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From: greg@gryphon.CTS.COM (Greg Laskin)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Please explain this MASM/LINK behavior...
Message-ID: <1423@gryphon.CTS.COM>
Date: Thu, 3-Sep-87 17:32:55 EDT
Article-I.D.: gryphon.1423
Posted: Thu Sep  3 17:32:55 1987
Date-Received: Sat, 5-Sep-87 17:23:17 EDT
References: <88@wjh12.HARVARD.EDU> <2474@bnrmtv.UUCP>
Reply-To: greg@gryphon.CTS.COM (Greg Laskin)
Organization: Trailing Edge Technology, Redondo Beach, CA
Lines: 81
Keywords: MASM LINK bug?

In article <2474@bnrmtv.UUCP> perkins@bnrmtv.UUCP (Henry Perkins) writes:
>In article <88@wjh12.HARVARD.EDU>, lotto@wjh12.HARVARD.EDU (Jerry Lotto) writes:
>> Code_Seg	Segment Para 'CODE'
>
>> 	Assume	CS:Code_Seg
>> 	Org	100H
>
>> Begin:	mov	ax, Code_Seg:[2ch]
>
>> Code_Seg	Ends
>> 	End	Begin
>
>> assembles fine, but generates a link error (invalid object module).
>The problem is with MASM not complaining about the
>
>	mov     ax, Code_Seg:[2ch]
>
>line.  This SHOULD be written as
>
>	mov     ax, cs:[2ch]
>
>if you want to make use of the ASSUME directive and specify the
>code segment.  There's no 8086 addressing mode that allows a
>segment to be specified as immediate data for a MOV instruction.

The assembler documentation asserts that the assembler will
resolve Code_Seg:[2ch] to CS:[2ch]. (see below)

>Of course, since you're making a .COM file, you can make use of
>the fact that CS and DS both already point to your one and only
>segment, and omit the segment override, which then defaults to
>using DS.  That's what you're doing in the example that works.

Accurate.

>If you had any data declarations in this segment, you'd also
>want to put in an
>
>	Assume  DS:Code_Seg
>
provided that you had assured that DS would contain the value of
Code_Seg at run-time.  The Assume directive is a cue to the assembler
about what segment addresses will be in the segment registers at
run-time.  This directive is a "guarantee" to the assembler.  If
DS is not actually pointing at Code_Seg, the assembler will (and
can not) do anything about it.

Ther MASM 4.0 reference manual says, in section 5.3.7, Segment-Override
Operator:

SYNTAX
segmentregister:expression
segmentname:expression
groupname:expression

... if either sementname or groupname is given, the name must have been
assigned to a segment register with a previous ASSUME directive and
defined using a SEGMENT or GROUP directive.  The expression can be an
absolute symbol or relocatable operand.  The segmentregister must be CS,
DS, SS or ES.

---- end of quotation ---

Code_Seg is a segmentname and a relocatable operand.  CS is a segmentregister
and is an absolute operand.

It would seem that one could reasonably expect to generate the code for
the sample correctly.  In fact, the code in the assembly listing IS
correct.  Once the assembler had resolved the relocatable Code_Seg to
the absolute CS, it should have stopped (while it was ahead). 

It doesn't seem unfair to expect the assembler to correctly generate
the sample code.

That said, it doesn't work.  Specify CS explicitly as a work-around.
-- 
Greg Laskin   
"When everybody's talking and nobody's listening, how can we decide?"
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