BBC BASIC for Windows

BBC BASIC for SDL 2.0

Keywords

Introduction to keywords

The syntax and any associated keywords are listed at the end of each section.

Syntax

Symbols

ABS

X = ABS(deficit)
length = ABS(X1-X2)

It is particularly useful when you want to know the difference between two values, but you don't know which is the larger. For instance, if X=6 and Y=10 then the following examples would give the same result.

difference = ABS(X-Y)
difference = ABS(Y-X)

IF answer>ideal-0.5 AND answer<ideal+0.5 THEN....

IF ABS(answer-ideal)<0.5 THEN....

Syntax

<n-var>=ABS(<numeric>)

Associated Keywords

SGN

ACS

If you know the cosine of the angle, this function will tell you the angle (in radians). Unfortunately, you cannot do this with complete certainty because two angles within the range +/− PI (+/− 180 degrees) can have the same cosine. This means that one cosine has two associated angles.

The following diagram illustrates the problem:

By convention, ACS gives a result in the top two quadrants (0 to PI; 0 to 180 degrees) and ASN and ATN in the right-hand two quadrants (−PI/2 to +PI/2; −90 to + 90 degrees).

In the example below, 'radian_angle' becomes equal to the angle (in radians) whose cosine is 'y'.

radian_angle=ACS(y)

degree_angle=DEG(ACS(y))

Syntax

<n-var>=ACS(<numeric>)

Associated Keywords

ASN, ATN, SIN, COS, TAN, RAD, DEG

ADVAL

ADVAL positive argument

ADVAL(1) returns the joystick X-position.
ADVAL(2) returns the joystick Y-position.
ADVAL(3) returns the joystick Z-position.

joyx=ADVAL(1)
level=ADVAL(chnumber)

ADVAL zero argument

ADVAL negative argument

ADVAL(−5) returns the number of free bytes in the channel 0 SOUND queue. If the sound queue is empty, ADVAL(−5) will return 16. If the queue is full, ADVAL(−5) will return 0. Each sound event uses 4 bytes.

ADVAL(−6) returns the number of free bytes in the channel 1 SOUND queue. If the sound queue is empty, ADVAL(−6) will return 16. If the queue is full, ADVAL(−6) will return 0. Each sound event uses 4 bytes.

ADVAL(−7) returns the number of free bytes in the channel 2 SOUND queue. If the sound queue is empty, ADVAL(−7) will return 16. If the queue is full, ADVAL(−7) will return 0. Each sound event uses 4 bytes.

ADVAL(−8) returns the number of free bytes in the channel 3 SOUND queue. If the sound queue is empty, ADVAL(−8) will return 16. If the queue is full, ADVAL(−8) will return 0. Each sound event uses 4 bytes.

Syntax

<n-var>=ADVAL(<numeric>)

Associated Keywords

SOUND, INPUT, GET

Input A	Input B	Output
0	0	0
0	1	0
1	0	0
1	1	1

answer=num1 AND num2
char=byte AND &7F

IF (num AND &F0)

test=(count=3 AND total=5)

byte=BGET#infile AND &7F
BPUT#outfile,byte

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 and B=3.

answer=(A=2 AND B=3)

The second example is similar to the first, but in the more familiar surroundings of an IF statement.

IF A=2 AND B=3 THEN 110

answer= A=2 AND B=3 (without brackets this time)
IF answer THEN 110

A=X AND 11

11111111 11111111 11111111 11101100
00000000 00000000 00000000 00001011
00000000 00000000 00000000 00001000  = 8

Syntax

<n-var>=<numeric> AND <numeric>
<n-var>AND=<numeric>

Associated Keywords

EOR, OR, FALSE, TRUE, NOT

ASC

A computer only understands numbers. In order to deal with characters, each character is assigned a code number. For example (in the ASCII code table) the character 'A' is given the code number 65 (decimal).

Different types of computer use different numbers for the characters. The codes used for PC Compatible computers are those defined by the American Standard Code for Information Interchange (ASCII). See the Table of ASCII codes section.

