There was a misleading in my knowledge of a conditional jump: It checks only the result of CMP and TEST instruments. So when it appears after other instruments like ADD or SUB, I can find no clue on how it works.

Actually, a conditional jump checks flags in the EFLAGS control register. From Intel’s manual, vol 1, 3.4.3:

The status flags (bits 0, 2, 4, 6, 7, and 11) of the EFLAGS register indicate the results of arithmetic instructions, such as the ADD, SUB, MUL, and DIV instructions. The status flag functions are:

CF (bit 0) Carry flag: Set if an arithmetic operation generates a carry or a borrow out of the most-significant bit of the result; cleared otherwise. This flag indicates an overflow condition for unsigned-integer arithmetic. It is also used in multiple-precision arithmetic.

PF (bit 2) Parity flag: Set if the least-significant byte of the result contains an even number of 1 bits; cleared otherwise.
AF (bit 4) Adjust flag: Set if an arithmetic operation generates a carry or a borrow out of bit 3 of the result; cleared otherwise. This flag is used in binary-coded decimal (BCD) arithmetic.

ZF (bit 6) Zero flag: Set if the result is zero; cleared otherwise.

SF (bit 7) Sign flag: Set equal to the most-significant bit of the result, which is the sign bit of a signed integer. (0 indicates a positive value and 1 indicates a negative value.)

OF (bit 11) Overflow flag: Set if the integer result is too large a positive number or too small a negative number (excluding the sign-bit) to fit in the destination operand; cleared otherwise. This flag indicates an overflow condition for signed-integer (two’s complement) arithmetic.

And again from vol 2a, section Jcc Jump if Condition is met, more details. I just copy content from here:

There are signed and unsigned versions when comparing: JA Vs JG, JB Vs JL etc.. Let’s take JA and JG to explain the difference. For JA, it’s clear that it requires CF=0(no borrow bit) and ZF=0(not equal). For JG, when two operands are both positive or negative, it requires ZF=0 and SF=OF=0. When two operands have different signs, it requires ZF=0 and the first operand is positive, thus requires SF=OF=1.

Note, the following 2 lines(AT&T syntax) are equivalent. CPU does arithmetic calculation, it does not care about whether it is signed or unsigned. It only set flags. It is we that make the signed or unsigned jump decision.

Last, I’d like to use ndisasm(install nasm package to get it) to illustrate how jump instruments are encoded, including short jump, near jump and far jump: