A string function which returns 'num' characters of the string starting from character 'start_posn'. If 'num' is not present or if there are insufficient characters in the string, then all the characters from 'start_posn' onwards are returned.

You can use this function to select any part of a string. For instance, if

then

would assign BASIC to 'part$'. If the last number is omitted or there are insufficient characters to the right of the specified position, MID$ returns with the right hand part of the string starting at the specified position. Thus,

would assign (Z80) to 'part$'.

For example,

would print

A binary operation giving the signed remainder of the integer division.

MOD is defined such that,

If you are doing integer division (DIV) of whole numbers it is often desirable to know the remainder. (A 'teach children to divide' program for instance.) For example, 23 divided by 3 is 7, remainder 2. Thus,

would print

You can use real numbers in these calculations, but they are truncated to their integer part before BBC BASIC (Z80) calculates the result. Thus,

would give exactly the same results as the previous example.

The MODE command sets the screen display mode. The screen is cleared and all the graphics and text parameters (colours, origin, etc) are reset to their default values.

The modes differ from those on the BBC series of microcomputers.

From MOS 1.03, they are as follows:

Modes with less than 64 colours per pixel are paletted; each logical colour can be assigned a different physical colour. See the COLOUR statement below for more information.

A statement which moves the graphics cursor to an absolute position without drawing a line.

Not implemented in the generic CP/M version of BBC BASIC (Z80)

A command which initialises the interpreter for a new program to be typed in. The old program may be recovered with the OLD command provided no new program lines have been typed in or deleted and no variables have been created.

This command effectively 'removes' a program from the computer’s memory. In reality, the program is still there, but BBC BASIC (Z80) has been told to forget about it.

If you have made a mistake, you can recover your old program by typing OLD. However, this won’t work if you have begun to enter a new program.

The statement delimiting FOR…​NEXT loops. NEXT takes an optional control variable.

If the control variable is present then FOR----NEXT loops may be 'popped' automatically in an attempt to match the correct FOR statement (this should not be necessary). If a matching FOR statement cannot be found, a 'Can’t match FOR' error will be reported.

Leaving out the control variable will make the program run quicker, but this is not to be encouraged.

See the keyword FOR for more details about the structure of FOR----NEXT loops.

This is a high priority unary operator (the same priority as unary -). It causes a bit-by-bit binary inversion of the numeric to its right. The numeric may be a constant, a variable, or a mathematical or boolean expression. Expressions must be enclosed in brackets.

NOT is most commonly used in an IF----THEN----ELSE statement to reverse the effect of the test.

BBC BASIC (Z80) does not have true boolean variables; it makes do with numeric variables. This can lead to confusion because the testable condition in an IF----THEN----ELSE statement is evaluated mathematically and can result in something other than -1 (TRUE) or 0 (FALSE).

When the test in an IF----THEN----ELSE is evaluated, FALSE=0 and anything else is considered to be TRUE. If you wish to use NOT to reverse the action of an IF statement it is important to ensure that the testable condition does actually evaluate to -1 for TRUE.

If the testable condition evaluates to 1, for example, the result of the test would be considered to be TRUE and the THEN part of the IF----THEN----ELSE statement would be carried out. However, using NOT in front of the testable condition would not reverse the action. NOT 1 evaluates to -2, which would also be considered to be TRUE.

A command which undoes the effect of NEW provided no lines have been typed in or deleted, and no variables have been created.

OLD works even if BBC BASIC (Z80) has been re-loaded and re-started from CP/M-80. However, it will only work if no other programs have been run and BBC BASIC (Z80) loads at the same address as before.

A statement controlling a multi-way switch. The line numbers in the list may be constants or calculated and the 'unwanted' ones are skipped without calculation. The ON statement is used in conjunction with four other key-words: GOTO, GOSUB, PROC and ERROR. (ON ERROR is explained separately.)

The ON statement alters the path through your program by transferring control to one of a selection of line numbers depending on the value of a variable. For example,

would send your program to line 1000 if 'number' was 1, to line 2000 if 'number' was 2, to line 500 if 'number' was 3 and to line 100 if 'number' was 4.

Exceptions may be trapped using the ELSE statement delimiter.

If there is no statement after the ELSE, the program will 'drop through' to the following line if an exception occurs. In the two following examples, the program would drop through to the error handling part of the program if 'choice' or 'B-46' was less than one or more than 3.

You can use ON…​GOTO, ON…​GOSUB, and ON…​PROC to execute the appropriate part of your program as the result of a menu selection. The following skeleton example offers a menu with three choices.

A statement controlling error trapping. If an ON ERROR statement has been encountered, BBC BASIC (Z80) will transfer control to it (without taking any reporting action) when an error is detected. This allows error reporting/recovery to be controlled by the program. However, the program control stack is still cleared when the error is detected and it is not possible to RETURN to the point where the error occurred.

ON ERROR OFF returns the control of error handling to BBC BASIC (Z80).

For example, the ON ERROR statement can be used to trap out the escape key to prevent a program being terminated at the wrong time by its accidental use.

