The FLAGS Register and Status Bits

The FLAGS register (EFLAGS in 32-bit, RFLAGS in 64-bit) is a special register where individual bits record the result of the most recent operation.

The CPU has no boolean type. Instead, instructions set flag bits, and conditional jumps read them. Flags are the glue between arithmetic and control flow.

ZF is set to 1 when the result of an operation is zero. It is the basis of equality tests.

CF is set when an unsigned operation overflows out of the most significant bit, e.g. an addition that exceeds the register width.

SF copies the most significant bit of the result, indicating a negative value in signed interpretation.

OF is set when a signed operation produces a result too large or small for the destination, i.e. signed overflow.

CMP a, b performs a - b but discards the result, keeping only the flags. This lets you test relationships without changing operands.

TEST a, b computes a bitwise AND, discards the result, and sets flags, commonly used to check if a value is zero or test specific bits.

Conditional jumps read flags:

• JE/JZ: jump if ZF=1
• JNE/JNZ: jump if ZF=0
• JC: jump if CF=1
• JG: signed greater (uses ZF, SF, OF)

Signed comparisons use SF and OF; unsigned comparisons use CF. Choosing the wrong jump (JA vs JG) is a classic bug.

Some instructions set or clear flags explicitly, like STC (set carry) and CLC (clear carry), useful before multi-precision arithmetic.

Test your understanding of CPU flags.

You learned the FLAGS register:

• ZF zero, CF unsigned overflow, SF sign, OF signed overflow
• CMP and TEST set flags without storing results
• Conditional jumps branch on flag state
• Match signed/unsigned jumps to the comparison