Defines a display character or performs a low level display operation.
Character stream (hex)
Defines a character or performs various miscellaneous VDU functions.
Defines a character or performs various miscellaneous VDU functions. Unrecognised or unimplemented commands are passed to VDUV, on platforms where VDUV is implemented.
VDU 23,32-255 - Define character
VDU 23,32-255,row1,row2,row3,row4,row5,row6,row7,row8 (OS 0.10 onwards)
In this type of command, all nine parameters are used.
The first byte of the bitmap (the second parameter) carries the top row of
the bitmap. The most significant bit corresponds to the leftmost column. A
binary 1 represents the foreground
COLOUR (usually white),
binary 0 the background
COLOUR (usually black.)
For example, the stick figure character printed in the User Guide (and above) can be defined with the following sequence:
When the character is printed, such as by
the figure will typically appear in white on a black background, that is, in the negative compared to the image above.
Depending on the font explode state a number of other characters will simultaneously be assigned that bitmap. The font is not ordinarily transferred to the printer.
MODE 7 the user defined character has no effect on the
Teletext hardware, but the definition is stored for later use. In the
MODEs the new definition does not change the
appearance of copies of that character already on screen; OSBYTE &87
will no longer recognise them as that character.
The character definition canbe read with OSWORD &0A. Note that character 127 can be defined with VDU 23,127, but can never be printed. The definition can be read with OSWORD &0A.
VDU 23,0 - CRTC register programming
VDU 23,0,register,value,0,0,0,0,0,0 (OS 0.10 onwards)
In this command,
VDU 23 programs the 6845 register <register>
(or emulated register) with value <value>. For the functions of each
register, see CRTC.
Non-6502 platforms, such as RISC OS and
BBC BASIC for Windows, partly emulate the CRTC so that
cursor appearance and screen interlace can be controlled with
23. The statement
VDU 23;8202;0;0;0; which appears in
many BBC Micro programs, also turns the cursor off in RISC OS, as it expands
to VDU 23,0,10,32;0;0;0.
Note that this command can 'unprogram' the current
that a full DRAM refresh is no longer performed. This coupled with a tight
machine code loop erases most of the I/O processor's memory, with disastrous
VDU 23,1 - Cursor control
VDU 23,1,state,0,0,0,0,0,0,0 (OS 1.00 onwards)
This subcommand turns the cursor on or off. The defined values of <state> are:
- 0 - off
- 1 - on
- 2 - flashing (MOS 3 onwards)
- 3 - steady (MOS 3 onwards)
Note that the cursor will re-appear if cursor editing is used.
VDU 23,2-5 - Define ECF pattern
VDU 23,2-5,row1,row2,row3,row4,row5,row6,row7,row8 (GXR, MOS 3.00 onwards, not BBFW)
This defines Extended Colour Fill patterns (ECFs) 1 to 4. They are selectable for plotting with GCOL 16*ECF,0.
As the fill patterns are mapped to the pixel layout of each screen mode, they are defined differently for different colour depths. On RISC OS, ECF patterns can be defined in BBC/Master compatible mode or in RISC OS native mode. To change between the modes, VDU 23,17,4 is used.
Numbering the pixels of each row as 76543210 give the following arrangement:
- 2-colour modes: 7,6,5,4,3,2,1,0 (BBC) 0,1,2,3,4,5,6,7 (RISC OS)
- 4-colour modes: 73,62,51,40 (BBC) 10,32,54,76 (RISC OS)
- 16-colour modes: 7531,6420 (BBC) 3210,7654 (RISC OS)
- 256-colour modes: 76543210 (BBC) 76543210 (RISC OS)
The ECFs are readable with OSWORD 10,2-5.
VDU 23,6 - Set dotted line pattern
VDU 23,6,n1,n2,n3,n4,n5,n6,n7,n8 (GXR, MOS 3.00 onwards, not BBFW)
This sets the dotted line pattern used by dotted PLOT commands. The pattern length is set with *FX163,242. The default is &AA, length 8, as prior to MOS 3.00. The pattern is readable with [[OSWORD &0A|OSWORD 10,6]], and the repeat length is read with OSBYTE 163,242.
VDU 23,7 - Scroll rectangle on screen
VDU 23,7,extent,direction,movement,0,0,0,0,0 (OS 3.00 onwards)
This command will scroll the whole screen, or the current text window, in any direction.
