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Flow Control Instructions

Flow Control Instructions

Program using the different instruction :- Conditional and Unconditional Jump InstructionsCompare InstructionLoop Instruction

Conditional Jump

Syntax: Jxxx destination_label Where xxx represents the condition

If condition is true, the next instruction to be executed is the one at destination_label.

If condition is false, the instruction immediately following the jump is done next.

Conditional Jump

The destination_label must precede the jump instruction by no more than 126 bytes.

To implement a conditional jump, the CPU looks at the FLAG register (set by last instruction executed by the processor).

JMP instructions themselves do not affect flags.

Unconditional JumpSyntax:

JMP destination_labelPurpose: To Transfer control to another part of the program.

Example: MOV AX, 5 MOV BX, 2 JMP LABEL_SUB ADD AX, BX ;this instruction will never execute LABEL_SUB: SUB AX, BX

Categories of Conditional Jump

Signed Jumps: This is used when a signed interpretation is being given to result.

Symbol Description Condition for Jumps

JG/JNLE Jump if greater thanJump if not less than or equal to

ZF = 0 and SF = OF

JGE/JNL Jump if greater than or equal toJump if not less than

SF = OF

JL/JNGE Jump if less thanJump if not greater than or equal to

SF <> OF

JLE/JNG Jump if less than or equal toJump if not greater than

ZF = 1 and SF <> OF

Categories of Conditional Jump

Unsigned Jumps: This is used for an unsigned interpretation of result

Symbol Description Condition for Jumps

JA/JNBE Jump if above thanJump if not below than or equal to

ZF = 0 and CF = 0

JAE/JNB Jump if above than or equal toJump if not below than

CF = 0

JB/JNAE Jump if below thanJump if not above than or equal to

CF = 1

JBE/JNA Jump if below than or equal toJump if not above than

ZF = 1 and CF = 1

Categories of Conditional JumpSingle-Flag Jumps: Which operate on settings of individual flags.

Compare InstructionCMP (compare) instruction performs an implied subtraction of a

source operand from destination operand. Neither operand is modified. CMP destination, source

Compare Instruction maintain three condition: Destination < Source Destination = Source Destination > Source

Examples

OUTPUT

High Level Language Structures

Branching Structure IF-THEN IF-THEN-ELSE CASE

IF-THEN

IF-THEN-ELSE

CASEA case is a multiway branch structure that tests a register,

variable, or expression for particular values or range of values. The general form is as follows:

Case Expression values_1: statements_1 values_2: statements_2

………….. values_n: statements_n END_CASE

CASE Examples

High Level Language Structures

Branching with Compound Conditions AND CONDITIONS OR CONDITIONS

AND CONDITION An AND condition is true if and only if condition_1 and

condition_2 are both true. If any condition is false, then the whole thing is false.

AND CONDITION EXAMPLES

OR CONDITION

If condition_1 OR condition_2 is true then an OR conditions is true. If both conditions are false, then the whole thing is false.

OR CONDITION EXAMPLES

High Level Language Structures

Looping StructuresFOR LOOPWHILE LOOPREPEAT LOOP

LOOPING STRUCTURES

FOR LOOP Executed at least once. If CX contains 0, the loop instruction decrements CX (CX = FFFFh) To prevent this, use instruction JCXZ before the loop. ;initialize CX JCXZ SKIP TOP: ;body of the loop LOOP TOPSKIP:

Examples For Looping Structures

LOOPING STRUCTURESWHILE LOOP

WHILE condition DO ;statements

END_WHILE

WHILE LOOP checks the terminating condition at the top of the loop, so don’t forget to initialize variables.

EXAMPLES WHILE STRUCTURES

LOOPING STRUCTURES

REPEAT LOOP REPEAT

;statements UNTIL condition

First statements are executed, then the condition is checked.If true, the loop terminates; if false, control branches to the top of the loop

Examples Repeat Structures

Any Question?

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