Draft_Assembly_Relocation_Table.xml

<?xml version="1.0" encoding="US-ASCII"?>
<!DOCTYPE rfc SYSTEM "rfc2629.dtd">
<?xml-stylesheet type='text/xsl' href='rfc2629.xslt' ?>
<?rfc strict="yes" ?>
<?rfc toc="yes"?>
<?rfc tocdepth="4"?>
<?rfc symrefs="yes"?>
<?rfc sortrefs="yes" ?>
<?rfc compact="yes" ?>
<?rfc subcompact="no" ?>
<?rfc private="RFC X____ (Draft-Asm)" ?>
<rfc ipr="none" number="____" category="info" docName="draft-assembly-relocation-table">
  <front>
    <title abbrev="Assembly Relocation Table">Assembly Relocation Table</title>

    <author fullname="James Rhodes" initials="J.R." role="editor"
            surname="Rhodes">
      <organization>Redpoint Software</organization>

      <address>
        <email>jrhodes@redpointsoftware.com.au</email>
        <uri>http://www.redpointsoftware.com.au/</uri>
      </address>
    </author>

    <date month="April" year="2012" />
    <area>Asm</area>
    <workgroup>0x10c Standards Committee</workgroup>
    <abstract>
      <t>This draft provides a formal structure for providing an assembly
      relocation table from within DCPU-16 programs.</t>
    </abstract>
  </front>

  <middle>
    <section title="Introduction">
      <t>As it stands, code generated by assemblers is either not relocatable,
      or the relocation format is not standardized.  Thus this document
      suggests a standard mechanism for providing a table of addresses that
      need relocating.</t>

      <section title="Requirements Language">
        <t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
        "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
        document are to be interpreted as described in <xref
        target="RFC2119">RFC 2119</xref>.</t>
      </section>
    </section>
    
    <section anchor="role-of-assemblers" title="Role of Assemblers">
      <t>For the purposes of this draft, the role of assemblers is to generate
      code from a defined syntax to DCPU-16 bytecode.</t>
      <t>In this case, assemblers SHOULD provide an option to generate
      relocatable code, but MUST NOT generate relocatable code unless the
      user indicates that they wish to do so.</t>
    </section>
    
    <section anchor="relocation-table-format" title="Relocation Table Format">
      <t>For purposes of future versioning, this document specifies version 1
         of the relocation table format.</t>
      <t>The format of the relocation table is as follows:</t>
      <texttable anchor="relocation-table" title="Relocation Table">
        <ttcol align="center">Contents of single Word</ttcol>
        <c>Magic number</c>
        <c>Version number</c>
        <c>Size of table</c>
        <c>Entry 1</c>
        <c>...</c>
        <c>Entry N</c>
      </texttable>
      <t>
        <list style="hanging">
          <t hangText="Magic number">The magic number consists of the string "RT"
            (stands for "Relocation Table") as a packed 7-bit ASCII literal, thus
            resulting in the value 0x5254.</t>
          <t hangText="Version number">The current version number is 0x0001.</t>
          <t hangText="Size of table">This field must be set to the total size of
            the table, in words. This includes the magic number, the version
            number and this field, too.</t>
          <t hangText="Entries">Each entry is a absolute address pointing to a
            single word in the file the relocation table is contained in, which
            should be relocated relatively to the position the code is loaded to.
          </t>
        </list>
      </t>
    </section>

    <section anchor="relocation-table-position" title="Relocation Table Positioning">
      <t>The assembly relocation table must be positioned inside the generated
      code, but have no effect on the program execution.</t>
      <t>When an assembler generates relocatable code, the first instruction
      MUST be a jump to the start of the actual program code.  This results
      in the first two words being:</t>
      <texttable anchor="relocation-setup-table" title="Relocation Setup Table">
        <ttcol align="center">Contents of single Word</ttcol>
        <c>SET PC, &lt;next word literally&gt;</c>
        <c>Location of first program instruction</c>
      </texttable>
      <t>It is important to note that assemblers will have to offset all label
      addresses by the size of the relocation table, plus the two words at
      the start.</t>
    </section>
    
    <section anchor="security" title="Security Considerations">
      <t>It is potentially possible for a malicious user to generate code
      which determines the offset of the resulting relocatable program when
      it is loaded into memory and executed.</t>
      <t>This is possible by creating a label with a predetermined address if
      the program was running at 0x0, and calculating the difference
      between the actual address that the program would jump to and the
      original value.</t>
    </section>
  </middle>

  <back>
    <references title="Normative References">
      <?rfc include="reference.RFC.2119.xml"?>
    </references>
  </back>

</rfc>