Bob Ammerman wrote:
> Please take the time to carefully read and understand the following. It w=
ill
> probably solve your problem. Perhaps more importantly, if you study it
> closely, you will have a much better handle on how the assembler and link=
er
> interact.
> </strong>
>
> If var were at a known address at assembly time (say in an absolute secti=
on)
> then you might think it would work because the assembler could resolve th=
e
> expression itself without involving the liker. Unfortunately that is not =
the
> case. Assume 'var' is in a absolutely addressed section like this:
>
> myseg	udata	H'20'
> var	res	1
>
> You would think the assembler should be able to determine that low(0-var)
> was simply H'E0'. However, even this won't work. The assembler does not
> 'know' in its symbol table that 'var' is in an absolutely addressed secti=
on
> and thus has an address that could be known at assembly time.
>
> The only way that I can think of to make this work would require a trick
> like this:
>
> work_abs 	equ H'30'
> work        udata work_abs
> work_base:
>
> adummy res 3
>
> var_abs =3D ($-work_base)+work_abs
> var res 1
>
>
> mycode     CODE
>
>     movlw    low (0-var_abs)
>
>
> This, no doubt, needs some explanation. Let's take it one line at a time.=
...
>
> [1] work_abs equ H'30'
>
> This is creating an assembly time symbol containing the absolute value
> H'30'. This is where we plan on placing our variables.
>
> [2] work udata work_abs
>
> This is declaring a data section, and tells the linker exactly where it h=
as
> to appear in memory. Now the linker and the assembler agree on where the
> data will go. It is critical to understand that for this to work our sect=
ion
> must be located at a location known at assembly time (ie: at 'work_abs').
>
> [3] work_base:
>
> This is declaring another symbol to the assembler. This one is a relocata=
ble
> symbol with the value "0 in the 'work' section".
>
> [4] adummy res 3
>
> This line is here just to show that other variables can be declared as
> normal in the 'work' section.
>
> [5] var_abs =3D ($-work_base)+work_abs
>
> This line contains the 'magic' that makes this whole scheme work. We'll t=
ake
> it apart to understand what it is doing:
>
> The dollar sign is a pseudo symbol for the current offset in the current
> section. Thus at this point it has the value "3 in the 'work' section".
>
> Work base, as described above is also a symbol with the value "0 in the
> 'work' section".
>
> Thus, $-work base is computed by the assembler as just plain 3, because t=
he
> "in the 'work' section" parts of the two symbols cancel in the subtractio=
n.
>
> Next we add in work_abs, which you will remember is the absolute value
> H'30'. This gives us the absolute value H'33', which of course is where t=
he
> linker is going to end up putting 'var'.
>
> [6] var res 1
>
> Allocates the 'var' inside the 'work' section. It is important to note th=
at
> 'var' is now a symbol with the relocatable value "3 in the 'work' section=
".
>
> [7] mycode CODE
>
> Just moving to the code section so we can put in instructions.
>
> [8] movlw low (0-var_abs)
>
> The argument to 'low' is the difference of two _absolute_ expressions. Th=
e
> result, of course, is also absolute, and so the assembler is happy with i=
t.
>
> -- Bob Ammerman
> RAm Systems
>

Thank you very much for your detailed explanation this was very helpful!=20
Now I understand the problem. Your words helped me to understand the way=20
the assembler and linker talk to each other.
Now I am left with the impression that there is something generally=20
wrong with this kind of one way communication between them.
Once again, thanks, your answer is highly appreciated!


Kind regards,
Svetlin

--=20
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