fpc/compiler/ogomf.pas

{
    Copyright (c) 2015 by Nikolay Nikolov

    Contains the binary Relocatable Object Module Format (OMF) reader and writer
    This is the object format used on the i8086-msdos platform.

    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program; if not, write to the Free Software
    Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.

 ****************************************************************************
}
unit ogomf;

{$i fpcdefs.inc}

interface

    uses
       { common }
       cclasses,globtype,
       { target }
       systems,
       { assembler }
       cpuinfo,cpubase,aasmbase,assemble,link,
       { OMF definitions }
       omfbase,
       { output }
       ogbase,
       owbase;

    type

      { TOmfRelocation }

      TOmfRelocation = class(TObjRelocation)
      private
        FOmfFixup: TOmfSubRecord_FIXUP;
        function GetGroupIndex(const groupname: string): Integer;
      public
        constructor CreateSection(ADataOffset:aword;aobjsec:TObjSection;Atyp:TObjRelocationType);
        destructor Destroy; override;

        procedure BuildOmfFixup;

        property OmfFixup: TOmfSubRecord_FIXUP read FOmfFixup;
      end;

      { TOmfObjSection }

      TOmfObjSection = class(TObjSection)
      private
        FClassName: string;
        FOverlayName: string;
        FCombination: TOmfSegmentCombination;
        FUse: TOmfSegmentUse;
        FPrimaryGroup: string;
        function GetOmfAlignment: TOmfSegmentAlignment;
      public
        constructor create(AList:TFPHashObjectList;const Aname:string;Aalign:shortint;Aoptions:TObjSectionOptions);override;
        property ClassName: string read FClassName;
        property OverlayName: string read FOverlayName;
        property OmfAlignment: TOmfSegmentAlignment read GetOmfAlignment;
        property Combination: TOmfSegmentCombination read FCombination;
        property Use: TOmfSegmentUse read FUse;
        property PrimaryGroup: string read FPrimaryGroup;
      end;

      { TOmfObjData }

      TOmfObjData = class(TObjData)
      private
        class function CodeSectionName(const aname:string): string;
      public
        constructor create(const n:string);override;
        function sectiontype2align(atype:TAsmSectiontype):shortint;override;
        function sectionname(atype:TAsmSectiontype;const aname:string;aorder:TAsmSectionOrder):string;override;
        procedure writeReloc(Data:aint;len:aword;p:TObjSymbol;Reloctype:TObjRelocationType);override;
      end;

      { TOmfObjOutput }

      TOmfObjOutput = class(tObjOutput)
      private
        FLNames: TOmfOrderedNameCollection;
        FSegments: TFPHashObjectList;
        FGroups: TFPHashObjectList;
        procedure AddSegment(const name,segclass,ovlname: string;
          Alignment: TOmfSegmentAlignment; Combination: TOmfSegmentCombination;
          Use: TOmfSegmentUse; Size: aword);
        procedure AddGroup(const groupname: string; seglist: array of const);
        procedure AddGroup(const groupname: string; seglist: TSegmentList);
        procedure WriteSections(Data:TObjData);
        procedure WriteSectionContentAndFixups(sec: TObjSection);

        procedure section_count_sections(p:TObject;arg:pointer);
        procedure WritePUBDEFs(Data: TObjData);
        procedure WriteEXTDEFs(Data: TObjData);

        property LNames: TOmfOrderedNameCollection read FLNames;
        property Segments: TFPHashObjectList read FSegments;
        property Groups: TFPHashObjectList read FGroups;
      protected
        function writeData(Data:TObjData):boolean;override;
      public
        constructor create(AWriter:TObjectWriter);override;
        destructor Destroy;override;
      end;

      TOmfAssembler = class(tinternalassembler)
        constructor create(smart:boolean);override;
      end;

implementation

    uses
       SysUtils,
       cutils,verbose,globals,
       fmodule,aasmtai,aasmdata,
       ogmap,owomflib,
       version
       ;

