# SINCLAIR ZX SPECTRUM BASIC Programming APPENDIX C (Part 2)
The BASIC
Numbers are stored to an accuracy of 9 or 10 digits. The largest number you can get is about 1038, and the smallest (positive) number is about 4*10-39.

A number is stored in the ZX Spectrum in floating point binary with one exponent byte e (l<=e<=255), and four mantissa bytes m (1/2<=m<l). This represents the number m*2e-128.

Since 1/2<=m<l, the most significant bit of the mantissa m is always 1. Therefore in actual fact we can replace it with a bit to show the sign - 0 for positive numbers, 1 for negative.

Small integers have a special representation in which the first byte is 0, the second is a sign byte (0 or FFh) and the third and fourth are the integer in twos complement form, the less significant byte first.

Numeric variables have names of arbitrary length, starting with a letter and continuing with letters and digits. Spaces and colour controls are ignored and all letters are converted to lower-case letters.

Control variables of FOR-NEXT loops have names a single letter long.

Numeric arrays have names a single letter long, which may be the same as the name of a simple variable. They may have arbitrarily many dimensions of arbitrary size. Subscripts start at 1.

Strings are completely flexible in length. The name of a string consists of a single letter followed by \$.

String arrays can have arbitrarily many dimensions of arbitrary size. The name is a single letter followed by \$ and may not be the same as the name of a string. All the strings in a given array have the same fixed length, which is specified as an extra, final dimension in the DIM statement. Subscripts start at 1.

Slicing: Substrings of strings may be specified using slicers. A slicer can be

 1 empty or 2 numerical expression or 3 optional numerical expression TO optional numerical expression

and is used in expressing a substring either by

(a) string expression (slicer)

(b) string array variable (subscript,..., subscript, slicer)

which means the same as

string array variable (subscript . . . , subscript) (slicer)

In (a), suppose the string expression has the value s\$.

If the slicer is empty, the result is s\$ considered as a substring of itself.

If the slicer is a numerical expression with value m, then the result is the mth character of s\$ (a substring of length 1).

If the slicer has the form (iii), then suppose the first numerical expression has the value m (the default value is 1), and the second, n (the default value is the length of s\$).

If 1<=m<=n<=the length of s\$ then the result is the substring of s\$ starting with the mth character and ending with the nth. If 0<=n&ltm then the result is the empty string. Otherwise, error 3 results.

Slicing is performed before functions or operations are evaluated, unless brackets dictate otherwise.

Substrings can be assigned to (see LET).

If a string quote is to be written in a string literal, then it must be doubled.

#### Functions

The argument of a function does not need brackets if it is a constant or a (possibly subscripted or sliced) variable.

