Arithmetic instructions in x86 not only compute results but also update processor flags that describe properties of the result.

These flags are stored in the EFLAGS register, a special register used by the CPU to track conditions such as carry, overflow, zero results, and sign.

Example Program

section .text
global _start
 
_start:
    MOV eax, 0x1
    MOV bl, 0b11111111
    MOV cl, 0b11
 
    ADD bl, cl
 
    ADC bh, bl
 
    MOV bl, 0x45
    MOV cl, 0x1F
    SUB bl, cl
 
    MOV bl, 0xA
    MOV cl, 0xA
    SUB bl, cl
 
    MOV bl, 0xD
    MOV cl, 0xF3
    SUB bl, cl
 
    INT 0x80

The EFLAGS Register

The EFLAGS register is a special-purpose register that stores various status, control, and system flags.

Each bit in this register represents a condition produced by the last executed instruction.

These flags are automatically modified by CPU instructions to record the outcome of operations.

Examples of commonly used flags:

Not all instructions affect all flags, but arithmetic operations typically modify several of them.

Example 1: Addition and Carry Flag

MOV bl, 0b11111111
MOV cl, 0b11
ADD bl, cl

Values:

bl = 11111111
cl = 00000011

Binary addition:

11111111
+00000011
---------
1 00000010

Result stored in bl:

00000010

Since bl is an 8-bit register, only the lower 8 bits are stored.

The extra bit (1) becomes a carry.

This carry does not propagate to the next register (bh).

Instead, the CPU records this event in the Carry Flag (CF) inside the EFLAGS register.

Flags raised by this operation include:

CF = 1
AF = 1

Using the Carry with ADC

The instruction:

ADC bh, bl

means:

bh = bh + bl + CF

Unlike ADD, ADC also includes the Carry Flag in the computation.

This instruction is commonly used when performing multi-byte arithmetic.

Example scenario:

16-bit addition implemented using two 8-bit operations

ADD low_byte
ADC high_byte

This allows carry from the lower byte to propagate into the higher byte.

Example 2: Subtraction

MOV bl, 0x45
MOV cl, 0x1F
SUB bl, cl

Calculation:

0x45 - 0x1F = 0x26

Result stored in bl:

bl = 0x26

No special flags of interest are triggered here.

Example 3: Zero Flag

MOV bl, 0xA
MOV cl, 0xA
SUB bl, cl

Calculation:

10 - 10 = 0

Result:

bl = 0

This raises the Zero Flag (ZF).

ZF = 1

The Zero Flag is frequently used for comparisons and conditional jumps.

Example 4: Subtraction with Signed Values

MOV bl, 0xD      ; 13
MOV cl, 0xF3     ; -13 (two's complement representation)
SUB bl, cl

Calculation:

13 - (-13) = 26

Result:

bl = 26

During this operation the Carry Flag (CF) is raised.

In subtraction, CF indicates that a borrow occurred.

Important Notes About Flags

Flags provide information about the result of the last instruction. Some common interpretations are:

These flags are often used with conditional jump instructions such as:

JE
JNE
JC
JZ
JG
JL

Summary

• Arithmetic instructions modify the EFLAGS register.
• The Carry Flag (CF) indicates carry or borrow during arithmetic.
• The Zero Flag (ZF) indicates that the result is zero.
• The Auxiliary Carry Flag (AF) indicates carry between lower nibbles.
• ADC allows arithmetic to include the carry flag, which is useful for multi-byte operations.
• Flags are essential for implementing conditional logic and comparisons in assembly.