The statement VDU X is equivalent to PRINT CHR$(X); and the statement VDU X,Y,Z is equivalent to PRINT CHR$(X);CHR$(Y);CHR$(Z);

However the VDU statement finds most common use when generating ASCII control codes and a detailed description of the effect of each control code is given in this chapter. The control codes are interpreted by part of the machine operating system called the VDU driver.

The VDU drivers interpret all 32 ASCII control character codes. Many of the ASCII control codes are followed by a number of bytes. The number of bytes which follow depends on the function to be performed. The VDU code table summarises all the codes and gives the number of bytes which follow the ASCII control code.

This code causes text to be written at the text cursor, i.e. in the normal fashion. A MODE change selects VDU 4, normal operation.

This code causes text to be written where the graphics cursor is. The position of the text cursor is unaffected. Normally the text cursor is controlled with statements such as

and the graphics cursor is controlled with statements like

Once the statement VDU5 has been given only one cursor is active (the graphics cursor). This enables text characters to be placed at any position on the screen. There are a number of other effects: text characters overwrite what is already on the screen so that characters can be superimposed; text and graphics can only be written in the graphics window and the colours used for both text and graphics are the graphics colours. In addition the page no longer scrolls up when at the bottom of the page. Note however that POS and VPOS still give you the position of the text cursor.

VDU6 is a complementary code to VDU21. VDU21 stops any further characters being printed on the screen and VDU6 re-enables screen output. A typical use for this facility would be to prevent a password appearing on the screen as it is being typed in.

This code, which can be entered in a program as VDU7 or directly from the keyboard as CTRL G, causes the computer to make a short 'beep'.

This code (VDU8 or CTRL H) moves the text cursor one space to the left. If the cursor was at the start of a line then it will be moved to the end of the previous line. It does not delete characters - unlike VDU127.

This code (VDU9 or CTRL I) moves the cursor forward one character position.

The statement (VDU10 or CTRL J) will move the cursor down one line. If the cursor is already on the bottom line then the whole display will normally be moved up one line.

This code (VDU11 or CTRL K) moves the text cursor up one line. If the cursor is at the top of the screen then the whole display will move down a line.

This code clears the screen - or at least the text area of the screen. The screen is cleared to the text background colour which is normally black. The BASIC statement CLS has exactly the same effect as VDU12 or CTRL L. This code also moves the text cursor to the top of the text window.

This code is produced by the RETURN key. However its effect on the screen display if issued as a VDU13 or PRINT CHR$(13); is to move the text cursor to the left hand edge of the current text line (but within the current text window, of course).

This code makes the screen display wait at the bottom of each page. It is mainly used when listing long programs to prevent the listing going past so fast that it is impossible to read. The computer will wait until a SHIFT key is pressed before continuing. This mode is called 'paged mode'. Paged mode is turned on with the CTRL N and off with CTRL O.

This code causes the computer to leave paged mode. See the previous entry (14) for more details.

This code (VDU16 or CTRL P) clears the graphics area of the screen to the graphics background colour and the BASIC statement CLG has exactly the same effect. The graphics background colour starts off as black but may have been changed with the GCOL statement. VDU16 does not move the graphics cursor - it just clears the graphics area of the screen.

VDU17 is used to change the text foreground and background colours. In BASIC, the statement COLOUR is used for an identical purpose. VDU17 is followed by one number which determines the new colour. See the BASIC keyword COLOUR for more details.

This code allows the definition of the graphics foreground and background colours. It also specified how the colour is to be placed on the screen. The colour can be plotted directly, ANDed, ORed or Exclusive-ORed with the colour already there, or the colour there can be inverted. In BASIC this is called GCOL.

The first byte specifies the mode of action as follows:

The second byte defines the logical colour to be used in future. If the byte is greater than 127 then it defines the graphics background colour (modulo the number of colours available). If the byte is less than 128 then it defines the graphics foreground colour (modulo the number of colours available).

This code is used to select the actual colour that is to be displayed for each logical colour. The statements COLOUR (and GCOL) are used to select the logical colour that is to be used for text (and graphics) in the immediate future. However the actual colour can be re-defined with VDU 19. For example:

will print all text in colour 1 which is red by default. However the addition of

will set logical colour 1 to actual colour 4 (blue). The three zeros after the actual colour in the VDU19 statement are for future expansion.

In MODE 5 there are four colours (0, 1, 2 and 3). An attempt to set colour 4 will in fact set colour 0 so that statement

is equivalent to

We say that logical colours are reduced modulo the number of colours available in any particular mode.

This code VDU20 or CTRL T sets default text and graphic foreground logical colours and also programs default logical to actual colour relationships. The default values are:

This code behaves in two different ways. If entered at the keyboard (as CTRL U) it can be used to delete the whole of the current line. It is used instead of pressing the DELETE key many times. If the code is generated from within a program by either VDU 21 or PRINT CHR$(21); it has the effect of stopping all further graphics or text output to the screen. The VDU is said to be disabled. It can be 'enabled' with VDU6.