{****************************************************************************
                                TOmfRelocation
****************************************************************************}

    function TOmfRelocation.GetGroupIndex(const groupname: string): Integer;
      begin
        if groupname='dgroup' then
          Result:=1
        else
          internalerror(2014040703);
      end;

    constructor TOmfRelocation.CreateSection(ADataOffset: aword; aobjsec: TObjSection; Atyp: TObjRelocationType);
      begin
        if not (Atyp in [RELOC_DGROUP,RELOC_DGROUPREL]) and not assigned(aobjsec) then
          internalerror(200603036);
        DataOffset:=ADataOffset;
        Symbol:=nil;
        OrgSize:=0;
        ObjSection:=aobjsec;
        ftype:=ord(Atyp);
      end;

    destructor TOmfRelocation.Destroy;
      begin
        FOmfFixup.Free;
        inherited Destroy;
      end;

    procedure TOmfRelocation.BuildOmfFixup;
      begin
        Writeln(typ, ' ', ObjSection<>nil, ' ', symbol<>nil);
        FreeAndNil(FOmfFixup);
        FOmfFixup:=TOmfSubRecord_FIXUP.Create;
        if ObjSection<>nil then
          begin
            FOmfFixup.LocationOffset:=DataOffset;
            if typ in [RELOC_ABSOLUTE,RELOC_RELATIVE] then
              FOmfFixup.LocationType:=fltOffset
            else if typ in [RELOC_SEG,RELOC_SEGREL] then
              FOmfFixup.LocationType:=fltBase
            else
              internalerror(2015041501);
            FOmfFixup.FrameDeterminedByThread:=False;
            FOmfFixup.TargetDeterminedByThread:=False;
            if typ in [RELOC_ABSOLUTE,RELOC_SEG] then
              FOmfFixup.Mode:=fmSegmentRelative
            else if typ in [RELOC_RELATIVE,RELOC_SEGREL] then
              FOmfFixup.Mode:=fmSelfRelative
            else
              internalerror(2015041401);
            if typ in [RELOC_ABSOLUTE,RELOC_RELATIVE] then
              begin
                FOmfFixup.TargetMethod:=ftmSegmentIndexNoDisp;
                FOmfFixup.TargetDatum:=ObjSection.Index;
                if TOmfObjSection(ObjSection).PrimaryGroup<>'' then
                  begin
                    FOmfFixup.FrameMethod:=ffmGroupIndex;
                    FOmfFixup.FrameDatum:=GetGroupIndex(TOmfObjSection(ObjSection).PrimaryGroup);
                  end
                else
                  FOmfFixup.FrameMethod:=ffmTarget;
              end
            else
              begin
                FOmfFixup.FrameMethod:=ffmTarget;
                FOmfFixup.TargetMethod:=ftmSegmentIndexNoDisp;
                FOmfFixup.TargetDatum:=ObjSection.Index;
                if TOmfObjSection(ObjSection).PrimaryGroup<>'' then
                  begin
                    FOmfFixup.TargetMethod:=ftmGroupIndexNoDisp;
                    FOmfFixup.TargetDatum:=GetGroupIndex(TOmfObjSection(ObjSection).PrimaryGroup);
                  end
                else
                  begin
                    FOmfFixup.TargetMethod:=ftmSegmentIndexNoDisp;
                    FOmfFixup.TargetDatum:=ObjSection.Index;
                  end;
              end;
          end
        else if symbol<>nil then
          begin
            FOmfFixup.LocationOffset:=DataOffset;
            if typ in [RELOC_ABSOLUTE,RELOC_RELATIVE] then
              FOmfFixup.LocationType:=fltOffset
            else if typ in [RELOC_SEG,RELOC_SEGREL] then
              FOmfFixup.LocationType:=fltBase
            else
              internalerror(2015041501);
            FOmfFixup.FrameDeterminedByThread:=False;
            FOmfFixup.TargetDeterminedByThread:=False;
            if typ in [RELOC_ABSOLUTE,RELOC_SEG] then
              FOmfFixup.Mode:=fmSegmentRelative
            else if typ in [RELOC_RELATIVE,RELOC_SEGREL] then
              FOmfFixup.Mode:=fmSelfRelative
            else
              internalerror(2015041401);
            FOmfFixup.TargetMethod:=ftmExternalIndexNoDisp;
            FOmfFixup.TargetDatum:=symbol.symidx;
            FOmfFixup.FrameMethod:=ffmTarget;
          end
        else if typ in [RELOC_DGROUP,RELOC_DGROUPREL] then
          begin
            FOmfFixup.LocationOffset:=DataOffset;
            FOmfFixup.LocationType:=fltBase;
            FOmfFixup.FrameDeterminedByThread:=False;
            FOmfFixup.TargetDeterminedByThread:=False;
            if typ=RELOC_DGROUP then
              FOmfFixup.Mode:=fmSegmentRelative
            else if typ=RELOC_DGROUPREL then
              FOmfFixup.Mode:=fmSelfRelative
            else
              internalerror(2015041401);
            FOmfFixup.FrameMethod:=ffmTarget;
            FOmfFixup.TargetMethod:=ftmGroupIndexNoDisp;
            FOmfFixup.TargetDatum:=GetGroupIndex('dgroup');
          end
        else
         internalerror(2015040702);
      end;

