Chapter 20 - The ZX81 Printer
If you have got a ZX81 printer, you will have some operating instructions with it. This chapter covers the
BASIC statements needed to make it work.
The first two,
LPRINT
&
LLIST
, are just kile
PRINT
and
LIST
, except that they use the printer instead of
the television. (The L is a historical accident. When BASIC was invented it usually usually used an
electronic typewriter instead of a television, so
PRINT
really did mean print. If you wanted messages of
output you would use a vry fast line printer attached to the computer, & an
LPRINT
statement meaning
'Line printer
PRINT
'.)
Try this program for example.
10
LPRINT
"THIS PROGRAM:",,,,
20
LLIST
30
LPRINT
,,"PRINTS OUT THE CHARACTER SET.",,,
40
FOR
N=0
TO
255
50
LPRINT CHR$
N;
60
NEXT
N
The third statement,
COPY
, prints out a copy of the television screen. For instance, get a listing on the
screen of the program above, & type
COPY
You can always stop the printer when it is running by pressing
BREAK
key (space).
If you execute these statements without the printer attached, it should just lose all the output & carry on
with the next statement. However, sometimes it might get stuck, & when this happens the break key will
bring it out.
Summary
Satements:
LPRINT
,
LLIST
,
COPY
Note
: None of these statements is standard BASIC, although
LPRINT
is used by some other computers.
Exercises
1. Try this:
10
FOR
N=31
TO
0
STEP
-1
20
PRINT AT
31-N,N;
CHR$
(
CODE
"0" +N);
30
NEXT
N
You will see a pattern of letters working down diagonally from the top right-hand corner until it reaches
the botrom of the screen, when the program stops with error report 5.
Now change '
AT
31-N,N' in line 20 to '
TAB
N'. The program will have exactly the same effect as before.
Now change
PRINT
in line 20 to
LPRINT
. This time there will be no error 5, which should not occur with
the printer, & the pattern will carry on an extra ten lines with the digits.
Now change '
TAB
N' to '
AT
21-N,N' stil using
LPRINT
. This time you will get just a single line of
symbols. The reason for the difference is that the output from the
LPRINT
is not printed straight away, but
arranged in a buffer store a picture one line long of what the computer will print when it gets round to it.
The printing takes place
(i) when the buffer is full,
(ii) after an
LPRINT
statement that does not end in a comma or semicolon.
(iii) when a comma or
TAB
item requires a new line.
or (iv) at the end of a program, if there is anything left unprinted.
(iii) explains why our program with
TAB
works the way it does. As for
AT
, the line number is ignored &
the
LPRINT
position (like the
PRINT
position, but for the printer instead of the television) is changed to the
column number. An
AT
item can never cause a line to be sent to the printer. (Actually, the line number
after
AT
is not completely ignored; it has to be between -21 & +21 or an error will result. For this reason it
is safest always to specify line 0. The item '
AT
21-N,N' in the final version of our program would be much
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