Color BASIC

Color BASIC is the implementation of Microsoft BASIC that is included in the ROM of the Tandy/Radio Shack TRS-80 Color Computers manufactured between 1980 and 1991. BASIC (Beginner's All-purpose Symbolic Instruction Code) is a high level language with simple syntax that makes it easy to write simple programs. Color BASIC is interpreted, that is, decoded as it is run.

Background
The nucleus of Color BASIC was Microsoft BASIC-69 which Tandy licensed from Microsoft. Color BASIC 1.0 was released with the original 4k TRS-80 Color Computer in 1980. It resides on 8k bytes of ROM, and is responsible for all 'housekeeping' duties on the system. This includes hardware initialization, memory management, interrupt processing, etc. Like most implementations of BASIC, each line of code starts with a line number and consists of one or more statements with variables and operators. 16k of memory is required for the next level of BASIC, Extended Color BASIC. ("ECB") Extended BASIC is required for the floppy disk controller, which then gives you Disk Extended Color BASIC. ("DECB") Emulators of the Color Computers running this interpreter and the others are available for modern computers, some of which require a "snapshot" file of the physical machine.

Variables
Color BASIC understands one type of numeric variable and string variables. Variable names in Color BASIC have the first two characters significant. The first character of the variable name must be a letter. The second can be either a letter or number. String variables are indicated by adding a dollar sign ($) after the variable name.

Examples
Numeric variables have only one type, a binary floating point implementation. Each numeric variable uses 5 bytes of memory and can be in the range from -1E+38 up to 1E+37.

Unlike most implementations of Microsoft BASIC, Color BASIC requires the user to reserve space for string variables via the CLEAR statement.

Multidimensional arrays are also supported with both numeric and string variables. In the case of an array, the element address is enclosed with a parenthesis: Multiple dimensions are separated by commas

Operators and Symbols
Color BASIC provides several operators for both mathematic and, to a lesser extent, string operations.


 * + can be used to concatenate strings or for mathematical addition
 * - is used for subtraction
 * * is used for multiplication
 * / is used for division

Parenthesis are used to override mathematical order of operation


 * AND is used for logical 'and' operations
 * OR is used for logical 'or' operations
 * NOT is used for logical 'not' operations

For testing, the following operators are used:


 * = is equal to
 * > is greater than
 * < is less than
 * >= is greater than or equal to (also => is acceptable)
 * <= is less than or equal to (also =< is acceptable)
 * <> is not equal to (also >< is acceptable)

Other symbols used in BASIC:


 * " " indicates string data is a constant (static)
 * : separates multiple commands on a single program line
 * A semicolon, when encountered in a PRINT function, will cause the output to remain on the same line
 * A comma, when encountered in a PRINT function, will tab to the next column

Key
num indicates a numeric expression is required. This can be a fixed number, a variable, or other operation or function that returns a numeric quantity.

str indicates a string expression is required. This can be a static string value (in quotes), a string variable, or other function or expression that returns a string of characters.

device number indicates a device. By default, device 0 (screen and keyboard) is assumed. In Color BASIC, device #-1 (cassette) and #-2 (printer) are available to the programmer.

Edit mode
If you make a mistake typing in a line, you can either retype it from scratch (or DEL it).. or you can EDIT it. When in EDIT mode, you get a reprint of the line, and a second copy that you SPACEbar across chars. You cannot use arrow keys. backspace takes you left, but does not actually erase it in the buffer. 'i' puts you in insert mode. pressing return gets you out of it. 'c' changes one char, 'd' deletes one char. 'x' takes you to end of line, allowing you to e'x'tend it. 'l' redraws the line. 's' searches for the next instance of a character. For the 's', 'c' and 'd' commands you can also enter a number (#) before pressing any of them which will: 's' - search for the # instance of the character, 'c' - allow you to change # of characters,  'd' - delete # amount characters.

Functions

 * : returns the absolute value of num
 * : returns the ASCII code of the first character in str
 * : returns a single string character with the ASCII code num
 * : returns 0 if the file has data, or -1 if at the end of the file
 * : returns a character if a key on the keyboard has been pressed, or null if nothing is pressed
 * : returns the integer portion of num
 * : searches for the first string, in the target str. startpos is optional.
 * : returns the position of the joystick axis (0-3) with a value from 0 to 63
 * : returns the first ("left") num characters of string str
 * : returns the length (in characters) of string str
 * : returns the available free memory in bytes
 * : returns a sub-string of string str beginning at position start num and length num characters long. Can also reassign by adding ="newvalue"
 * : returns the value of the memory location num (0-65535)
 * : returns the color of the semigraphics dot at position x numm (0-63) and y num (0-31)
 * : returns the end ("right") portion of string str beginning at character position num
 * : returns a random number (integer) between 1 and num
 * : returns the sign of a number num, 1 if positive, -1 if negative, 0 if 0
 * : returns the sine of num in radians
 * : returns a string of the number num
 * : calls a machine language subroutine whose address is stored in memory locations 275 and 276. num is passed to the routine, and a return value is assigned when the routine is done

