Conditional Jump Instructions 

Conditional jump instructions are the basic tools for creating selective structures like the IF..ENDIF statement and repetitive structures like loops.
A conditional jump tests one or more flags in the flags register
If the flag settings match the instruction, control transfers to the target location
If the match fails, the CPU ignores the conditional jump and execution continues with the next instruction.
Most of the time, a conditional jump is executed after a CMP instruction.
The CMP instruction sets the flags so that test can be carried out for less than, greater than, equality, etc.

Conditional jump instructions take the following form:

	Jcc label;

The “cc” in Jcc indicates that some character sequence, specifying the type of condition to be tested, must be substituted. For example, JS stands for jump if the sign flag is set and JC stands for jump if the carry flag is set.

Conditional jumps test the sign (S), zero (Z), carry (C), parity (P), and overflow (O) flags.

When comparing two numbers it is necessary to know whether these numbers are representing signed or unsigned numbers in order to establish a relationship between them.
For example, suppose that AL=FF and BL=01. If we execute the instruction CMP AL, BL, the result of the comparison will be different depending on whether the registers represent signed or unsigned numbers.
If unsigned numbers are represented, then AL=255 and BL=1 and hence AL is greater than BL. However, if signed numbers are represented, then AL=-1 and BL=1 and hence BL is greater than AL. Thus, we need conditional jump instructions for unsigned number comparison and conditional jump instructions for signed number comparison.

Conditional jump instructions are divided into three main types:

Single Flag Based Jump Instructions
Unsigned Conditional Jump Instructions
Signed Conditional Jump Instructions

Addressing Modes

Unlike the unconditional JMP instruction, the conditional jump instructions do not provide an indirect form.
The only form they allow is a branch to a statement label in your program.
Conditional jump instructions are only of type SHORT.

Since unconditional jump instructions can be only of type SHORT, if the target address is not within a range of -128 to +127 from the IP, then a remedy is needed. The following example shows two implementations of a program one when the target address is within a SHORT range and the other when it is in a Near range.

Example: Target address is SHORT.
	CMP AX, BX	; compare ax to bx to set flags
	JE THERE	; if equal, then skip next statement
	ADD AX, 0002H	; correction statement
THERE:	MOV CL, 07H	; load count

Example: Target address is Near.
	CMP AX, BX	; compare AX to BX to set flags
	JNE FIX		; if not equal, then goto label  FIX
	JMP THERE	; otherwise, unconditional jump to there
FIX:	ADD AX, 0002H	; correction statement
THERE:	MOV CL, 07H	; load count