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From: ralf+@cs.cmu.edu (Ralf Brown)
Newsgroups: comp.os.msdos.programmer
Subject: Re: TASM can't subtract ...
Message-ID: <1991Mar2.041832.28328@cs.cmu.edu>
Date: 2 Mar 91 04:18:32 GMT
References: <1991Mar1.014835.7651@athena.mit.edu>
Sender: netnews@cs.cmu.edu (USENET News Group Software)
Organization: School of Computer Science, Carnegie Mellon
Lines: 39

In article <1991Mar1.014835.7651@athena.mit.edu> mmshah@athena.mit.edu (Milan M Shah) writes:
}I am trying to assemble Fontedit.asm, a program published in PC-Magazine
}vol 7 no. 15 using Turbo Assembler 1.0 (Yup, its 1.0 alright). In the program,
}there's a couple of lines that go:
}
}LOADER 	LABEL	BYTE
}LOADER_OFFSET	EQU	100H - LOADER
}
}Simple enough, but TASM complains. It says 'Can't add relative addresses.'
}First off, 100H is a constant, and not a relative address. Second, I think
}its pretty ridiculous for TASM to give an error and die. Maybe a warning,
}but to refuse to assemble? Sheesh, here's a number, here's another, and I
}want TASM to subtract one from the other, and TASM can't do that? That's
}ridiculous, I sez.

It's actually a linker limitation.  LOADER doesn't necessarily end up in the
executable at the same offset as the assembler's location counter indicates.
You *can* say "EQU LOADER-100h", because the assembler can put a -100h in
the location and tell the linker to adjust it by the offset of LOADER.
Unfortunately, the assembler can't tell the linker to fixup a location
by the negative of an address....  The assembler used by FONTEDIT's author
probably has a bug that lets it do the arithmetic and store the value as
a constant in the .OBJ.  Unfortunately, you will get rather hard to find
errors on multi-file programs (I know, I once got bitten by such a problem 
with MASM 2.04) because the final executable will address the wrong
location.

BTW, TASM 2.0 has a more informative message: "Can't subtract dissimilar
relative quanties".  (If they were the same, i.e. LOADER1-LOADER2, where
both are in the same segment, the result would be a constant and would
be independent of any relocation performed by the linker)



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