Opcode Instruction Clocks Description 88 /r MOV r/m8,r8 2/2 Move byte register to r/m byte 89 /r MOV r/m16,r16 2/2 Move word register to r/m word 89 /r MOV r/m32,r32 2/2 Move dword register to r/m dword 8A /r MOV r8,r/m8 2/4 Move r/m byte to byte register 8B /r MOV r16,r/m16 2/4 Move r/m word to word register 8B /r MOV r32,r/m32 2/4 Move r/m dword to dword register 8C /r MOV r/m16,Sreg 2/2 Move segment register to r/m word 8D /r MOV Sreg,r/m16 2/5,pm=18/19 Move r/m word to segment register A0 MOV AL,moffs8 4 Move byte at (seg:offset) to AL A1 MOV AX,moffs16 4 Move word at (seg:offset) to AX A1 MOV EAX,moffs32 4 Move dword at (seg:offset) to EAX A2 MOV moffs8,AL 2 Move AL to (seg:offset) A3 MOV moffs16,AX 2 Move AX to (seg:offset) A3 MOV moffs32,EAX 2 Move EAX to (seg:offset) B0 + rb MOV reg8,imm8 2 Move immediate byte to register B8 + rw MOV reg16,imm16 2 Move immediate word to register B8 + rd MOV reg32,imm32 2 Move immediate dword to register C6 MOV r/m8,imm8 2/2 Move immediate byte to r/m byte C7 MOV r/m16,imm16 2/2 Move immediate word to r/m word C7 MOV r/m32,imm32 2/2 Move immediate dword to r/m dword
DEST := SRC;
If the destination operand is a segment register (DS, ES, SS, etc.), then data from a descriptor is also loaded into the register. The data for the register is obtained from the descriptor table entry for the selector given. A null selector (values 0000-0003) can be loaded into DS and ES registers without causing an exception; however, use of DS or ES causes a #GP(0), and no memory reference occurs.
A MOV into SS inhibits all interrupts until after the execution of the next instruction (which is presumably a MOV into eSP).
Loading a segment register under 80386 Protected Mode results in special checks and actions, as described in the following listing:
IF SS is loaded; THEN IF selector is null THEN #GP(0); FI; Selector index must be within its descriptor table limits else #GP(selector); Selector's RPL must equal CPL else #GP(selector); AR byte must indicate a writable data segment else #GP(selector); DPL in the AR byte must equal CPL else #GP(selector); Segment must be marked present else #SS(selector); Load SS with selector; Load SS with descriptor. FI; IF DS, ES, FS or GS is loaded with non-null selector; THEN Selector index must be within its descriptor table limits else #GP(selector); AR byte must indicate data or readable code segment else #GP(selector); IF data or nonconforming code segment THEN both the RPL and the CPL must be less than or equal to DPL in AR byte; ELSE #GP(selector); FI; Segment must be marked present else #NP(selector); Load segment register with selector; Load segment register with descriptor; FI; IF DS, ES, FS or GS is loaded with a null selector; THEN Load segment register with selector; Clear descriptor valid bit; FI;
Chapter 17 -- 80386 Instruction Set
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