{****************************************************************************
                                TOmfObjSection
****************************************************************************}

    function TOmfObjSection.GetOmfAlignment: TOmfSegmentAlignment;
      begin
        case SecAlign of
          1:
            result:=saRelocatableByteAligned;
          2:
            result:=saRelocatableWordAligned;
          4:
            result:=saRelocatableDWordAligned;
          16:
            result:=saRelocatableParaAligned;
          else
            internalerror(2015041504);
        end;
      end;

    constructor TOmfObjSection.create(AList: TFPHashObjectList;
          const Aname: string; Aalign: shortint; Aoptions: TObjSectionOptions);
      var
        dgroup: Boolean;
      begin
        inherited create(AList, Aname, Aalign, Aoptions);
        FCombination:=scPublic;
        FUse:=suUse16;
        if oso_executable in Aoptions then
          begin
            FClassName:='code';
            dgroup:=(current_settings.x86memorymodel=mm_tiny);
          end
        else if Aname='stack' then
          begin
            FClassName:='stack';
            FCombination:=scStack;
            dgroup:=current_settings.x86memorymodel in (x86_near_data_models-[mm_tiny]);
          end
        else if Aname='heap' then
          begin
            FClassName:='heap';
            dgroup:=current_settings.x86memorymodel in x86_near_data_models;
          end
        else if Aname='bss' then
          begin
            FClassName:='bss';
            dgroup:=true;
          end
        else if Aname='data' then
          begin
            FClassName:='data';
            dgroup:=true;
          end
        else if (Aname='debug_frame') or
                (Aname='debug_info') or
                (Aname='debug_line') or
                (Aname='debug_abbrev') then
          begin
            FClassName:='DWARF';
            FUse:=suUse32;
            dgroup:=false;
          end
        else
          begin
            FClassName:='data';
            dgroup:=true;
          end;
        if dgroup then
          FPrimaryGroup:='dgroup'
        else
          FPrimaryGroup:='';
      end;

{****************************************************************************
                                TOmfObjData
****************************************************************************}

    class function TOmfObjData.CodeSectionName(const aname: string): string;
      begin
{$ifdef i8086}
        if current_settings.x86memorymodel in x86_far_code_models then
          begin
            if cs_huge_code in current_settings.moduleswitches then
              result:=aname + '_TEXT'
            else
              result:=current_module.modulename^ + '_TEXT';
          end
        else
{$endif}
          result:='text';
      end;

    constructor TOmfObjData.create(const n: string);
      begin
        inherited create(n);
        CObjSection:=TOmfObjSection;
      end;

