The Basic - Sinclair ZX81 User Manual

symbol or compound token) being inserted just before the cursor. The cursor can be moved left with
(shifted 5) or right with
0). (Note:- the whole line can be deleted by typing EDIT (shifted 1) followed by NEWLINE ; or, if it is INPUT
data, just by typing EDIT .)
When NEWLINE is pressed, the line is executed, entered into the program, or used as INPUT data as
appropriate, unless it contains a syntax error. In this case the symbol
As program lines are entered, a listing is displayed in the top half of the screen. The manner in which the
listing is produced is rather complicated, and explained more fully in chapter 9, exercise 6. The last line to
be entered is called the current line and indicated by the symbol
keys (shifted 6) and
bottom part of the screen and can be edited.
When a command is executed or a program run, the screen is first of all cleared, & then output is
displayed in the top half of the screen and remains until a program line is entered, or NEWLINE is pressed
with an empty line, or
code showing what happened (see appendix B), & n is the number of the last line executed - or 0 for a
command. The report remains until a key is pressed (and indicates
In certain circumstances, the SPACE key acts as a BREAK , stopping the computer with report D. This is
recognized
(i) at the end of a statement while a program is running,
(ii) while the computer is looking for a program on tape,
or (iii) while the computer is using the printer (or by accident trying to use it when it is not there).

The BASIC

Numbers are stored to an accuracy of 9 or 10 digits. The largest number you can get is about 10
smallest (positive) number is about 4 * 10
A number is stored in the ZX81 in floating point binary with one exponent byte e (1
mantissa bytes m (½
Since ½ m 1, 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.
Zero has a special representation in which all 5 bytes are 0.
Numeric variables have names of arbitrary length, starting with a letter and continuing with letters and
digits. All these are significant, so that for instance LONGNAME & LONGNAMETOO are distinct names.
Spaces are ignored.
Control variables for 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 arbitraryily 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
(i) empty
or
(ii) numerical expression
or
(iii) optional numerical expression TO optional numerical expression
& is used in expressing a substring either by
(a) string expression (slicer)
or by
(b) string array variable (subscript,...,subscript, slicer)
which means the same as
string array variable (subscript,...,subscript)(slicer)
(shifted 8). The character before the cursor can be deleted with RUBOUT (shifted
(shifted 7). If EDIT (shifted 1) is pressed, the current line is brought down to the
or is pressed. In the bottom part appears a report of the form m/n where m is a
-39
m 1). This represents the number m * 2
.
e-128
appears just before the error.
, but this can be changed using the
mode).
.
38
, & the
e 255), & four