- extent=0 - scroll the current text window
- extent=1 - scroll the entire screen
- direction=0 - scroll right
- direction=1 - scroll left
- direction=2 - scroll down
- direction=3 - scroll up
- direction=4 - scroll in positive X direction
- direction=5 - scroll in negative X direction
- direction=6 - scroll in positive Y direction
- direction=7 - scroll in negative Y direction
- movement=0 - scroll by 1 character cell
- movement=1 - scroll by 1 character cell vertically, or 1 byte horizontally (not BBFW)
When scrolling by a byte horizontally, the movement depends on the colour depth of the current mode:
- 8 pixels in 2-colour modes
- 4 pixels in 4-colour modes
- 2 pixels in 16-colour modes
- 1 pixel in 256-colour modes
Note that the cursor is never moved.
VDU 23,8 - Clear block
VDU 23,8,t1,t2,x1,y1,x2,x2,0,0 (OS 3.00 onwards, not BBFW)
This will clear a block of the current text window to the current text background colour. <t1> and <t2> defines the type of block to be cleared, and <x1>,<y1>,<x2>,<y2> define the
<t1> - base position of start of block <t2> - base position of end of block
- 0 - top left of text window
- 1 - top of cursor column
- 2 - off top right of text window
- 4 - left end of cursor line
- 5 - cursor position
- 6 - off right of cursor line
- 8 - bottom left of text window
- 9 - bottom of cursor column
- 10 - off bottom right of text window
Note that 'left', 'up', etc are dependant on the direction of cursor movement as defined with VDU 23,16.
<x1>,<y1>,<x2>,<y2> are displacements from the positions specified by <t1> and <t2> and determine the start and end of the block to be cleared. If <t1> and <t2> are both 0, then <x1>,<y1>,<x2>,<y2> specify a block within the current text window in the same manner as VDU 28 defines the text window.
Values of <t1> and <t2> other than those listed produced undefined results. No clearing will occur if the end point os before the start point.
VDU 23,9 - Set flash mark period
VDU 23,9,period,0,0,0,0,0,0,0 (OS 3.00 onwards, not BBFW)
Set flash mark period (the time the first flash colour is visible) in the number of VSyncs (usually 1/50s), as with *FX9. If the period is 0, the first flash colour is displayed continously.
VDU 23,10 - Set flash space period
VDU 23,10,period,0,0,0,0,0,0,0 (OS 3.00 onwards, not BBFW)
Set flash space period (the time the second flash colour is visible) in the number of VSyncs (usually 1/50s), as with *FX10. If the period is 0, the second flash colour is displayed continuously.
VDU 23,11 - Set default ECF patterns
VDU 23,11,0,0,0,0,0,0,0,0 (GXR, OS 3.00 onwards, not BBFW)
This command resets all the Extended Colour Fill (ECF) patterns to the default for the current screen mode.
VDU 23,12-15 - Set simple ECF pattern
VDU 23,12-15,a,b,c,d,e,f,g,h (GXR, OS 3.00 onwards, not BBFW)
Defines Extended Colour Fill (ECF) patterns 1 to 4 in a simpler manner than VDU 23,2-5. The ECF is defined as a repeated block of four lines of two repeated pixels.
The first pixel line uses alternative pixels of colours a,b; the second pixel line uses alternative pixels of colours c,b, etc.
VDU 23,16 - Define cursor movement
VDU 23,16,x,y,0,0,0,0,0,0 (OS 3.00 onwards)
This allows the direction the cursor moves after a character is printed to be specified. Normal cursor movement is to the right, and then wrapping off the edge of the screen or window and then downwards. VDU 23,16 changes these directions, and all other commands that move or set the position of the text cursor.
The new direction is specified with (old status AND y) EOR x, and the new status is interpreted as follows:
- b7=0 - normal value
- b7=1 - undefined
- b6=0 - In VDU 5 (text at graphics cursor) mode, movement beyond the edge of the window causes an implicit carriage return/linefeed to the next positive Y line.
- b6=1 - In VDU 5 (text at graphics cursor) mode, movement beyond the edge of the window causes no action, and the text 'continues' invisibly off the edge of the window.
- b5=0 - Cursor moves in positive X direction after a character is printed.
- b5=1 - Cursor does not move after a character is printed.
