ia-32_instruction-set-ref_a-m

Result the of flag is undefined protected mode

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Unformatted text preview: . Compat/ Leg Mode Valid N.E. Valid Valid N.E. Valid Valid Description Decrement r/m8 by 1. Decrement r/m8 by 1. Decrement r/m16 by 1. Decrement r/m32 by 1. Decrement r/m64 by 1. Decrement r16 by 1. Decrement r32 by 1. NOTES: * In 64-bit mode, r/m8 can not be encoded to access the following byte registers if a REX prefix is used: AH, BH, CH, DH. Description Subtracts 1 from the destination operand, while preserving the state of the CF flag. The destination operand can be a register or a memory location. This instruction allows a loop counter to be updated without disturbing the CF flag. (To perform a decrement operation that updates the CF flag, use a SUB instruction with an immediate operand of 1.) This instruction can be used with a LOCK prefix to allow the instruction to be executed atomically. In 64-bit mode, DEC r16 and DEC r32 are not encodable (because opcodes 48H through 4FH are REX prefixes). Otherwise, the instruction's 64-bit mode default operation size is 32 bits. Use of the REX.R prefix permits access to additional registers (R8-R15). Use of the REX.W prefix promotes operation to 64 bits. See the summary chart at the beginning of this section for encoding data and limits. Operation DEST DEST 1; Flags Affected The CF flag is not affected. The OF, SF, ZF, AF, and PF flags are set according to the result. 3-260 Vol. 2 INSTRUCTION SET REFERENCE, A-M Protected Mode Exceptions #GP(0) If the destination operand is located in a non-writable segment. If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit. If the DS, ES, FS, or GS register contains a NULL segment selector. #SS(0) #PF(fault-code) #AC(0) If a memory operand effective address is outside the SS segment limit. If a page fault occurs. If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3. Real-Address Mode Exceptions #GP #SS If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit. If a memory operand effective address is outside the SS segment limit. Virtual-8086 Mode Exceptions #GP(0) #SS(0) #PF(fault-code) #AC(0) If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit. If a memory operand effective address is outside the SS segment limit. If a page fault occurs. If alignment checking is enabled and an unaligned memory reference is made. Compatibility Mode Exceptions Same exceptions as in Protected Mode. 64-Bit Mode Exceptions #SS(0) #GP(0) #PF(fault-code) #AC(0) If a memory address referencing the SS segment is in a noncanonical form. If the memory address is in a non-canonical form. If a page fault occurs. If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3. Vol. 2 3-261 INSTRUCTION SET REFERENCE, A-M DIV--Unsigned Divide Opcode F6 /6 Instruction DIV r/m8 DIV r/m8* 64-Bit Mode Valid Compat/ Leg Mode Valid Description Unsigned divide AX by r/m8, with result stored...
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