Error handling is explained more fully in the General Information section.

A function which opens a file for reading and returns the file handle of the file. This number must be used in subsequent references to the file with BGET#, INPUT#, EXT#, PTR#, EOF# or CLOSE#.

A returned value of zero signifies that the specified file was not found on the disk.

The example below reads data from disk into an array. If the data file does not exist, an error message is printed and the program ends.

A function which opens a file for writing and returns the file handle of the file. This number must be used in subsequent references to the file with BPUT#, PRINT#, EXT#, PTR# or CLOSE#. If the specified file does not exist it is created. If the specified file already exists it is truncated to zero length.

A returned value of zero indicates that the specified file could not be created.

You can also read from a file which has been opened using OPENOUT. This is of little use until you have written some data to it. However, once you have done so, you can move around the file using PTR# and read back previously written data.

Data is not written to the file at the time it is opened. Consequently, it is possible to successfully open a file on a full disk. Under these circumstances, a 'Disk full' error would be reported when you tried to write data to the file for the first time.

The example below writes the contents of two arrays (tables) to a file called 'TOPTEN.BBC'.

A function which opens a disk data file for update (reading and writing) and returns the file handle of the file. This number must be used in subsequent references to the file with BGET#, BPUT#, INPUT#, PRINT#, EXT#, PTR#, EOF# or CLOSE#.

A returned value of zero signifies that the specified file was not found on the disk.

See the random file examples (F-RAND?) in the BBC BASIC Disk Files section for examples of the use of OPENUP.

An assembler pseudo operation controlling the method of assembly. (See the Assembler section for more details.) OPT is followed by an expression with the following meanings:

The operation of bitwise integer logical OR between two items. The two operands are internally converted to 4 byte integers before the OR operation.

You can leave out the space between OR and a preceding constant, but it makes your programs difficult to read.

You can use OR as a logical operator or as a 'bit-by-bit' (bitwise) operator. The operands can be boolean (logical) or numeric.

Unfortunately, BBC BASIC does not have true boolean variables; it uses numeric variables and assigns the value 0 for FALSE and -1 for TRUE. This can lead to confusion at times. (See NOT for more details.)

In the example below, the operands are boolean (logical). In other words, the result of the tests (IF) A=2 and (IF) B=3 is either TRUE or FALSE. The result of this example will be TRUE if A=2 or B=3.

The brackets are not necessary, they have been included to make the example easier to follow.

The last example, uses the OR in a similar fashion to the numeric operators (+, -, etc).

Suppose X was -20 in the following example,

the OR operation would be:

This command allows a string expression to be passed to the operating system. It overcomes the problems caused by the exclusion of variables in the star (*) commands. Using this statement, you can, for instance, erase and rename files whose names you only know at run-time.

See the Operating System Interface section for more details.

A pseudo-variable controlling the starting address of the current user program area. It addresses the area where a program is (or will be) stored.

PAGE is automatically initialised by BBC BASIC (Z80) to the address of the lowest available page in RAM, but you may change it.

If you make PAGE less than its original value or greater than the original value of HIMEM, you will get a 'Bad program' error when you try to enter a program line and you may well crash BBC BASIC (Z80).

If you make PAGE greater than HIMEM, a 'No room' error will occur if the program exits to command level.

With care, several programs can be left around in RAM without the need for saving them.

USE WITH CARE.

A function returning 3.14159265.

You can use PI to calculate the circumference and area of a circle. The example below calculates the circumference and area of a circle of a given radius.

PI can also be used to convert degrees to radians and radians to degrees.

However, BBC BASIC (Z80) has two functions (RAD and DEG) which perform these conversions to a higher accuracy.

PLOT is a multi-purpose drawing statement. Three numbers follow the PLOT statement: the first specifies the mode, that is the type of point, line, triangle or circle to be drawn; the second and third give the X and Y coordinates to be used.

Plot supports the following operations:

BBC BASIC by default uses logical coordinates that run from 0 to 1280 left-to-right and from 0 to 1024 bottom to top. This logical screen is the same size in all screen modes with the same aspect ratio irrespective of the number of physical pixels. This means that you can run the same code under different modes and anything you draw will still appear in the same position on screen despite the physical resolution being different.

The point 640,512 will always appear at the centre of the screen.

A function which returns a number giving the physical colour of the screen at the coordinates specified. If the point is outside the graphics window, then -1 is returned.

Note, this differs from the BBC Basic standard which returns the logical colour not the physical colour.

A function returning the horizontal position of the cursor on the screen. The left hand column is 0 and the right hand column is one less than the width of the display.

COUNT will tell you the print head position of the printer. It is an uncertain indicator of the horizontal position of the cursor on the screen. (See the keyword COUNT for details.)

See VPOS for an example of the use of POS and VPOS.

A statement which prints characters on the VDU screen or printer (see *OPT). The printer may be turned on and off manually by typing ^P. All the characters displayed on the VDU screen after the first ^P will be echoed to the printer. A second ^P will turn the printer off.