Computer Hardware User's Manual

3-28 Vol. 2A ADC—Add with Carry
INSTRUCTION SET REFERENCE, A-M
Description
Adds the destination operand (first operand), the source operand (second operand),
and the carry (CF) flag and stores the result in the destination operand. The destina-
tion operand can be a register or a memory location; the source operand can be an
immediate, a register, or a memory location. (However, two memory operands
cannot be used in one instruction.) The state of the CF flag represents a carry from a
previous addition. When an immediate value is used as an operand, it is sign-
extended to the length of the destination operand format.
The ADC instruction does not distinguish between signed or unsigned operands.
Instead, the processor evaluates the result for both data types and sets the OF and
CF flags to indicate a carry in the signed or unsigned result, respectively. The SF flag
indicates the sign of the signed result.
The ADC instruction is usually executed as part of a multibyte or multiword addition
in which an ADD instruction is followed by an ADC instruction.
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
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 + CF;
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.
Opcode Instruction 64-Bit
Mode
Compat/
Leg Mode
Description
13 /r ADC r32, r/m32 Valid Valid Add with CF r/m32 to r32.
REX.W + 13 /r ADC r64, r/m64 Valid N.E. Add with CF r/m64 to r64.
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.