Code Examples

Code Examples of Conditional Jumps in Assembly Language

Here are some practical examples demonstrating conditional jumps in assembly. We'll use the x86 assembly language with MASM-style syntax to show how these instructions work. These examples assume you are using a 16-bit or 32-bit x86 environment, but the concepts apply to both.

---

Example 1: Jump if Equal (JE or JZ)

This example shows how to jump if two values are equal. The CMP instruction compares two values, and if they are equal, the Zero Flag (ZF) is set, causing the program to jump to the equal label.

section .data
    msg db 'Equal', 0
    msg_not_equal db 'Not Equal', 0

section .text
    ; Assume AX = 5, BX = 5
    MOV AX, 5
    MOV BX, 5

    CMP AX, BX        ; Compare AX with BX
    JE equal          ; Jump to "equal" if AX == BX (ZF = 1)

    ; If not equal
    MOV DX, msg_not_equal
    RET

equal:
    ; If equal
    MOV DX, msg
    RET

Explanation:

• MOV AX, 5 and MOV BX, 5 load both registers with the value 5.
• CMP AX, BX compares AX with BX. Since the values are equal, the Zero Flag (ZF) will be set to 1.
• JE equal will jump to the equal label because ZF = 1 (indicating the two values are equal).

---

Example 2: Jump if Not Equal (JNE or JNZ)

This example shows how to jump if two values are not equal. The CMP instruction compares the two values, and if they are not equal, the Zero Flag (ZF) is cleared, causing the program to jump to the not_equal label.

section .data
    msg db 'Equal', 0
    msg_not_equal db 'Not Equal', 0

section .text
    ; Assume AX = 5, BX = 10
    MOV AX, 5
    MOV BX, 10

    CMP AX, BX        ; Compare AX with BX
    JNE not_equal     ; Jump to "not_equal" if AX != BX (ZF = 0)

    ; If equal
    MOV DX, msg
    RET

not_equal:
    ; If not equal
    MOV DX, msg_not_equal
    RET

Explanation:

• MOV AX, 5 and MOV BX, 10 load AX with 5 and BX with 10.
• CMP AX, BX compares AX with BX. Since the values are different, the Zero Flag (ZF) is cleared (ZF = 0).
• JNE not_equal will jump to the not_equal label because ZF = 0 (indicating the values are not equal).

---

Example 3: Jump if Carry (JC)

This example demonstrates jumping if there was a carry. A carry can occur after an addition or subtraction that results in an overflow, but for simplicity, we'll use a subtraction.

section .data
    msg db 'Carry Detected', 0
    msg_no_carry db 'No Carry', 0

section .text
    ; Assume AX = 10, BX = 20
    MOV AX, 10
    MOV BX, 20

    CMP AX, BX          ; Compare AX and BX
    JC carry_detected   ; Jump to "carry_detected" if carry (CF = 1)

    ; No carry
    MOV DX, msg_no_carry
    RET

carry_detected:
    ; Carry occurred
    MOV DX, msg
    RET

Explanation:

• MOV AX, 10 and MOV BX, 20 load AX and BX with values 10 and 20, respectively.
• CMP AX, BX compares AX with BX. Since AX < BX, there will be a borrow (carry in unsigned terms).
• JC carry_detected will jump to carry_detected because the Carry Flag (CF) is set after the comparison.

---

Example 4: Jump if Overflow (JO)

This example demonstrates how to jump if there is an overflow. Overflow occurs in signed arithmetic operations when the result exceeds the maximum value that can be represented.

section .data
    msg db 'Overflow Detected', 0
    msg_no_overflow db 'No Overflow', 0

section .text
    ; Assume AX = 32767, BX = 1 (Overflow in signed addition)
    MOV AX, 32767      ; Largest positive signed 16-bit value
    MOV BX, 1          ; Small value to cause overflow

    ADD AX, BX         ; AX = AX + BX
    JO overflow_detected ; Jump to "overflow_detected" if overflow (OF = 1)

    ; No overflow
    MOV DX, msg_no_overflow
    RET

overflow_detected:
    ; Overflow detected
    MOV DX, msg
    RET

Explanation:

• MOV AX, 32767 and MOV BX, 1 load AX with the largest positive signed 16-bit value (32767) and BX with 1.
• ADD AX, BX adds AX and BX, resulting in 32768, which exceeds the 16-bit signed integer limit.
• JO overflow_detected will jump to the overflow_detected label because the Overflow Flag (OF) is set after the addition.

---

Example 5: Jump if Negative (JS)

This example shows how to jump if the result of a signed comparison is negative, based on the Sign Flag (SF).

section .data
    msg_negative db 'Negative', 0
    msg_non_negative db 'Non-negative', 0

section .text
    ; Assume AX = -5, BX = 5
    MOV AX, -5
    MOV BX, 5

    CMP AX, BX          ; Compare AX and BX
    JS negative         ; Jump to "negative" if AX < BX (SF = 1)

    ; Non-negative
    MOV DX, msg_non_negative
    RET

negative:
    ; Negative
    MOV DX, msg_negative
    RET

Explanation:

• MOV AX, -5 and MOV BX, 5 load AX with -5 and BX with 5.
• CMP AX, BX compares AX and BX. Since AX < BX in signed terms, the Sign Flag (SF) will be set (SF = 1).
• JS negative will jump to the negative label because SF = 1 indicates a negative result.

---

Example 6: Jump if No Sign (JNS)

This example demonstrates how to jump if the result of a signed operation is non-negative, i.e., if the Sign Flag (SF) is clear.

section .data
    msg_positive_or_zero db 'Positive or Zero', 0
    msg_negative db 'Negative', 0

section .text
    ; Assume AX = -5, BX = 5
    MOV AX, -5
    MOV BX, 5

    CMP AX, BX          ; Compare AX and BX
    JNS positive_or_zero ; Jump to "positive_or_zero" if AX >= BX (SF = 0)

    ; Negative
    MOV DX, msg_negative
    RET

positive_or_zero:
    ; Non-negative or Zero
    MOV DX, msg_positive_or_zero
    RET

Explanation:

• MOV AX, -5 and MOV BX, 5 load AX with -5 and BX with 5.
• CMP AX, BX compares AX and BX. Since AX is less than BX, the Sign Flag (SF) is set (SF = 1), meaning it's a negative result.
• JNS positive_or_zero will jump to the positive_or_zero label if the result is non-negative (SF = 0).

---

Summary of Conditional Jumps:

Jump Instruction	Condition	Flag Checked	Meaning
JE / JZ	Equal	ZF (Zero Flag)	Jump if the two values are equal
JNE / JNZ	Not Equal	ZF (Zero Flag)	Jump if the two values are not equal
JC	Carry	CF (Carry Flag)	Jump if there was a carry (unsigned)
JNC	No Carry	CF (Carry Flag)	Jump if there was no carry (unsigned)
JS	Negative	SF (Sign Flag)	Jump if the result was negative
JNS	Positive or Zero	SF (Sign Flag)	Jump if the result was non-negative
JO	Overflow	OF (Overflow Flag)