You could use this function to convert ASCII codes to some other coding scheme.

ascii_code=ASC("H")	Result would be 72
X=ASC("HELLO")	Result would be 72
name$="FRED"
ascii_code=ASC(name$)	Result would be 70
X=ASC"e"	Result would be 101
X=ASC(MID$(A$,position))	Result depends on A$ and position.

Syntax

<n-var>=ASC(<string>)

Associated Keywords

CHR$, STR$, VAL

ASN

By convention, the result will be in the range −PI/2 to +PI/2 (−90 to +90 degrees).

If you know the sine of the angle, this function will tell you the angle (in radians). Unfortunately, you cannot do this with complete certainty because one sine has two associated angles (see ACS for details).

In the example below, 'radian_angle' becomes equal to the angle (in radians) whose sine is 'y'.

radian_angle=ASN(y)

degree_angle=DEG(ASN(y))

Syntax

<n-var>=ASN(<numeric>)

Associated Keywords

ACS, ATN, SIN, COS, TAN, RAD, DEG

ATN

By convention, the result will be in the range −PI/2 to +PI/2 (−90 to +90 degrees).

If you know the tangent of the angle, this function will tell you the angle (in radians).

As the magnitude of the argument (tangent) becomes very large (approaches + or − infinity) the accuracy diminishes.

In the example below, 'radian_angle' becomes equal to the angle (in radians) whose tangent is 'y'.

radian_angle=ATN(y)

degree_angle=DEG(ATN(y))

Syntax

<n-var>=ATN(<numeric>)

Associated Keywords

ACS, ASN, SIN, COS, TAN, RAD, DEG

E=BGET#n
aux=BGET#3

You can use BGET# to read single bytes from a disk file. This enables you to read back small integers which have been 'packed' into fewer than 5 bytes (see BPUT#). It is also very useful if you need to perform some conversion operation on a file. Each byte read is numeric, but you can use CHR$(BGET#n) to convert it to a string.

The input file in the example below is a text file produced by a word-processor.

Words to be underlined are 'bracketed' with ^S. The program produces an output file suitable for a printer which expects such words to be bracketed by ^Y. You could, of course, perform several such translations in one program.

REM Open i/p and o/p files. End if error.
infile=OPENIN "WSFILE.DOC"
IF infile=0 THEN END
outfile=OPENOUT "BROTH.DOC"
IF outfile=0 THEN END
:
REM Process file, converting ^S to ^Y
REPEAT
  temp=BGET#infile :REM Read byte
  IF temp=&13 THEN temp=&19 :REM Convert ^S
  BPUT#outfile,temp :REM Write byte
UNTIL temp=&1A OR EOF#infile :REM ^Z
CLOSE#0 :REM Close all files
END

INPUT "Enter name of INPUT file " infile$
INPUT "Enter name of OUTPUT file " outfile$
REM Open i/p and o/p files. End if error.
infile=OPENIN(infile$)
IF infile=0 THEN END
outfile=OPENOUT(outfile$)
IF outfile=0 THEN END
:
REM Process file, converting ^S to ^Y
REPEAT
  temp=BGET#infile :REM Read byte
  IF temp=&13 THEN temp=&19 :REM Convert ^S
  BPUT#outfile,temp :REM Write byte
UNTIL temp=&1A OR EOF#infile :REM ^Z
CLOSE#0 :REM Close all files
END

Syntax

<n-var>=BGET#<numeric>

Associated Keywords

OPENIN, OPENUP, OPENOUT, CLOSE#, PRINT#, INPUT#, BGET#, EXT#, PTR#, EOF#, GET$#

BPUT#E,32
BPUT#staff_file,A/256
BPUT#chn,A$
BPUT#N,"a string";

You can use this statement to write single bytes to a disk file. The number that is sent to the file is in the range 0 to 255. Real numbers are converted internally to integers and the top three bytes are 'masked off'. Each byte written is numeric, but you can use ASC(character$) to convert (the first character of) 'character$' to a number.

Alternatively BPUT# can be used to write a character string to a file. The difference between PRINT#file,string$ and BPUT#file,string$ is that PRINT# appends a carriage-return character (CHR$13) whereas BPUT# appends a line-feed character (CHR$10). Adding a terminating semicolon (;) causes the contents of the string to be written to the file with nothing appended.

