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From: perkins@bnrmtv.UUCP (Henry Perkins)
Newsgroups: comp.sys.ibm.pc
Subject: Re: NOP
Message-ID: <2640@bnrmtv.UUCP>
Date: Fri, 25-Sep-87 15:21:01 EDT
Article-I.D.: bnrmtv.2640
Posted: Fri Sep 25 15:21:01 1987
Date-Received: Sun, 27-Sep-87 08:04:04 EDT
References: <2306@sphinx.uchicago.edu>
Organization: BNR Inc., Mountain View, California
Lines: 24
Summary: Two reasons why MASM puts NOPs in code: directives and short jumps

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?

It's not done to "slow down the processor"; it's done to make the
code size/location unambiguous.

There are two reasons why MASM will insert a NOP that isn't in the
original code: assembler directives and short jumps.  Directives
like EVEN and ORG specify where the next instruction starts; the
intervening space is padded with NOPs.  A single NOP is used after
a short jump when the assembler source did not specify (with the
SHORT pseudo-op) that the JMP was the shorter, 8-bit displacement
type.  Rather than use the slower 16-bit displacement jump, MASM
will use a short jump and add a NOP to preserve the offset of the
next instruction.  If you specify that the jump is to be short, of
course, no NOP is inserted.
-- 
{hplabs,amdahl,ames}!bnrmtv!perkins         --Henry Perkins

It is better never to have been born.  But who among us has such luck?
One in a million, perhap po