JCC Instruction Set

By Vesper Vei
9 minutes read
Table of Contents

Table of Contents

  1. JCC Instruction Set

JCC Instruction Set

JE / JZ

(jump if equal / jump if zero) Basic function
Jump when the most recent comparison/arithmetic result is “equal” or the result is 0. Jump condition (logic)

ZF == 1

RFLAGS (full diagram)

+-------------------------------+
| CF  | ZF  | SF  | OF  | PF  | AF  |
|carry|zero |sign |overflow|parity|aux|
+-------------------------------+
|  ?  | [1] |  ?  |  ?  |  ?  |  ?  |
+-------------------------------+

Equivalent expression

if (ZF == 1) jump;

Common uses
Branching after string or numeric equality checks (commonly used after cmp).

JNE / JNZ

(jump if not equal / jump if not zero) Basic function
Jump when the most recent comparison/arithmetic result is not equal or the result is non-zero. Jump condition

ZF == 0

RFLAGS (full diagram)

+-------------------------------+
| CF  | ZF  | SF  | OF  | PF  | AF  |
+-------------------------------+
|  ?  | [0] |  ?  |  ?  |  ?  |  ?  |
+-------------------------------+

Equivalent expression

if (ZF == 0) jump;

Common uses
Continue loops, continue after failed searches, etc.

JA / JNBE

(jump if above / jump if not below or equal) (unsigned a > b) Basic function
Used for unsigned comparison, indicating strictly greater than (a > b, unsigned).

Jump condition

(CF == 0) && (ZF == 0)

RFLAGS (full diagram)

+-------------------------------+
| CF  | ZF  | SF  | OF  | PF  | AF  |
+-------------------------------+
| [0] | [0] |  ?  |  ?  |  ?  |  ?  |
+-------------------------------+

Equivalent expression

if (!CF && !ZF) jump; // unsigned a > b

Common uses
Memory length, unsigned index comparisons, etc.

JAE / JNB / JNC

(jump if above or equal / jump if not below / jump if not carry) (unsigned a >= b) Basic function
Unsigned comparison >=: no borrow (carry).

Jump condition

CF == 0

RFLAGS (full diagram)

+-------------------------------+
| CF  | ZF  | SF  | OF  | PF  | AF  |
+-------------------------------+
| [0] |  ?  |  ?  |  ?  |  ?  |  ?  |
+-------------------------------+

Equivalent expression

if (!CF) jump; // unsigned a >= b

Common uses
Boundary checks (unsigned).

Note
JAE = JNB = JNC (common aliases).

JB / JNAE / JC

(jump if below / jump if not above or equal / jump if carry) (unsigned a < b) Basic function
Unsigned less than (with borrow).

Jump condition

CF == 1

RFLAGS (full diagram)

+-------------------------------+
| CF  | ZF  | SF  | OF  | PF  | AF  |
+-------------------------------+
| [1] |  ?  |  ?  |  ?  |  ?  |  ?  |
+-------------------------------+

Equivalent expression

if (CF == 1) jump; // unsigned a < b

Common uses
Error/boundary checks.
Note: JC (jump if carry) is an alias of JB.

JBE / JNA

(jump if below or equal / jump if not above) (unsigned a <= b) Basic function
Unsigned less than or equal.

Jump condition

(CF == 1) || (ZF == 1)

RFLAGS (full diagram)

+-------------------------------+
| CF  | ZF  | SF  | OF  | PF  | AF  |
+-------------------------------+
| [1] |  ?  |  ?  |  ?  |  ?  |  ?  |
|  OR | [1] |     |     |     |     |
+-------------------------------+

Equivalent expression

if (CF || ZF) jump; // unsigned a <= b

Common uses
Array/buffer boundary checks (unsigned).

JG / JNLE

(jump if greater / jump if not less or equal) (signed a > b)

Basic function
Used for signed integer comparison, indicating strictly greater than (a > b, signed).

Jump condition

(ZF == 0) && (SF == OF)

RFLAGS (full diagram)

+-------------------------------------------------+
| CF  | ZF  | SF  | OF  | PF  | AF  |
+-------------------------------------------------+
|  ?  | [0] | [SF]| [OF]|  ?  |  ?  |
|     |     | SF==OF (must be true)              |
+-------------------------------------------------+

Equivalent expression

if ((ZF == 0) && (SF == OF)) jump; // signed a > b

Typical uses
Signed comparison branches (such as signed integer sorting or conditional checks).

JGE / JNL

(jump if greater or equal / jump if not less) (signed a >= b)

Basic function
Signed >=.

Jump condition

SF == OF

RFLAGS (full diagram)

+-------------------------------+
| CF  | ZF  | SF  | OF  | PF  | AF  |
+-------------------------------+
|  ?  |  ?  | [SF] | [OF] |  ?  |  ?  |
|        (require SF == OF)           |
+-------------------------------+

Equivalent expression

if (SF == OF) jump; // signed a >= b

Typical uses
Signed boundary checks.