The example below is a program segment that 'packs' an integer number between 0 and 65535 (&FFFF) into two bytes, least significant byte first. The file must have already been opened for output and the channel number stored in 'fnum'. The integer variable number% contains the value to be written to the file.

BPUT#fnum,number% MOD 256
BPUT#fnum,number% DIV 256

Syntax

BPUT#<numeric>,<numeric>
BPUT#<numeric>,<string>[;]

Associated Keywords

OPENIN, OPENUP, OPENOUT, CLOSE#, PRINT#, INPUT#, BGET#, EXT#, PTR#, EOF#, GET$#

BY

BY is also used with the GET$#channel function to specify the number of bytes to be read from the file.

Syntax

DRAW BY <numeric>,<numeric>
FILL BY <numeric>,<numeric>
MOVE BY <numeric>,<numeric>
PLOT BY <numeric>,<numeric>

Associated Keywords

DRAW, FILL, GET$, MOVE, PLOT

CALL Muldiv,A,B,C,D
CALL &FFE3
CALL 12340,A$,M,J$

When BBC BASIC is running on a 32-bit x86 CPU the processor's EAX, EBX, ECX and EDX registers are initialised to the contents of A%, B%, C% and D% respectively (see also USR). The processor's flags register is initialised to the contents of F%; however, you cannot disable interrupts nor enter single-step mode by setting F% because this could affect the operation of the Operating System.

Your machine-code routine should return to BBC BASIC with a return instruction (RET).

Operating system interface

Parameter table

Variables included in the parameter list need not have been declared before the CALL statement.

The parameter types are:

Code	Parameter type	Example
0:	byte (unsigned 8 bits)	?A%
1:	byte (unsigned 8 bits)	A&
4:	32-bit signed integer	!A% or A%
40:	64-bit signed integer	A%%
5:	40-bit variant	A1
8:	64-bit variant	A2 or A#
10:	80-bit variant	A3
24:	structure	A{}
36:	function or procedure	FNa or PROCa
65:	byte array	A&()
68:	32-bit integer array	A%()
104:	64-bit integer array	A%%()
69:	40-bit variant array	A()1
72:	64-bit variant array	A()2 or A#()
74:	80-bit variant array	A()3
88:	structure array	A{()}
100:	function or procedure	FNa() or PROCa()
128:	fixed string (CR terminated)	$A%
129:	movable string, 6-byte descriptor	A$1,2
136:	movable string, 8-byte descriptor	A$3
130:	fixed string (NUL terminated)	$$A%
193:	string array, 6-byte descriptors	A$()1,2
200:	string array, 8-byte descriptors	A$()3

1.  BBC BASIC for Windows version 5.95a or earlier in *FLOAT 40 mode.
2.  BBC BASIC for Windows version 5.95a or earlier in *FLOAT 64 mode.
3.  BBC BASIC for Windows version 6.00a or later, or BBC BASIC for SDL 2.0.

On entry to the subroutine the parameter table contains the following values:

Number of parameters	1 byte (at EBP)
Parameter type	1 byte (at EBP+1)
Parameter address	4 bytes (at EBP+2 to EBP+5, LSB first)
Parameter type	) repeated as often
Parameter address	) as necessary.

Parameter formats

Byte variables are stored as a single unsigned byte.

Fixed strings are stored as the characters of the string followed by a carriage return (&0D) or NUL (&00).

Real variables are stored in 5-byte, 8-byte or 10-byte binary floating point format with their least significant byte first. See the Format of data in memory section for more details and examples.

In the case of a movable string (normal string variable), the parameter address points to a 6-byte or 8-byte 'string descriptor'. This descriptor gives the address of the start of the string (4 bytes, LSB first) and the current length of the string (2 bytes or 4 bytes, LSB first) in that order.

Running BASIC code in a separate file

CALL filename$

One use for this facility is to keep 'constant' and structure definitions (such as might be used, for example, by calls to OS API functions) in separate files rather than including them in every program that uses them. In this way it is easier to maintain and update the definitions, and your program size is reduced. This is similar to the use of library files for packaging functions which you might want to call from many programs.

