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From: resnicks@netcom.COM (Steve Resnick)
Newsgroups: comp.os.msdos.programmer
Subject: Re: TSR's - Must they be converted to .COM?
Keywords: TSR
Message-ID: <24856@netcom.COM>
Date: 20 Feb 91 17:54:55 GMT
References: <3188@unccvax.uncc.edu> <24314@netcom.COM> <1014@csource.oz.au>
Organization: What? Me organized? You gotta be kidding!
Lines: 38

In article <1014@csource.oz.au> david@csource.oz.au (david nugent) writes:
>In <24314@netcom.COM> resnicks@netcom.COM (Steve Resnick) writes:
>
>> Making an EXE TSR is a little more complicated, in that you need to know
>> where the code segment is relative to the PSP, and how  many paragraphs the 
>> TSR is going to take when you make the DOS call. I don't think the INT 27H
>> TSR interrupt will work for this, but rather DOS function 31H will do the TSR
>> in this "style'
>
>
>INT 27H will still work alright, provided you have CS set to the program's
>PSP.  That's often a little hard to arrange in a .EXE format, but it's
>nevertheless possible.  With an easier alternative around, it's no wonder
>that that sort of contortion is rarely, if ever, seen.
>
INT 27H will work, ONLY if these conditions are met:

	The total code is <64K
	The code segment is within the area kept resident by int 27H.

The latter can be tricky if the code segment is different than the 
PSP segment, which is usually the case with .EXE programs. Remember
that int27H will keep only 1 segment resident, whereas INT 21H, AH=31H
will keep any number of paragraphs (16 byte chunks) resident, up to
available memory. This makes it much easier to make an EXE TSR..

Cheers!
Steve

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