Function   Type of argument (x)   Result
ABS number Absolute magnitude
Error A if x not in the range -1 to +1
AND binary operation, right operand always a number.
 Numeric left operand: A AND B = A if B<>0 0 if B=0 String left operand:A\$ AND B = A\$ if B<>0 A\$ if B=0
Error A if x not in the range -1 to +1
ATTR two arguments, x and y, both numbers; enclosed in brackets A number whose binary form codes the attributes of line x, column y on the television. Bit 7 (most significant) is 1 for flashing, 0 for not flashing. Bit 6 is 1 for bright, 0 for normal. Bits 5 to 3 are the paper colour. Bits 2 to 0 are the ink colour. Error B unless 0<=x<=23 and 0<=y<=31
BIN   This is not really a function, but an alternative notation for numbers: BIN followed by a sequence of 0s and 1s is the number with such a representation in binary.
CHR\$ number The character whose code is x, rounded to the nearest integer
CODE string The code of the first character in x (or 0 if x is the empty string)
COS number (in radians) Cosine x
EXP number ex
FN   FN followed by a letter calls up a user-defined function (see DEF). The arguments must be enclosed in brackets; even if there are no arguments the brackets must still be present.
IN number The result of inputting at processor level from port x (0<=x<=FFFFh) (loads the bc register pair with x and does the assembly language instruction in a(c))
INKEY\$ none Reads the keyboard. The result is the character representing (in L or C mode) the key pressed if there is exactly one, else the empty string.
INT number Integer part (always rounds down)
LEN string Length
LN number Natural logarithm (to base e).
Error A if x<=0
NOT number 0 if x<>0, 1 if x=0. NOT has priority 4
OR binary operation, both operands numbers
 a OR b = 1 if b<>0 a if b=0
OR has priority 2
PEEK number The value of the byte in memory whose address is x (rounded to the nearest integer). Error B if x is not in the range 0 to 65535
PI none (3.14159265 . . . )
POINT Two arguments, x and y, both numbers; enclosed in brackets 1 if the pixel at (x,y) is ink colour. 0 if it is paper colour. Error B unless 0<=x<=255 and 0<=y<= 175
RND none The next pseudorandom number in a sequence generated by taking the powers of 75 modulo 65537, subtracting 1 and dividing by 65536. 0<=y< 1
SCREEN\$ Two arguments, x and y, both numbers; enclosed in brackets The character that appears, either normally or inverted, on the television at line x, column y. Gives the empty string, if the character is not recognised. Error B unless 0<=x<=23 and 0<=y<=31
SGN number Signum: the sign (-1 for negative, 0 for zero or +1 for positive) of x
SQR number Square root.
Error A if x<0
STR\$ number The string of characters that would be displayed if x were printed
USR number Calls the machine code subroutine whose starting address is x. On return, the result is the contents of the bc register pair
USR string The address of the bit pattern for the user-defined graphic corresponding to x.
Error A if x is not a single letter between a and u, or a user-defined graphic
VAL string Evaluates x (without its bounding quotes) as a numerical expression.
Error C if x contains a syntax error, or gives a string value. Other errors possible, depending on the expression
VAL\$ string Evaluates x (without its bounding quotes) as a string expression.
Error C if x contains a syntax error or gives a numeric value. Other errors possible, as for VAL
- number Negation

The following are binary operations:

 + Addition (on numbers), or concatenation (on strings) - Subtraction * Multiplication / Division Raising to a power. Error B if the left operand is negative = Equals > Greater than Both operands must be of the same type. The result is a number 1, if the comparison holds and 0 if it does not < Less than <= Less than or equal to >= Greater than or equal to <> Not equal to

Functions and operations have the following priorities:

 Operation Priority Subscripting and slicing 12 All functions except NOT and unary minus 11 10 Unary minus (i.e. minus just used to negate something) 0 *, / 8 +, - (minus used to subtract one number from another) 6 =, >, <, <=, >=, <> 5 NOT 4 AND 3 OR 2

#### Statements

In this list,

 a represents a single letter v represents a variable x, y, z represent numerical expressions m, n represent numerical expressions that are rounded to the nearest integer e represents an expression f represents a string valued expression s represents a sequence of statements separated by colons c represents a sequence of colour items, each terminated by commas, or semi-colons; a colour item has the form of a PAPER, INK, FLASH, BRIGHT, INVERSE or OVER statement.

Note that arbitrary expressions are allowed everywhere (except for the line number at the beginning of a statement).

All statements except INPUT, DEF and DATA can be used either as commands or in programs (although they can be more sensible in one than the other). A command or program line can have several statements, separated by colons (:). There is no restriction on whereabouts in a line any particular statement can occur - although see IF and REM.

