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[alokagarwal]

Can any one tell me what does NOP directive does with an example. I read that it actually does not do anything then what is its significance?

[kolusu]

alokagarwal,

NOP is No operation Command. AFAIK it does nothing but waste processor time. There are times when you want to delay the program execuetion. That is where you use NOP command.

Kolusu
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Kolusu
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[semigeezer]

Actually, the no operation does nothing. The processor interprets it, but does not send it to the execution unit (e-unit). Unless you string enough of them together to completely empty the pipeline at the i-unit stage, there will no performance penalty at all. It is normally used just to align the next instruction on a half word, word, or doubleword boundary.

[Bill Dennis]

A practice considered reckless today, the NOP would sometimes be modified to a BR instruction in an action called self-modifying code. You could control program flow by modifying the code itself. Very difficult to debug problems! Consider this:

[alokagarwal]

Hi Dennis,

In your example, when will the control come to the label OPENED, if it is coming to it, when the contol hits the label TEST second time, then why?

[Mervyn]

alokagarwal, the NOP command takes the form of a Branch instruction, with a condition operand of zero, indicating that the branch will never take place. The MVI command changes the condition operand to 15, indicating an unconditional branch.

In Bill's example, this prevents the file being opened twice, which would cause an error.
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[DaveyC]