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><H1
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><A
NAME="AEN520"
>3.9. Object Files and Sections</A
></H1
><P
>The object file target is very useful for large project because it allows
multiple files to be assembled independently and then linked into the final
binary at a later time. It allows only the small portion of the project
that was modified to be re-assembled rather than requiring the entire set
of source code to be available to the assembler in a single assembly process.
This can be particularly important if there are a large number of macros,
symbol definitions, or other metadata that uses resources at assembly time.
By far the largest benefit, however, is keeping the source files small enough
for a mere mortal to find things in them.</P
><P
>With multi-file projects, there needs to be a means of resolving references to
symbols in other source files. These are known as external references. The
addresses of these symbols cannot be known until the linker joins all the
object files into a single binary. This means that the assembler must be
able to output the object code without knowing the value of the symbol. This
places some restrictions on the code generated by the assembler. For
example, the assembler cannot generate direct page addressing for instructions
that reference external symbols because the address of the symbol may not
be in the direct page. Similarly, relative branches and PC relative addressing
cannot be used in their eight bit forms. Everything that must be resolved
by the linker must be assembled to use the largest address size possible to
allow the linker to fill in the correct value at link time. Note that the
same problem applies to absolute address references as well, even those in
the same source file, because the address is not known until link time.</P
><P
>It is often desired in multi-file projects to have code of various types grouped
together in the final binary generated by the linker as well. The same applies
to data. In order for the linker to do that, the bits that are to be grouped
must be tagged in some manner. This is where the concept of sections comes in.
Each chunk of code or data is part of a section in the object file. Then,
when the linker reads all the object files, it coalesces all sections of the
same name into a single section and then considers it as a unit.</P
><P
>The existence of sections, however, raises a problem for symbols even
within the same source file. Thus, the assembler must treat symbols from
different sections within the same source file in the same manner as external
symbols. That is, it must leave them for the linker to resolve at link time,
with all the limitations that entails.</P
><P
>In the object file target mode, LWASM requires all source lines that
cause bytes to be output to be inside a section. Any directives that do
not cause any bytes to be output can appear outside of a section. This includes
such things as EQU or RMB. Even ORG can appear outside a section. ORG, however,
makes no sense within a section because it is the linker that determines
the starting address of the section's code, not the assembler.</P
><P
>All symbols defined globally in the assembly process are local to the
source file and cannot be exported. All symbols defined within a section are
considered local to the source file unless otherwise explicitly exported.
Symbols referenced from external source files must be declared external,
either explicitly or by asking the assembler to assume that all undefined
symbols are external.</P
><P
>It is often handy to define a number of memory addresses that will be
used for data at run-time but which need not be included in the binary file.
These memory addresses are not initialized until run-time, either by the
program itself or by the program loader, depending on the operating environment.
Such sections are often known as BSS sections. LWASM supports generating
sections with a BSS attribute set which causes the section definition including
symbols exported from that section and those symbols required to resolve
references from the local file, but with no actual code in the object file.
It is illegal for any source lines within a BSS flagged section to cause any
bytes to be output.</P
><P
>The following directives apply to section handling.</P
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
>SECTION <CODE
CLASS="PARAMETER"
>name[,flags]</CODE
>, SECT <CODE
CLASS="PARAMETER"
>name[,flags]</CODE
>, .AREA <CODE
CLASS="PARAMETER"
>name[,flags]</CODE
></DT
><DD
><P
>Instructs the assembler that the code following this directive is to be
considered part of the section <CODE
CLASS="PARAMETER"
>name</CODE
>. A section name
may appear multiple times in which case it is as though all the code from
all the instances of that section appeared adjacent within the source file.
However, <CODE
CLASS="PARAMETER"
>flags</CODE
> may only be specified on the first
instance of the section.</P
><P
><CODE
CLASS="PARAMETER"
>flags</CODE
> is a comma separated list of flags. If a
flag is "bss", the section will be treated as a BSS section and no
statements that generate output are permitted.</P
><P
>If the flag is "constant",
the same restrictions apply as for BSS sections.  Additionally, all symbols
defined in a constant section define absolute values and will not be
adjusted by the linker at link time.  Constant sections cannot define
complex expressions for symbols; the value must be fully defined at assembly
time.  Additionally, multiple instances of a constant section do not
coalesce into a single addressing unit; each instance starts again at offset
0.</P
><P
>If the section name is "bss" or ".bss" in any combination of upper and
lower case, the section is assumed to be a BSS section. In that case,
the flag <CODE
CLASS="PARAMETER"
>!bss</CODE
> can be used to override this assumption.</P
><P
> If the section name is "_constants" or "_constant", in any
combination of upper and lower case, the section is assumed to be a constant
section.  This assumption can be overridden with the "!constant"
flag.</P
><P
>If assembly is already happening within a section, the section is implicitly
ended and the new section started. This is not considered an error although
it is recommended that all sections be explicitly closed.</P
></DD
><DT
>ENDSECTION, ENDSECT</DT
><DD
><P
>This directive ends the current section. This puts assembly outside of any
sections until the next SECTION directive. ENDSECTION is the preferred form.
Prior to version 3.0 of LWASM, ENDS could also be used to end a section but
as of version 3.0, it is now an alias for ENDSTRUCT instead.</P
></DD
><DT
><CODE
CLASS="PARAMETER"
>sym</CODE
> EXTERN, <CODE
CLASS="PARAMETER"
>sym</CODE
> EXTERNAL, <CODE
CLASS="PARAMETER"
>sym</CODE
> IMPORT</DT
><DD
><P
>This directive defines <CODE
CLASS="PARAMETER"
>sym</CODE
> as an external symbol.
This directive may occur at any point in the source code. EXTERN definitions
are resolved on the first pass so an EXTERN definition anywhere in the
source file is valid for the entire file. The use of this directive is
optional when the assembler is instructed to assume that all undefined
symbols are external. In fact, in that mode, if the symbol is referenced
before the EXTERN directive, an error will occur.</P
></DD
><DT
><CODE
CLASS="PARAMETER"
>sym</CODE
> EXPORT, <CODE
CLASS="PARAMETER"
>sym</CODE
> .GLOBL, EXPORT <CODE
CLASS="PARAMETER"
>sym</CODE
>, .GLOBL <CODE
CLASS="PARAMETER"
>sym</CODE
></DT
><DD
><P
>This directive defines <CODE
CLASS="PARAMETER"
>sym</CODE
> as an exported symbol.
This directive may occur at any point in the source code, even before the
definition of the exported symbol.</P
><P
>Note that <CODE
CLASS="PARAMETER"
>sym</CODE
> may appear as the operand or as the
statement's symbol. If there is a symbol on the statement, that will
take precedence over any operand that is present.</P
></DD
><DT
><CODE
CLASS="PARAMETER"
>sym</CODE
> EXTDEP</DT
><DD
><P
>This directive forces an external dependency on
<CODE
CLASS="PARAMETER"
>sym</CODE
>, even if it is never referenced anywhere else in
this file.</P
></DD
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author	lost@l-w.ca
date	Wed, 07 Sep 2011 20:45:20 -0600
parents	1e0a0e6cd918
children