INC

Increment by 1

Opcodes

Hex Mnemonic Encoding Long Mode Legacy Mode Description
40+ rd INC r32 B N.E. Valid Increment doubleword register by 1.
40+ rw ** INC r16 B N.E. Valid Increment word register by 1.
REX.W + FF /0 INC r/m64 A Valid N.E. Increment r/m quadword by 1.
FF /0 INC r/m32 A Valid Valid Increment r/m doubleword by 1.
FF /0 INC r/m16 A Valid Valid Increment r/m word by 1.
REX + FE /0 INC r/m8 * A Valid N.E. Increment r/m byte by 1.
FE /0 INC r/m8 A Valid Valid Increment r/m byte by 1.

Instruction Operand Encoding

Op/En Operand 0 Operand 1 Operand 2 Operand 3
A NA NA NA ModRM:r/m (r, w)
B NA NA NA reg (r, w)

Description

Adds 1 to 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. (Use a ADD instruction with an immediate operand of 1 to perform an increment operation that does updates the CF flag.)

This instruction can be used with a LOCK prefix to allow the instruction to be executed atomically.

In 64-bit mode, INC r16 and INC r32 are not encodable (because opcodes 40H through 47H 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.

Pseudo Code

DEST = DEST + 1;
AFlags Affected
The CF flag is not affected. The OF, SF, ZF, AF, and PF flags are set according to the result.

Exceptions

64-Bit Mode Exceptions

Exception Description
#UD If the LOCK prefix is used but the destination is not a memory operand.
#AC(0) If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3.
#PF(fault-code) If a page fault occurs.
#GP(0) If the memory address is in a non-canonical form.
#SS(0) If a memory address referencing the SS segment is in a non-canonical form.

Compatibility Mode Exceptions

Same exceptions as in protected mode.

Virtual-8086 Mode Exceptions

Exception Description
#UD If the LOCK prefix is used but the destination is not a memory operand.
#AC(0) If alignment checking is enabled and an unaligned memory reference is made.
#PF(fault-code) If a page fault occurs.
#SS(0) If a memory operand effective address is outside the SS segment limit.
#GP(0) If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit.

Real-Address Mode Exceptions

Exception Description
#UD If the LOCK prefix is used but the destination is not a memory operand.
#SS If a memory operand effective address is outside the SS segment limit.
#GP If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit.

Protected Mode Exceptions

Exception Description
#UD If the LOCK prefix is used but the destination is not a memory operand.
#AC(0) If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3.
#PF(fault-code) If a page fault occurs.
#SS(0) If a memory operand effective address is outside the SS segment limit.
#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 is used to access memory and it contains a NULLsegment selector.