    function TOmfObjData.sectiontype2align(atype: TAsmSectiontype): shortint;
      begin
        case atype of
          sec_stabstr:
            result:=1;
          sec_code:
            result:=1;
          sec_data,
          sec_rodata,
          sec_rodata_norel,
          sec_bss:
            result:=2;
          { For idata (at least idata2) it must be 4 bytes, because
            an entry is always (also in win64) 20 bytes and aligning
            on 8 bytes will insert 4 bytes between the entries resulting
            in a corrupt idata section.
            Same story with .pdata, it has 4-byte elements which should
            be packed without gaps. }
          sec_idata2,sec_idata4,sec_idata5,sec_idata6,sec_idata7,sec_pdata:
            result:=4;
          sec_debug_frame,sec_debug_info,sec_debug_line,sec_debug_abbrev:
            result:=4;
          sec_stack,
          sec_heap:
            result:=16;
          else
            result:=1;
        end;
      end;

    function TOmfObjData.sectionname(atype:TAsmSectiontype;const aname:string;aorder:TAsmSectionOrder):string;
      const
        secnames : array[TAsmSectiontype] of string[length('__DATA, __datacoal_nt,coalesced')] = ('','',
          'text',
          'data',
          'data',
          'rodata',
          'bss',
          'tbss',
          'pdata',
          'text','data','data','data','data',
          'stab',
          'stabstr',
          'idata2','idata4','idata5','idata6','idata7','edata',
          'eh_frame',
          'debug_frame','debug_info','debug_line','debug_abbrev',
          'fpc',
          '',
          'init',
          'fini',
          'objc_class',
          'objc_meta_class',
          'objc_cat_cls_meth',
          'objc_cat_inst_meth',
          'objc_protocol',
          'objc_string_object',
          'objc_cls_meth',
          'objc_inst_meth',
          'objc_cls_refs',
          'objc_message_refs',
          'objc_symbols',
          'objc_category',
          'objc_class_vars',
          'objc_instance_vars',
          'objc_module_info',
          'objc_class_names',
          'objc_meth_var_types',
          'objc_meth_var_names',
          'objc_selector_strs',
          'objc_protocol_ext',
          'objc_class_ext',
          'objc_property',
          'objc_image_info',
          'objc_cstring_object',
          'objc_sel_fixup',
          '__DATA,__objc_data',
          '__DATA,__objc_const',
          'objc_superrefs',
          '__DATA, __datacoal_nt,coalesced',
          'objc_classlist',
          'objc_nlclasslist',
          'objc_catlist',
          'obcj_nlcatlist',
          'objc_protolist',
          'stack',
          'heap'
        );
      begin
        if (atype=sec_user) then
          Result:=aname
        else if secnames[atype]='text' then
          Result:=CodeSectionName(aname)
        else
          Result:=secnames[atype];
      end;

    procedure TOmfObjData.writeReloc(Data:aint;len:aword;p:TObjSymbol;Reloctype:TObjRelocationType);
      var
        objreloc: TOmfRelocation;
        symaddr: AWord;
      begin
        Writeln(Reloctype);
        { RELOC_FARPTR = RELOC_ABSOLUTE+RELOC_SEG }
        if Reloctype=RELOC_FARPTR then
          begin
            if len<>4 then
              internalerror(2015041502);
            writeReloc(Data,2,p,RELOC_ABSOLUTE);
            writeReloc(0,2,p,RELOC_SEG);
            exit;
          end;

        if CurrObjSec=nil then
          internalerror(200403072);
        objreloc:=nil;
        if assigned(p) then
          begin
            { real address of the symbol }
            symaddr:=p.address;

            if p.bind=AB_EXTERNAL then
              begin
                objreloc:=TOmfRelocation.CreateSymbol(CurrObjSec.Size,p,Reloctype);
                CurrObjSec.ObjRelocations.Add(objreloc);
              end
            else
              begin
                objreloc:=TOmfRelocation.CreateSection(CurrObjSec.Size,p.objsection,Reloctype);
                CurrObjSec.ObjRelocations.Add(objreloc);
                inc(data,symaddr);
              end;
          end
        else if Reloctype in [RELOC_DGROUP,RELOC_DGROUPREL] then
            begin
              objreloc:=TOmfRelocation.CreateSection(CurrObjSec.Size,nil,Reloctype);
              CurrObjSec.ObjRelocations.Add(objreloc);
            end;
        CurrObjSec.write(data,len);
      end;