 BEEP x, y Sounds a note through the loudspeaker for x seconds at a pitch y semitones above middle C (or below if y is negative). BORDER m Sets the colour of the border of the screen and also the paper colour for the lower part of the screen.Error K is m not in the range 0 to 7. BRIGHT Sets brightness of characters subsequently printed. n=0 for normal, 1 for bright. 8 for transparent. Error K if n not 0, 1 or 8 CAT Does not work without Microdrive, etc CIRCLE x, y, z Draws an arc of a circle, centre (x,y), radius z CLEAR Deletes all variables, freeing the space they occupied.Does RESTORE and CLS, resets the PLOT position to the bottom left-hand corner and clears the GO SUB stack CLEAR n Like CLEAR. but if possible changes the system variable RAMTOP to n and puts the new GO SUB stack there CLOSE # Does not work without Microdrive, etc CLS (Clear Screen). Clears the display file CONTINUE Continues the program, starting where it left off last time it stopped with report other than 0. If the report was 9 or L, then continues with the following statement (taking jumps into account); otherwise repeats the one where the error occurred.If the last report was in a command line then CONTINUE will attempt to continue the command line and will either go into a loop if the error was in 0:1, give report 0 if it was in 0: 2, or give error N if it was 0: 3 or greater.CONTINUE appears as CONT on the keyboard COPY Sends a copy of the top 22 lines of display to the printer, if attached; otherwise does nothing. Note that COPY can not be used to print the automatic listings that appear on the screen.Report D if BREAK pressed DATA a1, a2, a3, ... Part of the DATA list. Must be in a program DEF FN a (a1 ... ak)=e User-defined function definition; must be in a program. Each of a and a1 to ak is either a single letter or a single letter followed by '\$' for string argument or result.Takes the form DEF FN a()=e if no arguments DELETE f Does not work without Microdrive, etc DIM a( n1, ... ,nk ) Deletes any array with the name a, and sets up an array a of numbers with k dimensions n1, ... ,nk. Initialises all the values to 0 DIM a\$( n1, ... ,nk ) Deletes any array or string with the name a\$, and sets up an array of characters with k dimensions nl ,...,nk. Initialises all the values to " ". This can be considered as an array of strings of fixed length nk , with k-l dimensions nl,...,nk-l . Error 4 occurs if there is no room to fit the array in.An array is undefined until it is dimensioned in a DIM satement DRAW x,y DRAW x,y,0 DRAW x,y,z Draws a line from the current plot position moving x horizontally and y vertically relative to it while turning through an angle z.Error B if it runs off the screen ERASE Does not work without Microdrive, etc. FLASH Defines whether characters will be flashing or steady. n=0 for steady, n=l for flash, n=8 for no change. FOR a=x TO y FOR a=x TO y STEP 1 FOR a=x TO y STEP z Deletes any simple variable a and sets up a control variable with value x, limit y, step z, and looping address referring to the statement after the FOR statement. Checks if the initial value is greater (if step>=0) or less (if step<0) than the limit, and if so then skips to statement NEXT a, giving error 1 if there is none. See NEXT.Error 4 occurs if there is no room for the control variable. FORMAT f Does not work without the Microdrive, etc GOSUB n Pushes the line number of the GOSUB statement onto a stack; then as GO TO n.Error 4 can occur if there are not enough RETURNs GO TO n Jumps to line n (or, if there is none, the first line after that) IF x THEN s If x true (non-zero) then s is executed. Note that s comprises all the statements to the end of the line.The form 'IF x THEN line number' is not allowed. INK n Sets the ink (foreground) colour of characters subsequently printed. n is in the range 0 to 7 for a colour, n=8 for transparent or 9 for contrast. See The television screen - Appendix B.Error K if n not in the range 0 to 9. INPUT The ' ... ' is a sequence of INPUT items, separated as in a PRINT statement by commas, semicolons or apostrophes. An INPUT item can be: i.    Any PRINT item not beginning with a letter ii.     A variable name, or iii.     LINE, then a string type variable name.The PRINT items and separators in (i) are treated exactly as in PRINT, except that everything is printed in the lower part of the screen. For (ii) the computer stops and waits for input of an expression from the keyboard; the value of this is assigned to the variable. The input is echoed in the usual way and syntax errors give the flashing ?. For string type expressions, the input buffer is initialised to contain two string quotes (which can be erased if necessary). If the first character in the input is STOP, the program stops with error H. (iii) is like (ii) except that the input is treated as a string literal without quotes, and the STOP mechanism doesn't work; to stop it you must type instead. INVERSE n