- b4=0 - Cursor movement in the positive Y direction past the bottom of the text window causes the text window to scroll in VDU 4 (text) mode, or moved to the opposite edge of the window in VDU 5 (text at graphics cursor) mode.
- b4=1 - Cursor movement in the positive Y direction past the bottom of the text window always causes the text cursor to be moved to the opposite edge of the window.
b3,b2,b1 define the cursor directon as follows:
- 0 0 0 - X direction is right, Y direction is down
- 0 0 1 - X direction is left, Y direction is down
- 0 1 0 - X direction is right, Y direction is up
- 0 1 1 - X direction is left, Y direction is up
- 1 0 0 - X direction is down, Y direction is right
- 1 0 1 - X direction is down, Y direction is left
- 1 1 0 - X direction is up, Y direction is right
- 1 1 1 - X direction is up, Y direction is left
- b0=0 - disables scroll protection, the text cursor moving off the positive edge of the window will always move directly to the next line, scrolling the window if neccessary.
- b0=1 - enables scroll protection, the text cursor moving off the positive edge of the window generates a pending newline, and waits at the edge of the window until the movement is needed. While the cursor is waiting, the cursor position returned by POS via OSBYTE &87 is the maximum column position plus one, ie the window width (eg 80 in MODE 0). If the cursor is moved elsewhere, the pending newline is abandonded, and no scrolling occurs.
VDU 23,17,0-3 - Set tint
VDU 23,17,action,tint,0,0,0,0,0,0 (Arthur 1.00 onwards, not BBFW)
Sets the additional tint for the specified colour. COLOUR and GCOL can only specify 6-bit logical colours, so in 256-colour screen modes, VDU 23,17 is used to set the lowest two bits of the colour.
- action=0 - sets tint for text foreground colour
- action=1 - sets tint for text background colour
- action=2 - sets tint for graphics foreground colour
- action=3 - sets tint for graphics background colour
The tint is passed in the top two bits of tint (allowing a further six bits of colour depth), so the normal range is &00, &40, &80, &80 darkest to lightest.
The BASIC keyword TINT sends a VDU 23,17 sequence.
VDU 23,17,4 - Select colour patterns
VDU 23,17,4,pattern,0,0,0,0,0,0 (Arthur 1.00 onwards, not BBFW)
Selects which colour patterns future VDU 23,2-5 calls use.
- pattern=0 - use BBC/Master compatible colour patterns
- pattern=1 - use native RISC OS colour patterns
VDU 23,17,5 - Swap text colours
VDU 23,17,5,0,0,0,0,0,0,0 (Arthur 1.00 onwards, not BBFW)
After this command, characters are effectively printed in inverse video. After repeating the command, characters are effectively printed back in normal video.
VDU 23,17,6 - Set ECF origin
VDU 23,17,6,x;y;0,0,0 (Arthur 1.00 onwards, not BBFW)
Set Extended Colour Fill origin. By default, EFC patterns fill aligned with the bottom lefthand corner of the screen. This command changes this so that the bottom lethand pixel of the fill pattern is aligned with the specified point.
VDU 23,17,7 - Set character size and spacing
VDU 23,17,7,flags,xsize;ysize;0,0 (Arthur 1.00 onwards, not BBFW)
This allows the character size and spacing of text printed in VDU 5 mode (text at graphics cursor) to be changed. If bit 1 of flags is set, the text size is changed, if bit 2 is set, the spacing is changed. xsize and ysize are specified in pixels.
VDU 23,18 - Set Teletext display characteristics
VDU 23,18,0 - Set Teletext transparency mode
VDU 23,18,0,mode,0,0,0,0,0,0 (RISC OS 5 onwards)
- mode = 0: "Text" mode: the whole display is set opaque (default)
- mode = 1: "Mix" mode: foreground colours, and both foreground and background of boxed text are opaque; non-boxed background colours are all transparent
- mode = 2: "Box" mode: boxed regions are opaque, others are transparent
- mode = 3: "TV" mode: the whole display is set transparent
VDU 23,18,1 - Set Teletext display update
VDU 23,18,1,flags,0,0,0,0,0,0 (RISC OS 5 onwards)
This sets how often the teletext screen is updated. Two bits are currently defined:
- bit 0 = suspend all automatic display upate
- bit 1 = update display after every character
VDU 23,18,2 - Set Teletext reveal state
VDU 23,18,2,flags,0,0,0,0,0,0 (RISC OS 5 onwards)
- bit 0 = if set, any concealed text will be displayed. If cleared,
concealed text is concealed.