You should normally prefix the filename with either @lib$ (for standard definitions) or @dir$ (for definitions specific to your program) so that the file will be loaded from the correct location irrespective of the current directory setting:

CALL @lib$+"windefs"
CALL @dir$+"mydefs"

The CALLed program will return control to the calling program when execution 'drops off the end' of the code, or if a RETURN statement is executed. CALL may be nested, that is program files executed using CALL may themselves include CALL statements.

Syntax

CALL <numeric>{,<n-var>|<s-var>|<array()>|<struct{}>}
CALL <string>

Associated Keywords

INSTALL, RETURN, USR

CASE

WHEN and ENDCASE must be the first thing on the program line and OF must be the last thing on the line (it cannot even be followed by a REMark).

CASE toss% OF
  WHEN 0 : coin$ = "tails"
  WHEN 1 : coin$ = "heads"
  OTHERWISE coin$ = "cheat"
ENDCASE

CASE direction$ OF
  WHEN "left","LEFT" :
  PRINT "turn left"
  WHEN "right","RIGHT" :
  PRINT "Turn right"
  OTHERWISE
  PRINT "Straight on"
ENDCASE

Syntax

CASE <numeric>|<string> OF
  WHEN <numeric>|<string>{,<numeric>|<string>} : {<stmt>}
  {<stmt>}
  WHEN <numeric>|<string>{,<numeric>|<string>} : {<stmt>}
  {<stmt>}
  OTHERWISE {<stmt>}
  {<stmt>}
ENDCASE

Associated Keywords

ENDCASE, IF, OF, ON, OTHERWISE, WHEN

CHAIN "GAME1"
CHAIN A$

All but the static variables @% to Z% are CLEARed.

CHAIN sets ON ERROR OFF before chaining the specified program.

RUN may be used as an alternative to CHAIN.

You can use CHAIN (or RUN) to link program modules together. This allows you to write modular programs which would, if written in one piece, be too large for the memory available or too unwieldy.

Passing data between CHAINed programs can be a bit of a problem because all but the static variables are cleared by CHAIN.

If you wish to pass large amounts of data between CHAINed programs, you should use a data file. However, if the amount of data to be passed is small and you do not wish to suffer the time penalty of using a data file, you can pass data to the CHAINed program by using the indirection operators to store them at known addresses. The safest way to do this is to move HIMEM down and store common data at the top of memory.

The following sample program segment moves HIMEM down 1000 bytes and stores the input and output file names in the memory above HIMEM. There is, of course, still plenty of room for other data in this area.

HIMEM=HIMEM-1000
$HIMEM=in_file$
$(HIMEM+256)=out_file$
CHAIN "NEXTPROG"

Syntax

CHAIN <string>

Associated Keywords

CALL, INSTALL, RUN

CHR$

A$=CHR$(72)
B$=CHR$(12)
C$=CHR$(A/200)

char$=CHR$(65)

CHR$(7)

PRINT "ERROR"+CHR$(7)

CHR$ is the complement of ASC.

Syntax

<s-var>=CHR$(<numeric>)

Associated Keywords

ASC, STR$, VAL, VDU

CIRCLE

The graphics origin (X=0, Y=0) is normally the bottom left of the 'screen' (BASIC's output window). The origin can be changed using the VDU29 command or the ORIGIN statement. See the Graphics and colours section for more details.

The circle or disc is drawn in the current graphics foreground colour. This colour can be changed using the GCOL statement.

CIRCLE x,y,r is equivalent to MOVE x,y : PLOT 145,r,0
CIRCLE FILL x,y,r is equivalent to MOVE x,y : PLOT 153,r,0

CIRCLE 200,300,40
CIRCLE FILL 300,400,100

Syntax

CIRCLE [FILL] <numeric>,<numeric>,<numeric>

Associated Keywords

ELLIPSE, FILL, GCOL, MOVE, PLOT

The CLEAR command tells BBC BASIC to 'forget' about ALL the dynamic variables used so far. This includes strings, arrays and structures, but the static variables (@% to Z%) are not altered.

You can use the indirection operators to store integers and strings at known addresses and these will not be affected by CLEAR. However, you will need to 'protect' the area of memory used. One way to do this is to move HIMEM down. See CHAIN for an example.

