procedure TDefault_Output_Filter.Write_To_Driver ;

var S : Ansistring ;
    UEC : TUnified_Exception ;

begin
    setlength( S, 1 ) ;
    while(
           ( ( TFiP_Stream( Term.Stream )._Device.Updated_Info.Status and DS_Output_Ready ) = DS_Output_Ready )
           and
           ( _Buffer.Length > 0 )
         ) do
    begin
        S[ 1 ] := Ansichar( _Buffer.Advance( 1 ) ) ;
        TFiP_Stream( Term.Stream ).Write_Data( PAnsiChar( S )[ 0 ], 1, UEC ) ;
    end ;
end ;

// Terminal output filter flags...
const TOFF_Null = 1 ; // Ignore output (throw it away)
const TOFF_Paused = 2 ; // Don't send output to device
const TOFF_Noformfeed	= 4 ; // Simulate FF with LFs (otherwise send FF)
const TOFF_Notab = 8 ; // UOS converts tabs to spaces (otherwise send TAB)
const TOFF_NoWrap = 16 ; // UOS doesn't output CRLF when input/output reaches the terminal width
const TOFF_NoVertical = 32 ; // Translate output controls to ^C format, otherwise send untranslated control chracters

ASCII (Decimal)	ASCII (Hex)	Character Glyph	ASCII Name	Character Name	HTML Entity	Comments
0	00		NUL	Null
1	01		SOH	Start of heading		Ctl+A
2	02		STX	Start of text		Ctl+B
3	03		ETX	End of text		Ctl+C
4	04		EOT	End of Transmission		Ctl+D
5	05		ENQ	Enquiry		Ctl+E
6	06		ACK	Acknowledge		Ctl+F
7	07		BEL	Bell		Ctl+G
8	08		BS	Backspace		Ctl+H
9	09		HT	Horizontal Tab	#TAB	Ctl+I
10	0A		LF	Linefeed	#RS	Ctl+J
11	0B		VT	Vertical Tab		Ctl+K
12	0C		FF	Form Feed		Ctl+L
13	0D		CR	Carriage Return	#RE	Ctl+M
14	0E		SO	Shift Out		Ctl+N
15	0F		SI	Shift In		Ctl+O
16	10		DLE	Data Link Escape		Ctl+P
17	11		DC1	Device Control 1		Ctl+Q
18	12		DC2	Device Control 2		Ctl+R
19	13		DC3	Device Control 3		Ctl+S
20	14		DC4	Device Control 4		Ctl+T
21	15		NAK	Negative Acknowledge		Ctl+U
22	16		SYN	Synchronous Idle		Ctl+V
23	17		ETB	End of Transmission Block		Ctl+W
24	18		CAN	Cancel		Ctl+X
25	19		EM	End of Medium		Ctl+Y
26	1A		SUB	Substitute		Ctl+Z
27	1B		ESC	Escape
28	1C		FS	File Separator
29	1D		GS	Group Separator
30	1E		RS	Record Separator
31	1F		US	Unit Separator
32	20		SPACE	Space
33	21	!		Exclamation
34	22	“		Quote
35	23	#
36	24	$		Dollar
37	25	%		Percent
38	26	&		Ampersand	amp
39	27	'		Apostrophe
40	28	(		Left Parenthesis
41	29	)		Right Parenthesis
42	2A	*		Asterisk
43	2B	+		Plus
44	2C	,		Comma
45	2D	-		Dash
46	2E	.		Period
47	2F	/		Slash
48	30	0		Zero
49	31	1		One
50	32	2		Two
51	33	3		Three
52	34	4		Four
53	35	5		Five
54	36	6		Six
55	37	7		Seven
56	38	8		Eight
57	39	9		Nine
58	3A	:		Colon
59	3B	;		Semicolon
60	3C	<		Less than	lt
61	3D	=		Equal
62	3E	>		Greater than	gt
63	3F	?		Question
64	40	@		Commercial at
65	41	A
66	42	B
67	43	C
68	44	D
69	45	E
70	46	F
71	47	G
72	48	H
73	49	I
74	4A	J
75	4B	K
76	4C	L
77	4D	M
78	4E	N
79	4F	O
80	50	P
81	51	Q
82	52	R
83	53	S
84	54	T
85	55	U
86	56	V
87	57	W
88	58	X
89	59	Y
90	5A	Z
91	5B	[		Left Bracket
92	5C	\		Backslash
93	5D	]		Right Bracket
94	5E	^		Caret
95	5F	_		Underscore
96	60	`		Grace accent
97	61	a
98	62	b
99	63	c
100	64	d
101	65	e
102	66	f
103	67	g
104	68	h
105	69	i
106	6A	j
107	6B	k
108	6C	l
109	6D	m
110	6E	n
111	6F	o
112	70	p
113	71	q
114	72	r
115	73	s
116	74	t
117	75	u
118	76	v
119	77	w
120	78	x
121	79	y
122	7A	z
123	7B	{		Left Brace
124	7C	|		Vertical Bar
125	7D	}		Right Brace
126	7E	~		Tilde
127	7F		DEL	Delete

