3-464 Vol. 2A INC—Increment by 1
INSTRUCTION SET REFERENCE, A-M
INC—Increment by 1
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 regis-
ters (R8-R15). Use of the REX.W prefix promotes operation to 64 bits.
Operation
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.
Opcode Instruction 64-Bit
Mode
Compat/
Leg Mode
Description
FE /0 INC r/m8 Valid Valid Increment r/m byte by 1.
REX + FE /0 INC r/m8
*
Valid N.E. Increment r/m byte by 1.
FF /0 INC r/m16 Valid Valid Increment r/m word by 1.
FF /0 INC r/m32 Valid Valid Increment r/m doubleword by 1.
REX.W + FF /0 INC r/m64 Valid N.E. Increment r/m quadword by 1.
40+ rw
**
INC r16 N.E. Valid Increment word register by 1.
40+ rd INC r32 N.E. Valid Increment doubleword register 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.
** 40H through 47H are REX prefixes in 64-bit mode.