CLEAR sets LOMEM equal to TOP.

Syntax

CLEAR

Associated Keywords

None

CLOSE#file_num
CLOSE#0

You can open and close a file several times within one program, but it is generally considered 'better form' not to close a file until you have finally finished with it. However, if you wish to CLEAR the variables, it is simpler if you close the data files first.

You should also close data files before chaining another program. CHAIN does not automatically close data files, but it does clear the variables in which the channel numbers were stored. You can still access the open file if you have used one of the static variables (A% to Z%) to store the channel number. Alternatively, you could reserve an area of memory (by moving HIMEM down for example) and use the byte indirection operator to store the channel number. See the keyword CHAIN for more details.

END or 'dropping off' the end of a program will also close all open data files. However, QUIT and STOP do not close data files.

CLOSE is also used in the ON CLOSE statement.

Syntax

CLOSE#<numeric>

Associated Keywords

OPENIN, OPENUP, OPENOUT, PRINT#, INPUT#, BPUT#, BGET#, EXT#, PTR#, EOF#, ON CLOSE

CLG

See the Graphics and colours section or GCOL for more information on graphics colours.

Syntax

CLG

Associated Keywords

CLS, GCOL

CLS

See the Graphics and colours section or COLOUR for more information on text colours.

Syntax

CLS

Associated Keywords

CLG, COLOUR

When COLOUR is followed by one value the text foreground or background colour is set. If the value is less than 128, the text foreground colour is set to that value. If the number is 128 or greater, the text background colour is set to value−128.

There are up to 16 logical colours (numbered 0 to 15), but the number available varies with the mode; see the section on text colours for details.

COLOUR 1 : REM sets the text foreground colour to 1
COLOUR 130 : REM sets the text background colour to 2

When COLOUR is followed by two values the palette is modified using a physical colour number. The first value specifies the logical colour to be changed (0-15)and the second value specifies the physical colour to which it should be mapped (0-15).

COLOUR 1,4 : REM set logical colour 1 to blue

When COLOUR is followed by four values the palette is modified using an RGB (Red, Green, Blue) colour. The first value specifies the logical colour to be changed (0-15) and the second, third and fourth values specify the red, green and blue components of the colour to which it should be mapped. In each case the value should be in the range 0 (none) to 255 (maximum).

COLOUR 1,128,0,128 : REM set logical colour 1 to dark purple

Syntax

COLOUR <numeric>
COLOUR <numeric>,<numeric>
COLOUR <numeric>,<numeric>,<numeric>,<numeric>

Associated Keywords

VDU, GCOL, MODE

COS

X=COS(angle)

Whilst the computer is quite happy dealing with angles expressed in radians, you may prefer to express angles in degrees. You can use the RAD function to convert an angle from degrees to radians.

The example below sets Y to the cosine of the angle 'degree_angle' expressed in degrees.

Y=COS(RAD(degree_angle))

Syntax

<n-var>=COS(<numeric>)

Associated Keywords

SIN, TAN, ACS, ASN, ATN, DEG, RAD

char_count=COUNT

Because control characters above 13 are included in COUNT, you cannot reliably use it to find the position of the cursor on the screen. If you need to know the cursor's horizontal position use the POS function.

The example below prints strings from the string array 'words$'. The strings are printed on the same line until the line length exceeds 65. When the line length is in excess of 65, a new-line is printed.

PRINT
FOR i=1 TO 1000
  PRINT words$(i);
  IF COUNT>65 THEN PRINT
NEXT

Syntax

<n-var>=COUNT

Associated Keywords

POS

As for INPUT, string values may be quoted or unquoted. However, quotes need to be used if the string contains commas or leading spaces.

Numeric values may include calculation so long as there are no keywords.

Data items in the list should be separated by a comma.

DATA 10.7,2,HELLO," THIS IS A COMMA,",1/3,PRINT
DATA " This is a string with leading spaces."

The following example program segment reads through a list of names looking for the name in 'name$'. If the name is found, the name and age are printed. If not, an error message is printed.

