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><H1
><A
NAME="OBJCHAP"
></A
>Chapter 6. Object Files</H1
><P
>LWTOOLS uses a proprietary object file format. It is proprietary in the sense
that it is specific to LWTOOLS, not that it is a hidden format. It would be
hard to keep it hidden in an open source tool chain anyway. This chapter
documents the object file format.</P
><P
>An object file consists of a series of sections each of which contains a
list of exported symbols, a list of incomplete references, and a list of
"local" symbols which may be used in calculating incomplete references. Each
section will obviously also contain the object code.</P
><P
>Exported symbols must be completely resolved to an address within the
section it is exported from. That is, an exported symbol must be a constant
rather than defined in terms of other symbols.</P
><P
>Each object file starts with a magic number and version number. The magic
number is the string "LWOBJ16" for this 16 bit object file format. The only
defined version number is currently 0. Thus, the first 8 bytes of the object
file are <FONT
COLOR="RED"
>4C574F424A313600</FONT
></P
><P
>Each section has the following items in order:</P
><P
></P
><UL
><LI
><P
>section name</P
></LI
><LI
><P
>flags</P
></LI
><LI
><P
>list of local symbols (and addresses within the section)</P
></LI
><LI
><P
>list of exported symbols (and addresses within the section)</P
></LI
><LI
><P
>list of incomplete references along with the expressions to calculate them</P
></LI
><LI
><P
>the actual object code (for non-BSS sections)</P
></LI
></UL
><P
>The section starts with the name of the section with a NUL termination
followed by a series of flag bytes terminated by NUL. There are only two
flag bytes defined. A NUL (0) indicates no more flags and a value of 1
indicates the section is a BSS section. For a BSS section, no actual
code is included in the object file.</P
><P
>Either a NULL section name or end of file indicate the presence of no more
sections.</P
><P
>Each entry in the exported and local symbols table consists of the symbol
(NUL terminated) followed by two bytes which contain the value in big endian
order. The end of a symbol table is indicated by a NULL symbol name.</P
><P
>Each entry in the incomplete references table consists of an expression
followed by a 16 bit offset where the reference goes. Expressions are
defined as a series of terms up to an "end of expression" term. Each term
consists of a single byte which identifies the type of term (see below)
followed by any data required by the term. Then end of the list is flagged
by a NULL expression (only an end of expression term).</P
><DIV
CLASS="TABLE"
><A
NAME="AEN945"
></A
><P
><B
>Table 6-1. Object File Term Types</B
></P
><TABLE
BORDER="1"
FRAME="border"
CLASS="CALSTABLE"
><COL><COL><THEAD
><TR
><TH
>TERMTYPE</TH
><TH
>Meaning</TH
></TR
></THEAD
><TBODY
><TR
><TD
>00</TD
><TD
>end of expression</TD
></TR
><TR
><TD
>01</TD
><TD
>integer (16 bit in big endian order follows)</TD
></TR
><TR
><TD
>02</TD
><TD
>	external symbol reference (NUL terminated symbol name follows)</TD
></TR
><TR
><TD
>03</TD
><TD
>local symbol reference (NUL terminated symbol name follows)</TD
></TR
><TR
><TD
>04</TD
><TD
>operator (1 byte operator number)</TD
></TR
><TR
><TD
>05</TD
><TD
>section base address reference</TD
></TR
><TR
><TD
>FF</TD
><TD
>This term will set flags for the expression. Each one of these terms will set a single flag. All of them should be specified first in an expression. If they are not, the behaviour is undefined. The byte following is the flag. Flag 01 indicates an 8 bit relocation. Flag 02 indicates a zero-width relocation (see the EXTDEP pseudo op in LWASM).</TD
></TR
></TBODY
></TABLE
></DIV
><P
>External references are resolved using other object files while local
references are resolved using the local symbol table(s) from this file. This
allows local symbols that are not exported to have the same names as
exported symbols or external references.</P
><DIV
CLASS="TABLE"
><A
NAME="AEN975"
></A
><P
><B
>Table 6-2. Object File Operator Numbers</B
></P
><TABLE
BORDER="1"
FRAME="border"
CLASS="CALSTABLE"
><COL><COL><THEAD
><TR
><TH
>Number</TH
><TH
>Operator</TH
></TR
></THEAD
><TBODY
><TR
><TD
>01</TD
><TD
>addition (+)</TD
></TR
><TR
><TD
>02</TD
><TD
>subtraction (-)</TD
></TR
><TR
><TD
>03</TD
><TD
>multiplication (*)</TD
></TR
><TR
><TD
>04</TD
><TD
>division (/)</TD
></TR
><TR
><TD
>05</TD
><TD
>modulus (%)</TD
></TR
><TR
><TD
>06</TD
><TD
>integer division (\) (same as division)</TD
></TR
><TR
><TD
>07</TD
><TD
>bitwise and</TD
></TR
><TR
><TD
>08</TD
><TD
>bitwise or</TD
></TR
><TR
><TD
>09</TD
><TD
>bitwise xor</TD
></TR
><TR
><TD
>0A</TD
><TD
>boolean and</TD
></TR
><TR
><TD
>0B</TD
><TD
>boolean or</TD
></TR
><TR
><TD
>0C</TD
><TD
>unary negation, 2's complement (-)</TD
></TR
><TR
><TD
>0D</TD
><TD
>unary 1's complement (^)</TD
></TR
></TBODY
></TABLE
></DIV
><P
>An expression is represented in a postfix manner with both operands for
binary operators preceding the operator and the single operand for unary
operators preceding the operator.</P
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author	William Astle <lost@l-w.ca>
date	Sat, 11 Aug 2012 15:18:58 -0600
parents	d389adbcc4ab
children