Character values 0 through 31, and 127, are non-printing characters with special meanings, although not all control values are used on all platforms. UOS generally considers the following control values to be significant:

Character	Significance
NUL	Specifically has no significance, so is often used as filler or to mark the end of data.
ETX	Used to request an interruption of program execution
EOT	Request status report
BEL	Sounds a tone when output
BS	Used to move the print position left one character. Sometimes used as a delete-previous-character feature
HT	Move to next horizontal tab position
LF	Move to following line
VT	Move to next vertical tab position (some printers)
FF	Move to next page (printer)
CR	Move to beginning of line, or as a means of submitting input (the Enter key)
SI	Turn output off
DC1	Unpause I/O
DC3	Pause I/O
NAK	Delete line
CAN	Used to force interruption of program execution
EM	Used to end input
ESC	Used to begin an escape sequence or as a delimiter

EOT, SI, DC1, and DC3 are generally processed as special characters - they are not considered data input.
LF, FF, CR, and ESC generally serve as input delimiters.
Insofar as the output filter flags are concerned, a FF (Form feed) character is normally sent to the device, where printers move to the top of the next page. However, some printers don't support this, so setting TOFF_NoFormFeed causes the output filter to translate FF into enough LFs (linefeeds) to move to the top of the next page.
Many devices will automatically position to the next tab stop when they receive a HT character. However, some do not support this, so if TOFF_Notab is set, the filter will output sufficient spaces to simulate moving to the next tab stop.
Finally, most devices will automatically move to the next line when the current line is filled with characters (automatic wrapping). Some devices, however, will simply overwrite the last character on the line if the line fills up. The TOFF_NoWrap flag causes the output filter to insert a CR and LF when the right margin is reached.

Now let's look at the Write method.

function TDefault_Output_Filter.Write( S : string ; Async : boolean ) : cardinal ;

var C : AnsiCHar ;
    I, Work, X : integer ;
    Translate : word ;

begin
    Result := 0 ;
    if( ( Flags and TOFF_Null ) <> 0 ) then
    begin
        Result := length( S ) ;
        exit ; // Throw output away
    end ;
    Term.Writes := Term.Writes + 1 ;
    if( Term.Tab_Stops = 0 ) then
    begin
        Term.Tab_Stops := 8 ;
    end ;
    if( Term.Char_Rows = 0 ) then
    begin
        Term.Char_Rows := 66 ;
    end ;

    I := 0 ;
    while( I < length( S ) ) do
    begin
        inc( I ) ;
        if(
            ( ( Flags and TOFF_NoWrap ) = 0 )
            and
            ( Term.Column >= Term.Char_Columns )
            and
            ( Term.Char_Columns > 0 ) // Columns are not undefined
            and
            Positive_Character_Movement( S[ I ] )
          ) then
        begin
            insert( CR, S, I ) ;
            insert( LF, S, I + 1 ) ;
        end ;
        Translate := Translate_Character( S[ I ] ) ;

        case Translate of
            CCC_HT : begin
                         Work := Term.Column + Term.Tab_Stops ;
                         Work := trunc( int( Term.Column div Term.Tab_Stops ) * Term.Tab_Stops ) ;
                         if( ( Flags and TOFF_Notab ) <> 0 ) then // Simulate tabs
                         begin
                             delete( S, I, 1 ) ;
                             for X := Term.Column to Work do
                             begin
                                 insert( ' ', S, I ) ; // Space to next column
                             end ;
                         end else
                         begin
                             Term.Column := Work ;
                         end ;
                     end ;