DATA FRED,17,BILL,21,ALLISON,21,NOEL,32
DATA JOAN,26,JOHN,19,WENDY,35,ZZZZ,0
REPEAT
READ list$,age
IF list$=name$ THEN PRINT name$,age
UNTIL list$=name$ OR list$="ZZZZ"
IF list$="ZZZZ" PRINT "Name not in list"

Syntax

DATA <s-const>|<n-const>{,<s-const>|<n-const>}

Associated Keywords

READ, RESTORE, LOCAL

DEF

If DEF is encountered during execution, the rest of the line is ignored. As a consequence, single line definitions can be put anywhere in the program.

Multi-line definitions must not be executed. The safest place to put multi-line definitions is at the end of the main program after the END statement.

There is no speed advantage to be gained by placing function or procedure definitions at the start of the program.

DEF FNMEAN ....
DEF PROCJIM ....

DEF FN_mean ....
DEF PROC_Jim$ ....

A procedure definition is terminated by the statement ENDPROC.

A function definition is terminated by a statement which starts with an equals (=) sign. The function returns the value of the expression to the right of the equals sign.

For examples of function and procedure declarations, see FN and PROC. For a general explanation refer to the Procedures and functions section.

Syntax

DEF PROC<name>[([RETURN]<s-var>|<n-var>{,[RETURN]<s-var>|<n-var>})]
DEF FN<name>[([RETURN]<s-var>|<n-var>{,[RETURN]<s-var>|<n-var>})]

Associated Keywords

ENDPROC, FN, PROC, RETURN

DEG

degree_angle=DEG(PI/2)
X=DEG(ATN(1))

Using DEG is equivalent to multiplying the radian value by 180/PI, but the result is calculated internally to a greater accuracy.

See ACS, ASN and ATN for further examples of the use of DEG.

Syntax

<n-var>=DEG(<numeric>)

Associated Keywords

RAD, SIN, COS, TAN, ACS, ASN, ATN, PI

DIM

Dimensioning arrays

DIM A(2),Ab(2,3),A$(2,3,4),A%(3,4,5,6)

The subscript base is 0, so DIM X(12) defines an array of 13 elements whose subscript value can be from 0 to 12 inclusive.

Arrays are like lists or tables. A list of names is a single dimension array. In other words, there is only one column - the names. Its single dimension in a DIM statement would be the maximum number of names you expected in the table less 1.

If you wanted to describe the position of the pieces on a chess board you could use a two dimensional array. The two dimensions would represent the row (numbered 0 to 7) and the column (also numbered 0 to 7). The contents of each 'cell' of the array would indicate the presence (if any) of a piece and its value.

DIM chess_board(7,7)

DIM chess_game(7,7,40)

Declaring structures

DIM mystruct{one,two%,three$,four(3),five%(1,2),six$(9),seven{a,b%}}
DIM structarray{(10)one,two%,three$}
DIM newstruct{}=protostruct{}
DIM multistruct{(3,2)}=protostruct{}

The 'DIM space' error will occur if you attempt to declare a structure or structure array for which there is insufficient memory. In this case you can (usually) increase the value of HIMEM to make more memory available.

Reserving an area of memory

The example below reserves 68 bytes of memory from the heap and sets a%% equal to the address of the first byte. Thus a%%?0 to a%%?67 are free for use by the program (68 bytes in all):

DIM a%% 67

DIM p%% -1
PRINT HIMEM-p%%

Memory reserved in this way causes irreversible loss of heap space, therefore you must be very careful not to reserve the memory multiple times when once will suffice. This is a particular danger when DIM is used within a function or procedure.

See the Assembler section for a more detailed description of the use of DIM for reserving memory for machine code programs.

Reserving a temporary area of memory

DIM a%% LOCAL 67

Note that the variable (a%% in this case) is not automatically made LOCAL to the function or procedure. In most cases you will want to do that as well:

LOCAL a%%
DIM a%% LOCAL 67

The amount of memory reserved is one byte greater than the value specified. DIM p%% LOCAL −1 is a special case; it reserves zero bytes of memory. This is of more use than you might think, since it tells you the current size of the stack:

DIM p%% LOCAL -1
PRINT HIMEM-p%%

DIM as a function

If an array is declared as follows:

DIM cuboid(4,5,7)

Discovering the size of an array is of particular interest when a whole array is passed as a parameter to a function or procedure. For example, if you write a general-purpose function to calculate the geometric mean of the values in a numeric array, the function must know the total number of elements in the array. See the section on passing arrays to functions/procedures for an example.