{****************************************************************************
                                TOmfObjOutput
****************************************************************************}

    procedure TOmfObjOutput.AddSegment(const name, segclass, ovlname: string;
        Alignment: TOmfSegmentAlignment; Combination: TOmfSegmentCombination;
        Use: TOmfSegmentUse; Size: aword);
      var
        s: TOmfRecord_SEGDEF;
      begin
        s:=TOmfRecord_SEGDEF.Create;
        Segments.Add(name,s);
        s.SegmentNameIndex:=LNames.Add(name);
        s.ClassNameIndex:=LNames.Add(segclass);
        s.OverlayNameIndex:=LNames.Add(ovlname);
        s.Alignment:=Alignment;
        s.Combination:=Combination;
        s.Use:=Use;
        s.SegmentLength:=Size;
      end;

    procedure TOmfObjOutput.AddGroup(const groupname: string; seglist: array of const);
      var
        g: TOmfRecord_GRPDEF;
        I: Integer;
        SegListStr: TSegmentList;
      begin
        g:=TOmfRecord_GRPDEF.Create;
        Groups.Add(groupname,g);
        g.GroupNameIndex:=LNames.Add(groupname);
        SetLength(SegListStr,Length(seglist));
        for I:=0 to High(seglist) do
          begin
            case seglist[I].VType of
              vtString:
                SegListStr[I]:=Segments.FindIndexOf(seglist[I].VString^);
              vtAnsiString:
                SegListStr[I]:=Segments.FindIndexOf(AnsiString(seglist[I].VAnsiString));
              vtWideString:
                SegListStr[I]:=Segments.FindIndexOf(AnsiString(WideString(seglist[I].VWideString)));
              vtUnicodeString:
                SegListStr[I]:=Segments.FindIndexOf(AnsiString(UnicodeString(seglist[I].VUnicodeString)));
              else
                internalerror(2015040402);
            end;
          end;
        g.SegmentList:=SegListStr;
      end;

    procedure TOmfObjOutput.AddGroup(const groupname: string; seglist: TSegmentList);
      var
        g: TOmfRecord_GRPDEF;
      begin
        g:=TOmfRecord_GRPDEF.Create;
        Groups.Add(groupname,g);
        g.GroupNameIndex:=LNames.Add(groupname);
        g.SegmentList:=Copy(seglist);
      end;

    procedure TOmfObjOutput.WriteSections(Data: TObjData);
      var
        i:longint;
        sec:TObjSection;
      begin
        for i:=0 to Data.ObjSectionList.Count-1 do
          begin
            sec:=TObjSection(Data.ObjSectionList[i]);
            WriteSectionContentAndFixups(sec);
          end;
      end;

