Loops

Assembly Language Programming 6

Loops

Unconditional Jump.

JMP

label

Jump to 'label'

Conditional Jump.

JZ	label	Jump if Zero flag is set.
JNZ	label	Jump if Zero flag is not set.
JC	label	Jump if Carry flag is set.
JNC	label	Jump if Carry flag is not set.

There are a number of other jump instructions. (See instruction set)

A Loop is a section of program which needs to be repeated a number of times.

The number of times a loop is repeated (iterations) may be counted by a Control Variable (Counter). This may be a store location or a register.

Example

Suppose we need a program which outputs a 'beep' three times.

The pseudocode may be...

1.	Set count to 3.
2.	Output a 'beep'.
3.	Decrement count.
4.	If count is not zero then go to step 2.
5.	End

We will use the [CL] register for the loop control variable, as this is not used by input or output interrupt routines.

Assembly program is :

	MOV	CL,3	{set count to 3}
BeepLoop:	MOV	AH,2
	MOV	DL,7	{ASCII code for a beep}
	INT	21H	{Outputs a beep}
	DEC	CL	{Decrement count}
	JNZ	Beeploop	{If not zero, repeat loop}