AMSTRAD CPC464 User Manual page 63

Similarly the displayed character has nothing to do with ' writing' the letter on the screen, it' s all
about numbers.
1.2.4
Please don' t worry if you do not understand all the technicalities or jargon used on this page. It is
important to fully explain how the computer deals with your instructions and comes up with the
required results, but it is also likely that only the technically minded will fully appreciate the
explanation, Ifyou find this section heavy going, skip to section 1.2.5.
For example, the code for the letter A is 97. The computer doesn' t understand 97 either, and this
number has to be translated from the human decimal code into a code that the computer can relate
to - it' s generally referred to as machine code, and the principles underlying this aspect of the
machine are covered in APPENDIX II.
At first, the translation from the decimal number notation we are used to in every day life to the
hexadecimal notation of the computer will seem heavy going. Thinking of numbers that are based on
the ten unit is so natural that to do otherwise is like trying to eat with your knife and fork in the
opposite hands.
A similar degree of mental dexterity must be aquired to understand hex notation (as HEXadecimal is
abbreviated), but once you do, many things about computing will fall into place and the elegant
structure of the numbering system will become apparent.
Once the computer has translated the striking of the A key into the type of number it understands, it
looks into that part of the memory indicated, and the result is another series of numbers that define
the character. That is to say the character you see displayed on your screen is built up from a block
of data, stored in memory as a numeric matrix:
Figure 5: A blank character grid Lower case a Upper case A
The elements of the matrix are rows and columns of dots. The character is displayed by turning the
required sequence of dots on or off - each dot is determined by data stored in the computer' s memory.
There are 8 rows and 8 columns in each character cell on the CPC464 display and if you don' t find a
character you want in the set of 256 that are provided, then you can redefine your own characters
using the instruc-tions that are given using the keywords SYMBOL and SYMBOL AFTER in
Chapter 8.
User defined characters can be made up using any combination of 0 to 64 dots, so the complete
character set that uses all possible combinations of this matrix would comprise many more different
elements (characters). Add to this the fact that you can group blocks of elements together to form
larger block characters, and possibilities for user-defined characters are only limited by your time
and ingenuity.