If a structure is declared as follows:

DIM mystruct{alpha%, beta$, gamma{l&,h&}}

Syntax

DIM <n-var>|<s-var>(<numeric>{,<numeric>})
DIM <name>{<member>{,<member>}}
DIM <struct{}>=<struct{}>
DIM <n-var> [LOCAL] <numeric>
<n-var> = DIM(<array()>[,<numeric>])
<n-var> = DIM(<struct{}>)

Associated Keywords

CLEAR, HIMEM, LOCAL, LOMEM

DIV

X=A DIV B
y=(top+bottom+1) DIV 2

21 DIV 4

Whilst it is possible to use DIV with real numbers, it is really intended for use with integers. If you do use real numbers, BBC BASIC converts them to integers by truncation before DIViding them.

Syntax

<n-var>=<numeric> DIV <numeric>
<n-var>DIV=<numeric>

Associated Keywords

MOD

DRAW

The start of the line is the current graphics cursor position, i.e. either the last point 'visited' (for example the end of the last line drawn) or a point explicitly specified by a MOVE statement.

The graphics origin (X=0, Y=0) is normally the bottom left of the 'screen' (BASIC's output window). The origin can be changed using the ORIGIN statement or the VDU 29 command.

The line is drawn in the current graphics foreground colour. This colour can be changed using the GCOL statement. DRAW only draws solid lines; you can draw dotted or dashed lines by using the PLOT statement.

DRAW x,y

MOVE 200,300
DRAW 640,800

Syntax

DRAW <numeric>,<numeric>
DRAW BY <numeric>,<numeric>

Associated Keywords

BY, MODE, PLOT, MOVE, CLG, VDU, GCOL

ELLIPSE

BBC BASIC for Windows and BBC BASIC for SDL 2.0 can draw only axis-aligned ellipses. To draw an angled ellipse use the ELLIPSE library routines.

The graphics origin (X=0, Y=0) is normally the bottom left of the 'screen' (BASIC's output window). The origin can be changed using the VDU29 command or the ORIGIN statement. See the Graphics and colours section for more details.

The ellipse is drawn in the current graphics foreground colour. This colour can be changed using the GCOL statement.

ELLIPSE x,y,a,b is equivalent to MOVE x,y : PLOT 0,a,0 : PLOT 193,0,b
ELLIPSE FILL x,y,a,b is equivalent to MOVE x,y : PLOT 0,a,0 : PLOT 201,0,b

ELLIPSE 200,300,40,50
ELLIPSE FILL 300,400,100,80

Syntax

ELLIPSE [FILL] <numeric>,<numeric>,<numeric>,<numeric>

Associated Keywords

CIRCLE, FILL, GCOL, MOVE, PLOT

In an IF statement, if the conditional expression evaluates to zero, the statements after ELSE will be executed. This makes the following work:

IF A=B THEN B=C ELSE B=D
IF A=B THEN B=C:PRINT"WWW" ELSE B=D:PRINT"QQQ"
IF A=B THEN B=C ELSE IF A=C THEN...............

IF x=3 THEN IF a=b THEN PRINT a$ ELSE PRINT er$

IF x=3 THEN
  IF a=b THEN PRINT a$ ELSE PRINT "Bad"
ENDIF

IF x=3 THEN
  PRINT "x is 3"
ELSE
  PRINT "x is not 3"
ENDIF

IF x=3 THEN
  PRINT "x is 3"
ELSE IF x=4 THEN
    PRINT "x is 4"
  ELSE
    PRINT "x is neither 3 nor 4"
  ENDIF
ENDIF

You can use ELSE with ON...GOSUB, ON...GOTO and ON...PROC statements to prevent an out of range control variable causing an 'ON range' error.

