inc
Increment
INC r/m
Increments the operand by 1.
Details
Increments operand by 1 and stores result in operand. Sets OF, SF, ZF, AF, PF; does not affect CF. Supports 8/16/32/64-bit operand sizes in all modes. Useful for loop counters and pointer arithmetic.
Pseudocode Operation
result ← dest + 1; dest ← result; OF ← overflow; ZF ← (result == 0); SF ← (result < 0); AF ← (result & 0xF); PF ← parity(result)Example
INC rbx
Encoding
Binary Layout
FF
+0
ModRM
+1
Operands
-
dest
Register or memory operand
Reference (Intel® SDM)
Instruction Forms
| Opcode | Instruction | Op/En | 64/32-bit Mode | CPUID | Description |
|---|---|---|---|---|---|
| FE /0 | INC r/m8 | M | Valid Valid | Increment r/m byte by 1. | |
| FF /0 | INC r/m16 | M | Valid Valid | Increment r/m word by 1. | |
| FF /0 | INC r/m32 | M | Valid Valid | Increment r/m doubleword by 1. | |
| REX.W + FF /0 | INC r/m64 | M | Valid N.E. | Increment r/m quadword by 1. | |
| 40+ rw2 | INC r16 | O | N.E. Valid | Increment word register by 1. | |
| 40+ rd | INC r32 | O | N.E. Valid | Increment doubleword register 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 registers (R8-R15). Use of the REX.W prefix promotes operation to 64 bits.
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.
Exceptions
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 is used to access memory and it contains a NULLsegment
selector.
#SS(0) If a memory operand effective address is outside the SS segment limit.
#PF(fault-code) If a page fault occurs.
#AC(0) If alignment checking is enabled and an unaligned memory reference is made while the
current privilege level is 3.
#UD If the LOCK prefix is used but the destination is not a memory operand.
Real-Address Mode Exceptions
#GP If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit.
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#SS If a memory operand effective address is outside the SS segment limit.
#UD If the LOCK prefix is used but the destination is not a memory operand.
Virtual-8086 Mode Exceptions
#GP(0) If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit.
#SS(0) If a memory operand effective address is outside the SS segment limit.
#PF(fault-code) If a page fault occurs.
#AC(0) If alignment checking is enabled and an unaligned memory reference is made.
#UD If the LOCK prefix is used but the destination is not a memory operand.
Compatibility Mode Exceptions
Same exceptions as in protected mode.
64-Bit Mode Exceptions
#SS(0) If a memory address referencing the SS segment is in a non-canonical form.
#GP(0) If the memory address is in a non-canonical form.
#PF(fault-code) If a page fault occurs.
#AC(0) If alignment checking is enabled and an unaligned memory reference is made while the
current privilege level is 3.
#UD If the LOCK prefix is used but the destination is not a memory operand.
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