Controls inversion of characters subsequently printed.If n=0, characters are printed in normal video, as ink colour on paper colour.If n=1, characters are printed in inverse video, i.e. paper colour on ink colour. See The television screen - Appendix B.Error K if n is not 0 or 1 LET v=e Assigns the value of e to the variable v. LET cannot be omitted. A simple variable is undefined until it is assigned to in a LET, READ or INPUT statement. If v is a subscripted string variable, or a sliced string variable (substring), then the assignment is Procrustean (fixed length): the string value of e is either truncated or filled out with spaces on the right, to make it the same length as the variable v LIST LIST 0 LIST n Lists the program to the upper part of the screen, starting at the first line whose number is at least n, and makes n the current line LLIST LLIST 0 LLIST n Like LIST, but using the printer LOAD f Loads program and variables LOAD f DATA() Loads a numeric array LOAD f DATA \$() Loads character array \$ LOAD f CODE m,n Loads at most n bytes, starting at address m LOAD f CODE m Loads bytes starting at address m LOAD f CODE Loads bytes back to the address they were saved from. LOAD f SCREEN\$ LOAD f CODE 16384,6912. Searches for file of the right sort on cassette tape and loads it, deleting previous versions in memory. See Chapter 20 LPRINT Like PRINT but using the printer MERGE f Like LOAD f. but does not delete old program lines and variables except to make way for new ones with the same line number or name. MOVE f1,f2 Does not work without the Microdrive, etc NEW Starts the BASIC system off anew, deleting program and variables, and using the memory up to and including the byte whose address is in the system variable RAMBOT and preserves the system variables UDG, P RAMT, RASP and PIP NEXT a 1.     Finds the control variable 2.     Adds its step to its value 3.     If the step>=0 and the value>the limit; or if the step<0 and the value=1013, then it is printed using scientific notation. The mantissa part has up to eight digits (with no trailing zeros), and the decimal point (absent if only one digit) is after the first. The exponent part is E, followed by + or -, followed by one or two digits. Otherwise x is printed in ordinary decimal notation with up to eight significant digits, and no trailing zeros after the decimal point. A decimal point right at the beginning is always followed by a zero, so for instance .03 and 0.3 are printed as such.0 is printed as a single digit 0.a string expression The tokens in the string are expanded, possibly with a space before or after. Control characters have their control effect. Unrecognized characters print as ?. AT m,n Outputs an AT control character followed by a byte for m (the line number) and a byte for n (the column number). TAB n Outputs a TAB control character followed by two bytes for n (less significant byte first), the TAB stop. A colour item, which takes the form of a PAPER, INK, FLASH, BRIGHT, INVERSE or OVER statement RANDOMIZE RANDOMIZE 0 RANDOMIZE n Sets the system variable (called SEED) used to generate the next value of RND. If n<>0, SEED is given the value n; if n=0 then it is given the value of another system variable (called FRAMES) that counts the frames so far displayed on the television, and so should be fairly random. RANDOMIZE appears as RAND on the keyboard. Error B occurs if n is not in the range 0 to 65535 READ vl , v2 , ... vk Assigns to the variables using successive expressions in the DATA list. Error C if an expression is the wrong type. Error E if there are variables left to be read when the DATA list is exhausted REM . . . No effect. ' ... ' can be any sequence of characters except ENTER. This can include : , so no statements are possible after the REM statement on the same line RESTORE RESTORE 0 RESTORE n Restores the DATA pointer to the first DATA statement in a line with number at least n: the next READ statement will start reading there RETURN Takes a reference to a statement off the GO SUB stack, and jumps to the line after it. Error 7 occurs when there is no statement reference on the stack. There is some mistake in your program; GO SUBs are not properly balanced by RETURNs RUN RUN 0 RUN n CLEAR, and then GO TO n SAVE f Saves the program and variables SAVE f LINE m Saves the program and variables so that if they are loaded there is an automatic jump to line m SAVE f DATA() Saves the numeric array SAVE f DATA\$() Saves the character array \$ SAVE f CODE m,n Saves n bytes starting at address m SAVE f SCREEN\$ SAVE f CODE 16384,6912. Saves information on cassette, giving it the name f Error F if f is empty or has length eleven or more. See Chapter 20 STOP Stops the program with report 9. CONTINUE will resume with the following statement VERIFY The same as LOAD except that the data is not loaded into RAM, but compared against what is already there. Error R if one of the comparisons shows different bytes. 