    procedure TOmfObjOutput.WriteSectionContentAndFixups(sec: TObjSection);
      const
        MaxChunkSize=$3fa;
      var
        RawRecord: TOmfRawRecord;
        ChunkStart,ChunkLen: DWord;
        ChunkFixupStart,ChunkFixupEnd: Integer;
        SegIndex: Integer;
        NextOfs: Integer;
        I: Integer;
      begin
        if (oso_data in sec.SecOptions) then
          begin
            if sec.Data=nil then
              internalerror(200403073);
            for I:=0 to sec.ObjRelocations.Count-1 do
              TOmfRelocation(sec.ObjRelocations[I]).BuildOmfFixup;
            SegIndex:=Segments.FindIndexOf(sec.Name);
            RawRecord:=TOmfRawRecord.Create;
            sec.data.seek(0);
            ChunkFixupStart:=0;
            ChunkFixupEnd:=-1;
            ChunkStart:=0;
            ChunkLen:=Min(MaxChunkSize, sec.Data.size-ChunkStart);
            while ChunkLen>0 do
            begin
              { find last fixup in the chunk }
              while (ChunkFixupEnd<(sec.ObjRelocations.Count-1)) and
                    (TOmfRelocation(sec.ObjRelocations[ChunkFixupEnd+1]).DataOffset<(ChunkStart+ChunkLen)) do
                inc(ChunkFixupEnd);
              { check if last chunk is crossing the chunk boundary, and trim ChunkLen if necessary }
              if (ChunkFixupEnd>=ChunkFixupStart) and
                ((TOmfRelocation(sec.ObjRelocations[ChunkFixupEnd]).DataOffset+
                  TOmfRelocation(sec.ObjRelocations[ChunkFixupEnd]).OmfFixup.LocationSize)>(ChunkStart+ChunkLen)) then
                begin
                  ChunkLen:=TOmfRelocation(sec.ObjRelocations[ChunkFixupEnd]).DataOffset-ChunkStart;
                  Dec(ChunkFixupEnd);
                end;
              { write LEDATA record }
              RawRecord.RecordType:=RT_LEDATA;
              NextOfs:=RawRecord.WriteIndexedRef(0,SegIndex);
              RawRecord.RawData[NextOfs]:=Byte(ChunkStart);
              RawRecord.RawData[NextOfs+1]:=Byte(ChunkStart shr 8);
              Inc(NextOfs,2);
              sec.data.read(RawRecord.RawData[NextOfs], ChunkLen);
              Inc(NextOfs, ChunkLen);
              RawRecord.RecordLength:=NextOfs+1;
              RawRecord.CalculateChecksumByte;
              RawRecord.WriteTo(FWriter);
              { write FIXUPP record }
              if ChunkFixupEnd>=ChunkFixupStart then
                begin
                  RawRecord.RecordType:=RT_FIXUPP;
                  NextOfs:=0;
                  for I:=ChunkFixupStart to ChunkFixupEnd do
                    begin
                      TOmfRelocation(sec.ObjRelocations[I]).OmfFixup.DataRecordStartOffset:=ChunkStart;
                      NextOfs:=TOmfRelocation(sec.ObjRelocations[I]).OmfFixup.WriteAt(RawRecord,NextOfs);
                    end;
                  RawRecord.RecordLength:=NextOfs+1;
                  RawRecord.CalculateChecksumByte;
                  RawRecord.WriteTo(FWriter);
                end;
              { prepare next chunk }
              Inc(ChunkStart, ChunkLen);
              ChunkLen:=Min(MaxChunkSize, sec.Data.size-ChunkStart);
              ChunkFixupStart:=ChunkFixupEnd+1;
            end;
            RawRecord.Free;
          end;
      end;

    procedure TOmfObjOutput.section_count_sections(p: TObject; arg: pointer);
      begin
        TOmfObjSection(p).index:=pinteger(arg)^;
        inc(pinteger(arg)^);
      end;

    procedure TOmfObjOutput.WritePUBDEFs(Data: TObjData);
      var
        PubNamesForSection: array of TFPHashObjectList;
        i: Integer;
        objsym: TObjSymbol;
        PublicNameElem: TOmfPublicNameElement;
        RawRecord: TOmfRawRecord;
        PubDefRec: TOmfRecord_PUBDEF;
        PrimaryGroupName: string;
      begin
        RawRecord:=TOmfRawRecord.Create;
        SetLength(PubNamesForSection,Data.ObjSectionList.Count);
        for i:=0 to Data.ObjSectionList.Count-1 do
          PubNamesForSection[i]:=TFPHashObjectList.Create;

        for i:=0 to Data.ObjSymbolList.Count-1 do
          begin
            objsym:=TObjSymbol(Data.ObjSymbolList[i]);
            if objsym.bind=AB_GLOBAL then
              begin
                PublicNameElem:=TOmfPublicNameElement.Create(PubNamesForSection[objsym.objsection.index-1],objsym.Name);
                PublicNameElem.PublicOffset:=objsym.offset;
              end;
          end;

