Mathematical Operations Used in Level I Basic

Where x and y must be either a numeric variable or an array element.  Testing of a string's equality is not supported

Level I BASIC.

Multiplication and division are evaluated prior to addition and subtraction. And, all four of these math operations are

evaluated before the <, =, and > comparisons. Parantheses can be used to override this order of operations.

  For example:

Note for programmers from other languages: The IF statement will execute the statement that follows the THEN only

if the expression evaluated is non-zero. Since a*b will be non-zero only if a and b are both non-zero, "*" works like a

logical AND. Similarly, since comparisons return an unsigned number a+b will be non-zero when either of a or b are

non-zero. So, "+" can be used as a logical OR. 

Using the PRINT statement:

PRINT is a very flexible command. At its basic level it can output a message or the contents of a variable. For

example:

If you end your PRINT statement with a semicolon, the next PRINT statement (or INPUT statement) will continue on

the same line, rather than starting on the next line. e.g. PRINT"HELLO ";:PRINT"THERE" will print the message

"HELLO THERE" on a single line.

You can also combine PRINT statements with the semicolon. For example A=3:PRINT"A=";A will print the message

"A= 3 ". (There is a space between the equal sign and the 3 as that is where the sign would go if the number were

negative.)

TABs: You can also include the function TAB(x) which will position the cursor to column x of the current line before it

prints anything. For example, PRINT TAB(29);"CENTRE" will print the word CENTRE in the centre of the line. (Each

line is 64 characters wide and x can be in the range 0-63. If the cursor is already to the right of the requested

column, the text will print at the cursor position. Remember, if you don't end your PRINT statement with a ";",

subsequent PRINTs start at the beginning of a new line).

Positioning the cursor anywhere: There are 16 lines in the Simulator window, each 64 characters wide. You can

use the statement PRINT AT x;... to position the cursor anywhere on the screen prior to printing. In this case x is a

number from 0 to 1023. 0 represents the upper-left corner, 64 the beginning of the second line, 128 the beginning of

the third line, etc. down to 1023 which is the lower-right corner. (Note: if you position near the bottom there may not

be enough room for what you wish to print. If this happens the screen will scroll.) The general formula is x = 64*r + c

where r is the row (from 0-15) and c is the column (from 0-63). PRINT AT 541;"MIDDLE" will print the word MIDDLE

starting 8 rows down from the top, 29 columns from the left. You can also use PRINT AT to position where the

INPUT prompt is printed (e.g. "PRINT AT 541;:INPUT A$"). 

Colour Chart:

Below is a chart of the colour values used by the COL. command and returned by the CPOINT(x,y) function. The

colours follow a pattern analogous to the CGA graphics palette.

*Colour 0 (black) is a valid return value for CPOINT(x,y) but it may not be used as a parameter for the COL.

command. If you want to draw pixels in black, use RESET instead of SET. If you want to draw text in black on

another colour background, use INVERSE. If you want to draw black text on a black background, use spaces

instead and pretend.