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From: bright@dataio.Data-IO.COM (Walter Bright)
Newsgroups: comp.sys.ibm.pc
Subject: Re: NOP
Message-ID: <1377@dataio.Data-IO.COM>
Date: Mon, 28-Sep-87 14:14:09 EDT
Article-I.D.: dataio.1377
Posted: Mon Sep 28 14:14:09 1987
Date-Received: Tue, 29-Sep-87 06:35:19 EDT
References: <2306@sphinx.uchicago.edu>
Reply-To: bright@dataio.UUCP (Walter Bright)
Organization: Data I/O Corporation; Redmond, WA
Lines: 16
Keywords: NOP

In article <2306@sphinx.uchicago.edu> cjdb@sphinx.uchicago.edu (Charles Blair) writes:
>In debugging code written in assembly-language, I notice that MASM
>periodically inserts NOP's in places where they don't appear in the
>source. I imagine this is done to slow down the processor, but how
>does MASM "decide" the appropriate circumstances?

The only place I have seen it done is following a JMP instruction where
the target address is a forward reference. The reason is that MASM is
2 passes, on the first pass the values of all symbols must be determined.
JMP can be 2 or 3 bytes, depending on how far away the target is. Since
the target is forward referenced, MASM assumes worst case and allocates
enough space for a 3 byte JMP. On pass 2, if the target is close enough
that it can be a 2 byte JMP, MASM creates a 2 byte JMP followed by a NOP
to fill in the 'hole'.

If you want only a 2 byte JMP, use the JMP SHORT mnemonic.