Jumps and Labels

Turbo Pascal generates jumps in an interesting way. Since most jumps will jump to the same place, Turbo Pascal keeps them in a linked list.

Each near jump (within segment) can be part of a jump list. This procedure creates intermediate code for a jump instruction and adds it to the list.

Procedure GenerateCodeForNearJump (Var JumpRecordList: Word; OpCode: Byte);
Var Saved_PreviousJumpRecord: Word;
begin
  Saved_PreviousJumpRecord := JumpRecordList;
  JumpRecordList := SymbolTable [stIntermediateCode].UsedSize;
  With PIntermediateCodeRecord (IncreaseSymbolTable (stIntermediateCode, 8))^ do
    begin
      RecordType := icJumpNear;
      JumpOpCode := OpCode;
      PreviousJumpRecord := Saved_PreviousJumpRecord;
    end
end;

Once the jumps are generated, the compiler can generate a label. This procedure generates label intermediate code and sets all jumps in a list to point to it.

Procedure GenerateLabelAndSetJumpsToIt (Var JumpRecordList: Word);
Var LabelRecord: PIntermediateCodeRecord;
    JumpRecord: PIntermediateCodeRecord;
    JumpRecordOfs: Word absolute JumpRecord;
    CurrentJumpRecord: Word;
begin
  CurrentJumpRecord := JumpRecordList;
  JumpRecordList := 0;
  If CurrentJumpRecord = 0 then Exit;
  LabelRecord := IncreaseSymbolTable (stIntermediateCode, 3);
  LabelRecord^.Recordtype := icLabel;
  JumpRecord := Ptr (Seg (LabelRecord^), 0);
  Repeat
    JumpRecordOfs := CurrentJumpRecord;
    CurrentJumpRecord := JumpRecord^.PreviousJumpRecord;
    JumpRecord^.LabelRecordOffset := Ofs (LabelRecord^) + 1;
  until CurrentJumpRecord = 0;
end;