The fundamental difference between the ADD and ADDC instructions lies in how they handle the CPU's Carry Flag. While both perform addition, ADDC incorporates the state of the Carry Flag into its calculation, whereas ADD does not.
What is the Difference Between the ADD and ADDC Instructions?
The ADD instruction performs a standard binary addition of two operands, storing the result in the destination operand. In contrast, the ADDC (Add with Carry) instruction performs an addition that includes not only the two specified operands but also the current value of the Carry Flag (CF) from the CPU's status register.
This distinction is crucial for performing multi-precision arithmetic, where numbers larger than the processor's native word size need to be added.
Understanding the ADD Instruction
The ADD instruction is a basic arithmetic operation that computes the sum of two source operands and places the result in the destination operand. It sets the CPU's flag register bits, including the Carry Flag, based on the outcome of the addition.
- Operation:
Destination = Destination + Source - Carry Flag (CF): The Carry Flag is set if the addition results in an unsigned overflow (i.e., the sum is too large to fit in the destination register). It is reset otherwise. However, the
ADDinstruction itself does not use the CF from a previous operation as part of its calculation.
Understanding the ADDC Instruction
The ADDC instruction is similar to ADD but with one critical addition: it adds the value of the Carry Flag (CF) from the previous arithmetic operation to the sum of its two operands.
- Operation:
Destination = Destination + Source + CarryFlag - Carry Flag (CF): The
ADDCinstruction uses the current state of the Carry Flag (0 or 1) as an implicit third operand. It also updates the Carry Flag based on its own result, allowing a chain of additions to propagate carries correctly.
Key Differences Summarized
| Feature | ADD Instruction | ADDC Instruction |
|---|---|---|
| Basic Operation | Adds two operands. | Adds two operands plus the Carry Flag. |
| Carry Flag Input | Does not use the Carry Flag as an input to its sum. | Uses the Carry Flag as an input (0 or 1) to its sum. |
| Primary Use Case | Single-word (native size) addition. | Multi-precision (multi-word) addition, chained operations. |
| Calculation | Result = Operand1 + Operand2 |
Result = Operand1 + Operand2 + CarryFlag_Input |
Practical Application: Multi-Precision Addition
The ADDC instruction is indispensable for adding numbers that exceed the native word size of the processor. For instance, if you need to add two 64-bit numbers on a 32-bit processor, you would typically break down each 64-bit number into two 32-bit parts (high and low words).
Consider adding two 64-bit numbers, A and B, composed of A_high:A_low and B_high:B_low respectively.
- Add the low-order words:
ADD A_low, B_low ; Adds the least significant 32 bits. ; If this operation produces a carry, the Carry Flag (CF) is set to 1. - Add the high-order words with carry:
ADDC A_high, B_high ; Adds the most significant 32 bits, ; AND the value of the Carry Flag from the previous ADD operation.
In this sequence, the ADDC instruction ensures that any overflow from the addition of the lower 32-bit words is correctly propagated and added to the higher 32-bit words, resulting in an accurate 64-bit sum. This capability extends to any number of words, allowing for arithmetic operations on arbitrarily large integers.
For more detailed information on CPU instruction sets and flags, you can refer to processor architecture manuals such as those from Intel or AMD.
