Jumps Based on Specific Condition

Jumps Based on Specific Condition in Assembly Language

In assembly language, jumps based on a specific condition allow you to alter the flow of a program depending on the result of an operation. These conditional jumps are typically used after a comparison operation (CMP) or any arithmetic/logical operation that affects the flags in the EFLAGS register. Depending on the state of the flags (such as the Zero Flag (ZF), Carry Flag (CF), Sign Flag (SF), etc.), you can control the program's execution path.

Overview of Conditional Jump Instructions

Conditional jumps are instructions that cause a jump to a specific label if a certain condition (based on the processor's flags) is met. The key flags used for these jumps are set by the previous instructions and dictate the flow.

Here’s a breakdown of common conditional jumps based on specific conditions:

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1. Jump if Zero (JE or JZ)

• Condition: Jump if the Zero Flag (ZF) is set.
• Meaning: This happens if the result of the last operation (e.g., CMP, SUB, etc.) was zero. This indicates that two compared values are equal.

Instruction:

JE  label   ; Jump if Zero Flag (ZF) is set (i.e., if the values are equal)
JZ  label   ; Same as JE

Example:

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

MOV DX, 'Not Equal'
RET

equal:
MOV DX, 'Equal'
RET

Here, CMP AX, BX compares the contents of AX and BX. If they are equal, the Zero Flag (ZF) is set, and the program will jump to the equal label.

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2. Jump if Not Zero (JNE or JNZ)

• Condition: Jump if the Zero Flag (ZF) is clear.
• Meaning: This happens if the result of the last operation was non-zero. This indicates that two compared values are not equal.

Instruction:

JNE label   ; Jump if Zero Flag (ZF) is clear (i.e., if the values are not equal)
JNZ label   ; Same as JNE

Example:

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

MOV DX, 'Equal'
RET

not_equal:
MOV DX, 'Not Equal'
RET

In this example, the program jumps to not_equal if the values in AX and BX are different (i.e., ZF = 0).

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3. Jump if Carry (JC)

• Condition: Jump if the Carry Flag (CF) is set.
• Meaning: This usually occurs after an arithmetic operation like addition or subtraction. The Carry Flag is set if there was an overflow during an unsigned operation (like when adding two large numbers or subtracting with a borrow).

Instruction:

JC label   ; Jump if Carry Flag (CF) is set (i.e., there was an unsigned overflow or borrow)

Example:

CMP AX, BX    ; Compare AX and BX (sets flags)
JC  carry_set ; Jump to "carry_set" if there was a carry (CF = 1)

MOV DX, 'No Carry'
RET

carry_set:
MOV DX, 'Carry Occurred'
RET

Here, the CMP instruction compares AX and BX, and the jump will happen if there is a carry flag set (i.e., an unsigned overflow or borrow).

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4. Jump if No Carry (JNC)

• Condition: Jump if the Carry Flag (CF) is clear.
• Meaning: This happens if there was no carry during the previous arithmetic operation.

Instruction:

JNC label   ; Jump if Carry Flag (CF) is clear (i.e., no carry occurred)

Example:

CMP AX, BX    ; Compare AX and BX (sets flags)
JNC no_carry  ; Jump to "no_carry" if no carry occurred (CF = 0)

MOV DX, 'Carry Occurred'
RET

no_carry:
MOV DX, 'No Carry'
RET

This example demonstrates how to check if there was no carry after a comparison.

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5. Jump if Positive (JS)

• Condition: Jump if the Sign Flag (SF) is set.
• Meaning: The Sign Flag (SF) is set if the result of the previous operation was negative (in a signed operation).

Instruction:

JS label   ; Jump if Sign Flag (SF) is set (i.e., the result was negative)

Example:

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

MOV DX, 'Positive or Zero'
RET

negative:
MOV DX, 'Negative Result'
RET

Here, the jump to negative occurs if the comparison shows that AX is less than BX (i.e., SF = 1 after a signed comparison).

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6. Jump if No Sign (JNS)

• Condition: Jump if the Sign Flag (SF) is clear.
• Meaning: This happens if the result of the previous operation was non-negative (in a signed operation).

Instruction:

JNS label   ; Jump if Sign Flag (SF) is clear (i.e., the result was non-negative)

Example:

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

MOV DX, 'Negative Result'
RET

positive_or_zero:
MOV DX, 'Positive or Zero'
RET

This example shows how to check for non-negative results in signed comparisons.

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7. Jump if Overflow (JO)

• Condition: Jump if the Overflow Flag (OF) is set.
• Meaning: The Overflow Flag (OF) is set if there was an overflow in a signed arithmetic operation, meaning the result exceeded the representable range for the data type.

Instruction:

JO label   ; Jump if Overflow Flag (OF) is set (i.e., signed overflow occurred)

Example:

CMP AX, BX    ; Compare AX and BX (sets flags)
JO  overflow  ; Jump to "overflow" if signed overflow occurred (OF = 1)

MOV DX, 'No Overflow'
RET

overflow:
MOV DX, 'Overflow Occurred'
RET

Here, the program jumps to overflow if there was a signed overflow in the comparison.

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8. Jump if No Overflow (JNO)

• Condition: Jump if the Overflow Flag (OF) is clear.
• Meaning: This happens if there was no overflow during the last signed arithmetic operation.

Instruction:

JNO label   ; Jump if Overflow Flag (OF) is clear (i.e., no signed overflow)

Example:

CMP AX, BX    ; Compare AX and BX (sets flags)
JNO no_overflow ; Jump to "no_overflow" if no signed overflow (OF = 0)

MOV DX, 'Overflow Detected'
RET

no_overflow:
MOV DX, 'No Overflow'
RET

In this case, the program jumps to no_overflow if there was no overflow in the signed comparison.

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Summary of Common 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)	Jump if there was signed overflow
JNO	No Overflow	OF (Overflow Flag)	Jump if there was no signed overflow

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Example: Complete Program with Conditional Jumps

section .data
    msg db 'Comparison Result: ', 0

section .text
    MOV AX, 5
    MOV BX, 10
    CMP AX, BX          ; Compare AX and BX

    JE  equal           ; Jump if equal (ZF = 1)
    JNE not_equal       ; Jump if not equal (ZF =