JL / JNGE

(jump if less / jump if not greater or equal) (signed a < b) Basic function
Signed less than. Jump condition

SF != OF

RFLAGS (full diagram)

+-------------------------------+
| CF  | ZF  | SF  | OF  | PF  | AF  |
+-------------------------------+
|  ?  |  ?  | [SF] | [OF] |  ?  |  ?  |
|       (require SF != OF)             |
+-------------------------------+

Equivalent expression

if (SF != OF) jump; // signed a < b

Typical uses
Signed comparison branches (negative-number-related checks).

JLE / JNG

(jump if less or equal / jump if not greater) (signed a <= b) Basic function
Signed less than or equal. Jump condition

(ZF == 1) || (SF != OF)

RFLAGS (full diagram)

+------------------------------------------------+
| CF  | ZF  | SF  | OF  | PF  | AF  |
+------------------------------------------------+
|  ?  | [1] | [SF]| [OF]|  ?  |  ?  |
|  OR    (or SF != OF)                          |
+------------------------------------------------+

Equivalent expression

if (ZF || (SF != OF)) jump; // signed a <= b

Typical uses
Signed range checks (<=).

JO

(jump if overflow) Basic function
Jump when the previous arithmetic operation caused overflow (signed overflow).

Jump condition

OF == 1

RFLAGS (full diagram)

+-------------------------------+
| CF  | ZF  | SF  | OF  | PF  | AF  |
+-------------------------------+
|  ?  |  ?  |  ?  | [1] |  ?  |  ?  |
+-------------------------------+

Equivalent expression

if (OF == 1) jump;

Common uses
Detect signed overflow (for example, when an addition/subtraction result cannot be represented in the target bit width).

JNO

(jump if not overflow) Basic function
Jump when there is no overflow.

Jump condition

OF == 0

RFLAGS (full diagram)

+-------------------------------+
| CF  | ZF  | SF  | OF  | PF  | AF  |
+-------------------------------+
|  ?  |  ?  |  ?  | [0] |  ?  |  ?  |
+-------------------------------+

Equivalent expression

if (OF == 0) jump;

JS

(jump if sign) Basic function
Jump when the result is negative (highest bit is 1), checking the sign bit.

Jump condition

SF == 1

RFLAGS (full diagram)

+-------------------------------+
| CF  | ZF  | SF  | OF  | PF  | AF  |
+-------------------------------+
|  ?  |  ?  | [1] |  ?  |  ?  |  ?  |
+-------------------------------+

Equivalent expression

if (SF == 1) jump; // result negative (signed)

Common uses
Detect negative numbers or branch on negative results in signed arithmetic.

JNS

(jump if not sign) Basic function
Jump when the result is non-negative (highest bit is 0).

Jump condition

SF == 0

RFLAGS (full diagram)

+-------------------------------+
| CF  | ZF  | SF  | OF  | PF  | AF  |
+-------------------------------+
|  ?  |  ?  | [0] |  ?  |  ?  |  ?  |
+-------------------------------+

Equivalent expression

if (SF == 0) jump;

JP / JPE

(jump if parity / jump if parity even) Basic function
Jump when the parity of the most recent arithmetic or logical result is even (that is, PF == 1). PF indicates that the number of 1 bits in the low 8 bits of the result is even.

Jump condition

PF == 1

RFLAGS (full diagram)

+-------------------------------+
| CF  | ZF  | SF  | OF  | PF  | AF  |
+-------------------------------+
|  ?  |  ?  |  ?  |  ?  | [1] |  ?  |
+-------------------------------+

Equivalent expression

if (PF == 1) jump;

Common uses
Used in some parity-check logic in legacy code/protocols.

JNP / JPO

(jump if not parity / jump if parity odd) Basic function
Jump when PF == 0 (odd parity).

Jump condition

PF == 0

RFLAGS (full diagram)

+-------------------------------+
| CF  | ZF  | SF  | OF  | PF  | AF  |
+-------------------------------+
|  ?  |  ?  |  ?  |  ?  | [0] |  ?  |
+-------------------------------+

Equivalent expression

if (PF == 0) jump;

JCXZ / JECXZ / JRCXZ

(jump if CX/ECX/RCX == 0) Basic function
Check whether register CX/ECX/RCX is 0 (bitwise compare against immediate 0); if it is 0, jump. Commonly used for fast branching when the loop counter is 0 (short jump).

Jump condition

(CX == 0) 或 (ECX == 0) 或 (RCX == 0)

RFLAGS (full diagram)
(These instructions do not directly depend on RFLAGS such as CF/ZF, but logically they are equivalent to comparing whether the register is 0, so they can be understood as depending on the register value rather than FLAGS; for consistency, FLAGS are still illustrated)

+-------------------------------+
| CF  | ZF  | SF  | OF  | PF  | AF  |
+-------------------------------+
|  ?  |  ?  |  ?  |  ?  |  ?  |  ?  |
+-------------------------------+

Equivalent expression

if (RCX == 0) jump;

Common uses
Short loops, special-case branches in string/block processing.

Relationship Graph

Loading graph...