VDU 23,18,3 - Set Teletext black enable
VDU 23,18,3,enable,0,0,0,0,0,0 (RISC OS 5 onwards)
Enable/disable black foreground colour control codes
- enable = 0: control codes &80 and &90 do nothing (default)
- enable = 1: control code &80 selects black text, control code &90 selects black graphics
VDU 23,18,16 - Set Teletext display characteristics
VDU 23,18,16,flags,0,0,0,0,0,0 (RISC OS 5 onwards)
- bit 0 = render high quality text
VDU 23,19-21 - Unused
Reserved for future expansion, passed to VDUV.
VDU 23,22 - Select user defined screen mode
VDU 23,22,xpixels;ypixels;xchars,ychars,colours,flags (BBFW)
This command selects a specified user-defined mode. Flags specified the following:
- b0-b3=0 - ANSI character set
- b0-b3=1 - OEM (IBM PC) character set
- b0-b3=8 - UTF-8 (Unicode) character set
- b7=0 - white text on black background
- b7=1 - black text on white background
VDU 23,23,0 - Set linedraw thickness
VDU 23,23,0,t,0,0,0,0,0,0 (BBFW 5.80)
This command sets the width in pixels of lines draw with PLOT commands.
VDU 23,24 - Unused
Reserved for future expansion, passed to VDUV.
VDU 23,25,0-4 - Set anti-aliasing transfer function
VDU 23,25,bitsperpixel,threshold1,...,theshold7 (Communicator, Arthur 1.00 onwards, not BBFW)
bitsperpixel specify the number of bits per pixel to use when anit-alising fonts. threshold1 to threshold7 are used to decide which output bits are produced from the original values (0 to 15). There are either one, three or seven of them depending on the number of bits per pixel. For example:
translates the values 0-15 to 0,0,1,1,2,2,3,3,4,4,5,5,6,6,7,7.
VDU 23,25,&80+ - Set anti-aliasing pallette
VDU 23,25,&80+logical background colour,logical foreground colour,start R,start G,start B,end R,end G,end B (Communicator, Arthur 1.00 onwards, not BBFW)
This command sets the physical colour range to use when anti-aliasing between the two specified logical colours.
VDU 23,25,&FF - Set anti-aliasing pallette in 256-colour modes
VDU 23,25,&FF,logicalcolour,start R,start G,start B,end R,end G,end B (Communicator, Arthur 1.00 onwards, not BBFW)
This command sets the physical colour range to use when anti-aliasing between the background and the specified logical foreground colour in 256-colour screen modes.
VDU 23,26 - Select font
VDU 23,26,fontnumber,size,xppi,yppi,xscale,yscale,0,0,fontname,13 (Communicator, Arthur 1.00 onwards, not BBFW)
This selects the name font at the specified size for future use when selecting font fontnumber. The font name is sent as a CR-terminated sequence immediately after the VDU 23 sequence.
- fontnumber - the font handle that will be used to select the specified font
- size - font size in points
- xppi - if nonzero, specifies the number of horizontal pixels per inch
- yppi - if nonzero, specifies the number of vertical pixels per inch
- xscale - if nonzero, multiplier for font size width
- yscale - if nonzero, multiplier for font size height
xscale and yscale allow non-integer font sizes to be selected, and allow the horizontal and vertical font sizes to be different. For example, using VDU 23,26,fontnumber,1,xppi,yppi,10,12,0,0,fontname,13 will set the horizontal point size to 1*10 and the vertical font size to 1*12.
VDU 23,27 - Sprite operation
VDU 23,27,m,n,0,0,0,0,0,0 (GXR, SpriteROM, Arthur 1.00 onwards, not BBFW)
This command performs various sprite operations, depending on the value of m.
If m=0, then sprite n is selected to be plotted by subsequent PLOT &EC+k commands.
If m=1, then the rectangular area between the last two graphics points is uses to define sprite n.
VDU 23,28-31 - User calls
VDU 23,28-31,a,b,c,d,e,f,g,h (OS 0.10 onwards)
Reserved for user calls. These are defined to vector through VDUV, and will never be used for inbuilt functions.