and data segment descriptors are valid for LAR.

The valid special segment and gate descriptor types for LAR are given in the following table:




Type   Name                     Valid/Invalid

  0    Invalid                  Invalid
  1    Available 80286 TSS      Valid
  2    LDT                      Valid
  3    Busy 80286 TSS           Valid
  4    80286 call gate          Valid
  5    80286/80386 task gate    Valid
  6    80286 trap gate          Valid
  7    80286 interrupt gate     Valid
  8    Invalid                  Invalid
  9    Available 80386 TSS      Valid
  A    Invalid                  Invalid
  B    Busy 80386 TSS           Valid
  C    80386 call gate          Valid
  D    Invalid                  Invalid
  E    80386 trap gate          Valid
  F    80386 interrupt gate     Valid

Flags Affected

ZF as described above

Protected Mode Exceptions

#GP(0) for an illegal memory operand effective address in the CS, DS, ES, FS, or GS segments; #SS(0) for an illegal address in the SS segment; #PF(fault-code) for a page fault

Real Address Mode Exceptions

Interrupt 6; LAR is unrecognized in Real Address Mode

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode

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