        for i:=0 to Data.ObjSectionList.Count-1 do
          if PubNamesForSection[i].Count>0 then
            begin
              PubDefRec:=TOmfRecord_PUBDEF.Create;
              PubDefRec.BaseSegmentIndex:=i+1;
              PrimaryGroupName:=TOmfObjSection(Data.ObjSectionList[i]).PrimaryGroup;
              if PrimaryGroupName<>'' then
                PubDefRec.BaseGroupIndex:=Groups.FindIndexOf(PrimaryGroupName)
              else
                PubDefRec.BaseGroupIndex:=0;
              PubDefRec.PublicNames:=PubNamesForSection[i];
              while PubDefRec.NextIndex<PubDefRec.PublicNames.Count do
                begin
                  PubDefRec.EncodeTo(RawRecord);
                  RawRecord.WriteTo(FWriter);
                end;
              PubDefRec.Free;
            end;

        for i:=0 to Data.ObjSectionList.Count-1 do
          FreeAndNil(PubNamesForSection[i]);
        RawRecord.Free;
      end;

    procedure TOmfObjOutput.WriteEXTDEFs(Data: TObjData);
      var
        ExtNames: TFPHashObjectList;
        RawRecord: TOmfRawRecord;
        i,idx: Integer;
        objsym: TObjSymbol;
        ExternalNameElem: TOmfExternalNameElement;
        ExtDefRec: TOmfRecord_EXTDEF;
      begin
        ExtNames:=TFPHashObjectList.Create;
        RawRecord:=TOmfRawRecord.Create;

        idx:=1;
        for i:=0 to Data.ObjSymbolList.Count-1 do
          begin
            objsym:=TObjSymbol(Data.ObjSymbolList[i]);
            if objsym.bind=AB_EXTERNAL then
              begin
                ExternalNameElem:=TOmfExternalNameElement.Create(ExtNames,objsym.Name);
                objsym.symidx:=idx;
                Inc(idx);
              end;
          end;

        if ExtNames.Count>0 then
          begin
            ExtDefRec:=TOmfRecord_EXTDEF.Create;
            ExtDefRec.ExternalNames:=ExtNames;
            while ExtDefRec.NextIndex<ExtDefRec.ExternalNames.Count do
              begin
                ExtDefRec.EncodeTo(RawRecord);
                RawRecord.WriteTo(FWriter);
              end;
            ExtDefRec.Free;
          end;

        ExtNames.Free;
        RawRecord.Free;
      end;

    function TOmfObjOutput.writeData(Data:TObjData):boolean;
      var
        RawRecord: TOmfRawRecord;
        Header: TOmfRecord_THEADR;
        Translator_COMENT: TOmfRecord_COMENT;
        LinkPassSeparator_COMENT: TOmfRecord_COMENT;
        LNamesRec: TOmfRecord_LNAMES;
        ModEnd: TOmfRecord_MODEND;
        I: Integer;
        SegDef: TOmfRecord_SEGDEF;
        GrpDef: TOmfRecord_GRPDEF;
        DGroupSegments: TSegmentList;
        nsections: Integer;
      begin
        { calc amount of sections we have and set their index, starting with 1 }
        nsections:=1;
        data.ObjSectionList.ForEachCall(@section_count_sections,@nsections);
        { maximum amount of sections supported in the omf format is $7fff }
        if (nsections-1)>$7fff then
          internalerror(2015040701);

        { write header record }
        RawRecord:=TOmfRawRecord.Create;
        Header:=TOmfRecord_THEADR.Create;
        Header.ModuleName:=Data.Name;
        Header.EncodeTo(RawRecord);
        RawRecord.WriteTo(FWriter);
        Header.Free;

        { write translator COMENT header }
        Translator_COMENT:=TOmfRecord_COMENT.Create;
        Translator_COMENT.CommentClass:=CC_Translator;
        Translator_COMENT.CommentString:='FPC '+full_version_string+
        ' ['+date_string+'] for '+target_cpu_string+' - '+target_info.shortname;
        Translator_COMENT.EncodeTo(RawRecord);
        RawRecord.WriteTo(FWriter);
        Translator_COMENT.Free;

        LNames.Clear;
        LNames.Add('');  { insert an empty string, which has index 1 }
        FSegments.Clear;
        FSegments.Add('',nil);
        FGroups.Clear;
        FGroups.Add('',nil);

        for i:=0 to Data.ObjSectionList.Count-1 do
          with TOmfObjSection(Data.ObjSectionList[I]) do
            AddSegment(Name,ClassName,OverlayName,OmfAlignment,Combination,Use,Size);


