IMPORT S:=SYSTEM, ..
TYPE
  Req = RECORD
    opcode : BYTE;
    mode    : BYTE;
    length  : SHORTINT;
    window  : INTEGER;
  END;
  AReq = ARRAY SIZE(Req) OF BYTE;

VAR rq:Req;
  ...
  rq.opcode := opcode;
  rq.mode  := mode;
  rq.length := length;
  rq.window := window;

  stream.WriteBytes(S.VAL(AReq, rq), 0,SIZE(Req),res);

CONST szReq =8;
VAR rq: ARRAY szReq OF BYTE;
    opcode : BYTE;
    mode    : BYTE;
    length  : SHORTINT;
    window  : INTEGER;

  ix:=0;
  S.PUT(S.ADR(rq[0]),opcode);
  S.PUT(S.ADR(rq[1]),mode);
  S.PUT(S.ADR(rq[2]),length);
  S.PUT(S.ADR(rq[4]),window);
  stream.WriteBytes(rq, 0,szReq,res);

CONST szReq =8;

  PROCEDURE WInt8(VAR b:ARRAY OF BYTE; VAR i: INTEGER; x: INTEGER);
  BEGIN
    b[i]:= SHORT(SHORT(x)); INC(i)
  END WInt8;
  ...

  VAR rq: ARRAY szReq OF BYTE;
    ix: INTEGER;

  ix:=0;
  WInt8(rq,ix,opcode);
  WInt8(rq,ix,mode);
  WInt16(rq,ix,length);
  WInt32(rq,ix,window);
  stream.WriteBytes(rq, 0,szReq,res);

TYPE
  Writer* = RECORD
    offset*: INTEGER
  END;

  PROCEDURE (VAR this: Writer) WriteByte* (VAR dst: ARRAY OF BYTE; value: BYTE), NEW;
  BEGIN dst[this.offset] := value; INC(this.offset)
  END WriteByte;

 PROCEDURE WRec(VAR b:ARRAY OF BYTE; VAR ix: INTEGER; IN r:ANYREC);
 VAR size: INTEGER;
 BEGIN
   S.GET(S.TYP(r),size);
   S.MOVE(S.ADR(r), S.ADR(b[ix]), size);
   INC(ix,size);
 END WRec;
 

TYPE
  Packet* = RECORD
    length-: INTEGER;
    pos-: INTEGER;
    buff-: ARRAY 32768 OF BYTE (* столько хватит? *)
  END;

  PROCEDURE (VAR this: Packet) Clear*, NEW;
    VAR i: INTEGER;
  BEGIN this.length := 0; this.pos := 0;
    FOR i := 0 TO LEN(this.buff)-1 DO this.buff[i] := 0 END
  END Clear;

  PROCEDURE (VAR this: Packet) WriteByte* (value: BYTE), NEW;
  BEGIN ASSERT(this.pos < LEN(this.buff));
    this.buff[this.pos] := value; INC(this.pos);
    IF this.length < this.pos THEN this.length := this.pos END
  END WriteByte;

  PROCEDURE (VAR this: Packet) SetPos* (pos: INTEGER), NEW;
  ...

TYPE
  Packet* = RECORD
    pos- : INTEGER; (* оно же length, обойдёмся без Seek() *)
    data- : POINTER TO ARRAY OF BYTE;
  END;

PROCEDURE (VAR p : Packet) Init* (length : INTEGER), NEW;
PROCEDURE (VAR p : Packet) WriteByte* (x : BYTE), NEW;
...
PROCEDURE SendPacket* (stream : Stream; p : Packet);
...