        CCC_FF : begin
                     Work := Term.Line + Term.Char_Rows ;
                     Work := trunc( int( Term.Line div Term.Char_Rows ) * Term.Char_Rows ) ;
                     if( ( Flags and TOFF_Noformfeed ) <> 0 ) then // Simulate FF
                     begin
                         delete( S, I, 1 ) ;
                         for X := Term.Line to Work do
                         begin
                             insert( LF, S, I ) ; // lF to next line
                         end ;
                     end else
                     begin
                         Term.Line := Work ;
                     end ;
                 end ;

        CCC_VT : begin
                     if( Term.VTab_Stops > 0 ) then
                     begin
                         Work := Term.Line + Term.Char_Rows ;
                         Work := trunc( int( Term.Line div Term.VTab_Stops ) * Term.VTab_Stops ) ;
                         if( ( Flags and TOFF_Noverticaltab ) <> 0 ) then // Simulate VT
                         begin
                             delete( S, I, 1 ) ;
                             for X := Term.Line to Work do
                             begin
                                 insert( LF, S, I ) ; // lF to next line
                             end ;
                         end else
                         begin
                             Term.Line := Work ;
                         end ;
                     end ;
                 end ;

        CCC_CC : begin
                     C := AnsiChar( ord( S[ I ] ) + 64 ) ;
                     insert( '^', S, I ) ;
                     insert( C, S, I + 1 ) ;
                     delete( S, I, 1 ) ;
                 end ;
    end ; // case Translate

        Translate := Translate_Character( S[ I ] ) ;
        case Translate of
            CCC_BS : Term.Column := Term.Column - 1 ;
            CCC_CR : Term.Column := 0 ;
            CCC_LF : Term.Line := Term.Line + 1 ;
            CCC_HT, CCC_VT, CCC_FF, _DEL : ; // No further processing
            else if( ( Translate >= 32 ) and ( Translate <> 127 ) ) then
                 begin
                     Term.Column := Term.Column + 1 ;
                 end else
                 if( Translate >= 0 ) then
                 begin
                     S[ I ] := AnsiChar( Translate ) ;
                 end ;
        end ;

        if( _Buffer.Length >= _Buffer.Size - 1 ) then // Output buffer is full
        begin
            if( Async ) then
            begin
                Result := length( S ) - I + 1 ;
                exit ;
            end else
            begin
                Term.Kernel.USC.Block( 0, PS_IOW, Term ) ;
            end ;
        end else
        begin
            _Buffer.Append( ord( S[ I ] ), 1 ) ;
            inc( Result ) ;
            if( ( Flags and TOFF_Paused ) = 0 ) then // Output not paused
            begin
                if( ( TFiP_Stream( Term.Stream )._Device.Info.Status and DS_Output_Ready ) = DS_Output_Ready ) then
                begin
                    Write_To_Driver ;
                end ;
            end ;
        end ;
    end ; // while ( I <= length( S ) )
end ; // TDefault_Output_Filter.Write

    function Translate_Character( S : AnsiChar ) : word ;

    begin
        if( S = DEL ) then
        begin
            Translate := _Controls[ 0 ] ;
        end else
        if( ( S < ' ' ) and ( S <> NUL ) ) then
        begin
            Translate := _Controls[ ord( S ) ] ;
        end else
        begin
            Translate := ord( S ) ;
        end ;
    end ;

    function Positive_Character_Movement( S : Ansichar ) : boolean ;

    var Translate : word ;

    begin
        if( S > #127 ) then
        begin
            Result := True ;
            exit ;
        end ;
        Translate := Translate_Character( S ) ;
        if( ( Translate >= 32 ) and ( Translate <> 127 ) ) then
        begin
            Result := True ;
            exit ;
        end ;
        case Translate of
            CCC_HT, CCC_CC, _HT : Result := True ;
            else Result := False ;
        end ;
    end ;