ON action GOTO 100, 200, 300 ELSE PRINT "Error"
ON number GOSUB 100,200,300 ELSE PRINT "Error"
ON value PROCa,PROCb,PROCc ELSE PRINT "Error"

Syntax

IF <t-cond> THEN <stmt> ELSE <stmt>
ELSE [<stmt>]
ON <n-var> GOTO <l-num>{,<l-num>} ELSE <stmt>
ON <n-var> GOSUB <l-num>{,<l-num>} ELSE <stmt>
ON <n-var> PROC<name>{,PROC<name>} ELSE <stmt>

Associated Keywords:

IF, THEN, ON, ENDIF

END

END tells BBC BASIC that it has reached the end of the program. You don't have to use END, just 'running out of program' will have the same effect, but it's a bit messy.

You can use END within, for instance, an IF...THEN...ELSE statement to stop your program if certain conditions are satisfied. You should also use END to stop BBC BASIC 'falling into' any procedure or function definitions at the end of your program.

END can also be used as a function. It returns the address of the first byte above BASIC's dynamic variable area (the heap). These two statements are equivalent:

e%% = END
DIM e%% -1

Syntax

END
<n-var> = END

Associated Keywords

QUIT, STOP, CLOSE#

ENDCASE

CASE die% OF
  WHEN 1,2,3 : bet$ = "lose"
  WHEN 4,5,6 : bet$ = "win"
  OTHERWISE bet$ = "cheat"
ENDCASE

Syntax

ENDCASE

Associated Keywords

CASE, OF, OTHERWISE, WHEN

ENDIF

Syntax

ENDIF

Associated Keywords

ELSE, IF, THEN

ENDPROC

All LOCAL or PRIVATE variables and the dummy arguments are restored at ENDPROC and the program returns to the statement after the calling statement.

Syntax

ENDPROC

Associated Keywords

DEF, FN, LOCAL, PRIVATE, PROC

ENDWHILE

 WHILE LEFT$(A$,1) = " " A$ = MID$(A$,2) : ENDWHILE

Syntax

ENDWHILE

Associated Keywords

REPEAT, UNTIL, WHILE

ENVELOPE

During its 'life' the pitch and amplitude (volume) of a sound may change. The variation of pitch with time is called the pitch envelope and the variation of amplitude with time is called the amplitude envelope. In BBC BASIC there are four phases of amplitude (the attack phase, the decay phase, the sustain phase and the release phase) and three phases of pitch. The four amplitude phases and the three pitch phases are defined separately based on a common 'step' duration.

You may use the ENVELOPE statement to define up to 16 different envelopes. The envelopes are referred to by number in the SOUND statement.

The ENVELOPE statement has 14 parameters. The syntax of the ENVELOPE statement is:

ENVELOPE <parameter list>

N,T,PI1,PI2,PI3,PN1,PN2,PN3,AA,AD,AS,AR,ALA,ALD

The durations of the three phases of the pitch envelope are explicitly specified (PN1, PN2 and PN3 respectively) but the durations of the phases of the amplitude envelope are determined indirectly. See *TEMPO for more information about the pitch envelope.

The duration of the 'attack' phase is determined by the target level (ALA) and the rate of attack (AA), i.e. the duration is equal to ALA/AA or the total duration specified in the SOUND statement, whichever is less.

The duration of the 'decay' phase is determined by the target level (ALD) and the rate of decay (AD), i.e. the duration is equal to (ALD-ALA)/AD or what remains of the total duration specified in the SOUND statement, whichever is less.

The duration of the 'sustain' phase is determined by what remains of the total duration specified in the SOUND statement. For example if the total duration is two seconds, and the attack and decay phases lasted a total of one second, the sustain phase will last one second.

The 'release' phase is terminated as soon as another note is ready to play on that channel, otherwise it continues indefinitely or until the amplitude has reduced to zero. A SOUND statement with the 'hold' bit set can be used to force some or all of the release phase to sound even if there are more notes ready to be played.

Syntax

ENVELOPE <numeric>, <numeric>, <numeric>, <numeric>, <numeric>, <numeric>, <numeric>, <numeric>, <numeric>, <numeric>, <numeric>, <numeric>, <numeric>, <numeric>

Associated Keywords

SOUND

CONTENTS

CONTINUE