Arithmetic Instructions

x86 provides a compact but complete set of arithmetic instructions: ADD, SUB, INC, DEC, NEG, MUL, IMUL, DIV, and IDIV. Every one of them (except INC/DEC's treatment of CF, discussed below) computes a result and simultaneously updates the FLAGS register -- setting the Zero flag (ZF) if the result is 0, the Sign flag (SF) to the result's high bit, the Carry flag (CF) if an unsigned operation wrapped around, and the Overflow flag (OF) if a signed operation produced a mathematically invalid result. Later conditional jumps like JZ, JC, or JO branch based on exactly these flags.

Real programs frequently need to add or subtract numbers wider than a single register, such as 64-bit values on a 32-bit CPU. ADC (add with carry) and SBB (subtract with borrow) solve this by incorporating the Carry flag left over from a previous ADD or SUB. The standard pattern is to ADD the lowest-order words first, which correctly establishes CF based on that word's own overflow, and then use ADC for each successively higher-order word, folding in the carry from the word below it.

Multiplication and division on x86 always involve the EDX:EAX register pair for their one-operand forms. MUL ebx multiplies the unsigned value in EAX by ebx, producing a full 64-bit product spread across EDX (high) and EAX (low); IMUL performs the signed equivalent. DIV ebx divides the 64-bit value in EDX:EAX by ebx, leaving the quotient in EAX and the remainder in EDX; IDIV does the same for signed operands. Dividing by zero raises a CPU exception rather than returning an error code, so a zero-divisor check before DIV is essential.

INC and DEC increment or decrement a single operand by one and are notable for deliberately leaving CF unchanged, even though they update ZF, SF, OF, and PF -- this lets a loop counter use INC/DEC without disturbing an ongoing multi-word carry chain built from ADC/SBB elsewhere in the same routine. NEG computes the two's complement of its operand, effectively negating a signed value, and internally sets CF unless the operand was zero. Understanding exactly which flags each instruction touches is essential for writing correct conditional logic afterward.