This VDU code is used to change MODE. It is followed by one number which is the new mode. Thus VDU 22,6 is exactly equivalent to MODE6 (except that it does not change HIMEM).

This code is used to re-program displayed characters. The ASCII code assigns code numbers of each displayed letter and number. The normal range of displayed characters includes all upper and lower case letters, numbers and punctuation marks as well as some special symbols. These characters occupt ASCII codes 32 to 126. If the user wishes to define his or her own characters or shapes then ASCII codes 224 to 255 are left available for this purpose. In fact you can re-define any character that is displayed, but extra memory must be set aside if this is done, and this is explained in Appendix D.

ASCII codes 0 to 31 are interpreted as VDU control codes - and this chapter is explaining the exact function of each. Thus the full ASCII set consists of all the VDU control codes, all the normal printable characters and a user defined set of characters.

For example if the user wishes to define ASCII code 240 to be a small triangle then the following statement would have to be executed.

As explained above the user may define any ASCII code in the range 224 to 255. To display the resultant shape on the screen the user can type:

PRINT CHR$(240)   or
VDU240

In the unlikely event of the user wishing to define more than the 32 characters mentioned above (ASCII 244 to 25), it will be necessary to allocate more RAM for the purpose.

This code enables the user to define the graphics window - that is, the area of the screen inside which graphics can be drawn with the DRAW and PLOT statements. The graphics screen is addressed with the following coordinates:

IMAGE MISSING

Thus the coordinates of A would be approximately 1000,200.

When defining a graphics window four coordinates must be given; the left, bottom, right and top edges of the graphics area. Suppose that we wish to confine all graphics to the area shown below:

IMAGE MISSING

The left hand edge of the graphics area has an X value of (about) 150. The bottom of the area has a Y value of 300. The right hand side has X = 1100 and the top has Y = 700. The full statement to set this area is:

Notice that the edges must be given in the order left X, bottom Y, right X, top Y and that when defining graphics windows the numbers must be followed by a semi-colon.

For those who wish to know why trailing semi-colons are used the reason is as follows: X and Y graphic coordinates have to be sent to the VDU software as two bytes since the values may well be greater than 255. The semi-colon punctuation in the VDU statement sends the number as a two byte pair with low byte first followed by the high byte.

This VDU code is identical to the BASIC PLOT statement. Only those writing machine code graphics will need to use it. VDU25 is followed by five bytes. The first gives the value of A referred to in the explanation of PLOT in the BASIC keywords chapter. The next two bytes give the X coordinate and the last two bytes give the Y coordinate. Refer to the entry for VDU24 for an explanation of the semi-colon syntax used. Thus

would move to absolute position 100,500.

The above is completely equivalent to:

The code VDU26 (CTRL X) returns both the graphics and text windows to their initial values where they occupy the whole screen. This code re-positions the text cursor at the top left of the screen, the graphics cursor at the bottom left and sets the graphics origin to the bottom left of the screen. In this state it is possible to write text and to draw graphics anywhere on the screen.

This code (VDU28) is used to set a text window. Initially it is possible to write text anywhere on the screen but establishing a text window enables the user to restrict all future text to a specific area of the screen. The format of the statement is:

VDU28,leftX,bottomY,rightX,topY

IMAGE MISSING

For the example shown the statement would be

Note that the units are character positions and the maximum values will depend on the mode in use. The example above refers to MODE1 and MODE4. In MODEs 2 and 5 the maximum values would be 19 for X and 31 for Y since these modes have only 20 characters per line.

IMAGE MISSING

This code is used to move the graphics origin. The statement VDU29 is followed by two numbers giving X and Y coordinates of the new origin. The graphics screen is addressed

IMAGE MISSING

Thus to move the origin to the centre of the screen the statement

should be executed. Note that the X and Y values should be followed by semi-colons. See the entry for VDU24 if you require an explanation of the trailing semi-colons. Note also that the graphics cursor is not affected by VDU29.

This code (VDU30 or CTRL ^) moves the text cursor to the top left of the text area.

The code VDU31 enables the text cursor to be moved to any character position on the screen. The statement VDU31 is followed by two numbers which give the X and Y coordinates of the desired position.

Thus to move the text cursor to the centre of the screen in MODE7 one would execute the statement

Notice that the maximum values of X and Y depend on the mode selected and that both X and Y are measured from the edges of the current text window not the edges of the screen.

These codes generate the full set of letters and numbers in the ASCII set.

This code moves the text cursor back one character and deletes the character at that position. VDU127 has exactly the same effect as the DELETE key.

These characters are normally undefined and will produce random shapes.

These characters may be defined by the user using the statement VDU23. It is thus possible to have 32 user defined shapes such as

Try typing each of the lines above, remembering to press the RETURN key after each definition. To display any of the new definitions, type in the appropriate VDU code. For example, to display the heart, type

Characters definitions 224 to 255 are stored in a block of memory reserved for them in the computer. If however, you need more characters, or your want to re-define some of the keyboard characters, the best way to do this is to tell the computer to set aside extra memory to store them. (If you don't, you may run into problems). The Operating System call *FX20 described in Appendix D enables you to do this.