        { create group "dgroup" }
        SetLength(DGroupSegments,0);
        for i:=0 to Data.ObjSectionList.Count-1 do
          with TOmfObjSection(Data.ObjSectionList[I]) do
            if PrimaryGroup='dgroup' then
              begin
                SetLength(DGroupSegments,Length(DGroupSegments)+1);
                DGroupSegments[High(DGroupSegments)]:=index;
              end;
        AddGroup('dgroup',DGroupSegments);

        { write LNAMES record(s) }
        LNamesRec:=TOmfRecord_LNAMES.Create;
        LNamesRec.Names:=LNames;
        while LNamesRec.NextIndex<=LNames.Count do
          begin
            LNamesRec.EncodeTo(RawRecord);
            RawRecord.WriteTo(FWriter);
          end;
        LNamesRec.Free;

        { write SEGDEF record(s) }
        for I:=1 to Segments.Count-1 do
          begin
            SegDef:=TOmfRecord_SEGDEF(Segments[I]);
            SegDef.EncodeTo(RawRecord);
            RawRecord.WriteTo(FWriter);
          end;

        { write GRPDEF record(s) }
        for I:=1 to Groups.Count-1 do
          begin
            GrpDef:=TOmfRecord_GRPDEF(Groups[I]);
            GrpDef.EncodeTo(RawRecord);
            RawRecord.WriteTo(FWriter);
          end;

        { write PUBDEF record(s) }
        WritePUBDEFs(Data);

        { write EXTDEF record(s) }
        WriteEXTDEFs(Data);

        { write link pass separator }
        LinkPassSeparator_COMENT:=TOmfRecord_COMENT.Create;
        LinkPassSeparator_COMENT.CommentClass:=CC_LinkPassSeparator;
        LinkPassSeparator_COMENT.CommentString:=#1;
        LinkPassSeparator_COMENT.NoList:=True;
        LinkPassSeparator_COMENT.EncodeTo(RawRecord);
        RawRecord.WriteTo(FWriter);
        LinkPassSeparator_COMENT.Free;

        { write section content, interleaved with fixups }
        WriteSections(Data);

        { write MODEND record }
        ModEnd:=TOmfRecord_MODEND.Create;
        ModEnd.EncodeTo(RawRecord);
        RawRecord.WriteTo(FWriter);
        ModEnd.Free;

        RawRecord.Free;
        result:=true;
      end;

    constructor TOmfObjOutput.create(AWriter:TObjectWriter);
      begin
        inherited create(AWriter);
        cobjdata:=TOmfObjData;
        FLNames:=TOmfOrderedNameCollection.Create;
        FSegments:=TFPHashObjectList.Create;
        FSegments.Add('',nil);
        FGroups:=TFPHashObjectList.Create;
        FGroups.Add('',nil);
      end;

    destructor TOmfObjOutput.Destroy;
      begin
        FGroups.Free;
        FSegments.Free;
        FLNames.Free;
        inherited Destroy;
      end;

{****************************************************************************
                               TOmfAssembler
****************************************************************************}

    constructor TOmfAssembler.Create(smart:boolean);
      begin
        inherited Create(smart);
        CObjOutput:=TOmfObjOutput;
        CInternalAr:=TOmfLibObjectWriter;
      end;

{*****************************************************************************
                                  Initialize
*****************************************************************************}
{$ifdef i8086}
    const
       as_i8086_omf_info : tasminfo =
          (
            id     : as_i8086_omf;
            idtxt  : 'OMF';
            asmbin : '';
            asmcmd : '';
            supported_targets : [system_i8086_msdos];
            flags : [af_outputbinary,af_no_debug];
            labelprefix : '..@';
            comment : '; ';
            dollarsign: '$';
          );
{$endif i8086}

initialization
{$ifdef i8086}
  RegisterAssembler(as_i8086_omf_info,TOmfAssembler);
{$endif i8086}
end.