Vol. 2A 3-31
INSTRUCTION SET REFERENCE, A-M
ADD—Add
Description
Adds the destination operand (first operand) and the source operand (second
operand) and then stores the result in the destination operand. The destination
operand can be a register or a memory location; the source operand can be an imme-
diate, a register, or a memory location. (However, two memory operands cannot be
used in one instruction.) When an immediate value is used as an operand, it is sign-
extended to the length of the destination operand format.
The ADD instruction performs integer addition. It evaluates the result for both signed
and unsigned integer operands and sets the OF and CF flags to indicate a carry (over-
flow) in the signed or unsigned result, respectively. The SF flag indicates the sign of
the signed result.
This instruction can be used with a LOCK prefix to allow the instruction to be
executed atomically.
In 64-bit mode, the instruction’s default operation size is 32 bits. Using a REX prefix
in the form of REX.R permits access to additional registers (R8-R15). Using a REX a
REX prefix in the form of REX.W promotes operation to 64 bits. See the summary
chart at the beginning of this section for encoding data and limits.
Operation
DEST ← DEST + SRC;
Flags Affected
The OF, SF, ZF, AF, CF, and PF flags are set according to the result.
Protected Mode Exceptions
#GP(0) If the destination 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 NULL segment 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.