Commands

 * : Connects or disconnects cassette audio from the TV sound
 * : reserves memory for string variables, and optionally, a machine language program
 * : loads BASIC program from cassette. If no name is specified, the next program is loaded
 * : loads machine language program from cassette. If no name is specified, the next program is loaded
 * : closes a device (in Color BASIC this can only be #-1, the cassette)
 * : clears the screen. An optional color num (0-8) can be specified
 * : continues a program after pressing BREAK or a STOP statement
 * : saves a BASIC program to cassette with optional name
 * : stores data in a BASIC program for retrieval with the READ command
 * : dimensions an array and reserves memory space for it
 * : indicates the end of a BASIC program
 * : [memory address] executes the machine language program at memory address. If none specified, the execute address of the program loaded off tape is used
 * : Waits for input from device number. If not specified, device 0 (keyboard) is assumed. An optional prompt can be printed on the screen for the input statement
 * : lists line(s) of your program. Either start or end can be omitted, or if both are omitted, the entire program will be listed
 * : works like LIST, but outputs to the printer
 * : turns the cassette motor on or off
 * : erases contents of memory (program and variable)
 * : evaluates expression num and calls the numth subroutine listed
 * : evaluates expression num and jumps to the numth line listed
 * : opens a device for communication
 * : writes data (0-255) into memory address (0-65535)
 * : prints data to device specified. If omitted, #0 (screen) is assumed
 * : works like PRINT, but prints at the location specified (0-511)
 * : reads the next variable(s) from the BASIC program embedded with DATA statements
 * : renumbers each line (optional Oldstart ) (with optional NewStart) of the program : at multiples of (optional num).: If all options are omitted, the list will use 10 for increment. : With a program with 10 lines, with 10 separation. 10,20,30,40,50,60,70,80,90,100: renum 100, 40, 2.  This will renumber, Starting at line number 40. It will change all line number following and including 40 by 2. lines now 10,20,30,100,102,104,106,108,110, 112
 * : sets the semigraphics pixel at location x (0-63) and y (0-31) to black
 * : resets the READ pointer back to the first DATA statement
 * : returns from a subroutine
 * : runs the BASIC program, optionally, at the line number specified
 * : sets the semigraphics pixel at location x (0-63) y (0-31) to color (0-8)
 * : skips over BASIC programs on tape until the program name specified is found
 * : sounds a tone with frequency (1-255) and duration (1-255)
 * : causes the program to stop executing
 * : tabs to the column specified (used with PRINT)
 * : returns the numeric value of a string that contains a number in string form

Control flow
FOR {num} = {number} TO {number} [STEP {number}] ... NEXT (num)
 * : calls the subroutine at the line number specified
 * : jumps to the program's line number specified
 * : performs conditional test. If the test is true THEN commands are executed, otherwise (ELSE) other commands are executed. If the no ELSE is specified, and the test is false, the next line of the program will be run
 * creates a loop where the numeric variable (num) runs from start number to end number in increments of number (STEP). If step is omitted, 1 is assumed

Error Messages

 * : division by zero
 * : file specified is already open
 * : bad subscript. subscript is out of DIM range
 * : can't continue (see CONT command)
 * : attempt to redimension an array
 * : invalid device number
 * : direct statement error (program has no line numbers)
 * : illegal function call: function contains a value that is out of range
 * : bad file data: attempt to read a number into a string value, etc.
 * : bad file mode, attempt to INPUT data to a file open for OUTPUT, etc.
 * : illegal direct: the specified command can only be run in a program
 * : input past end of file. See EOF
 * : input/output error
 * : long string: strings can only have 255 characters
 * : NEXT without FOR
 * : file not open
 * : out of data: attempt to read beyond the last DATA in the program
 * : out of memory
 * : out of string space: see CLEAR
 * : overflow: the number is out of range
 * : RETURN without GOSUB
 * : syntax error
 * : string operation too complex
 * : type mismatch (A$=3, A="CAT")
 * : attempt to GOTO or GOSUB to a line that doesn't exist

Documented ROM subroutines
There are a few subroutines available for machine language programs in the Color BASIC ROM that are available for general purpose programming.


 * : address [$A000]: polls keyboard for a character
 * : address [$A002]: outputs a character to screen or device
 * : address [$A004]: starts cassette and prepares for reading
 * : address [$A006]: reads a block from cassette
 * : address [$A008]: writes a block to cassette
 * : address [$A00A]: reads joystick values