AMSTRAD CPC464 User Manual page 59

Some of the characters that you will see are not directly accessible by pressing the keys on the
keyboard, but only available for display using the PRINT CHR$(<number>) statement described
later on in this guide.
This is because each element stored in the computer is stored in the unit of data known as the ' byte' -
and as you will see if you work through Appendix II, a byte has 256 different possible combinations
of value. But as the computer has to use at least one whole byte per character stored (whether we
want it to or not, it' s the smallest denomination that the CPC464 appreciates), we might as well use
all 256 possible combinations,,rather than simply be satisfied with the 96 or so standard characters
that are printed on most typewriters - and throw away the spare 160 possibilities.
The ' standard' range of characters is known as a ' subset' . It is classified throughout the computer
world as the ' ASCII' display system, a term derived from:
American
Standard
Code for
Information
Interchange
. ..it' s primarily a system that ensures the data sent from one computer to another is in a
recognisable form. It' s about the only aspect of computing that is truly universal, so we strongly
advise that you become familiar with all aspects of ASCII. Appendix III lists the ASCII display
range, together with the additional characters available on the CPC464 and their numeric codes.
Some of the other ' unprintable' characters can be displayed by using a combination of the CONTROL
(marked on the keyboard as [CTRL]) key and the other keys on the keyboard - but don' t worry about
that just yet, since until you understand the control key function, you can do more harm than good
by testing it at random.
1.2.2
To see exactly what these characters look like on the screen for yourself, type in the following
program, and we' ll exercise your curiosity and the CPC464. This program will also help establish
your confidence in both the simplicity of programming, and the fact that as long as you can get to the
' wake up' message on switching on, then you are not likely to encounter any further ' hardware'
problems or misunderstanding, and the CPC464 is simply waiting to be programmed with the right
information to process.
(If you make an error when you are typing this program, skip to 1.2.7 to see if you can correct the
error without needing to restart entering the program from scratch.) When typing the program on
the next page into the computer, it doesn' t matter if you use the lower case letters (a b c >, or the
[SHIFT]' ed uppercase letters (A B C), the computer will sort out the following program either way.
You MUST delimit the words using spaces or other delimiters (commas, colons etc. as appropriate) at
the positions shown, since AMSTRAD BASIC permits the use of the reserved words (fully listed in
Appendix VIII) within variable names.