AMSTRAD cpc 6128 User Instruction page 390

(Each character has a corresponding number, and these are listed in part 3 of the
chapter entitled 'For your reference .... '.)
Similarly the displayed character has nothing to do with 'writing' the letter on the
screen; once again it's all about numbers.
For example, the ASCII code for the letter A is 97. The computer doesn't understand
97 either (awkward blighter, eh?), and this number has to be translated from the
human decimal code into a code that computer can relate to - it's generally referred to
as 'machine code', and the principles underlying this aspect of the machine are
discussed earlier in this chapter.
At first, the translation from the decimal number notation we are used to in everyday
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 degree of mental dexterity must be acquired to understand hex notation, but once
you do, many things about computing will fall into place and the elegant structure of
the numbering system will become apparent.
If you are unsure about the binary and hexadecimal numbering systems, we suggest
that you thoroughly read part 1 of this chapter (if you have not already done so).
Once the computer has translated the pressing 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. Hence the character that you see
displayed on your screen, is built up from a block of data, stored in the memory as a
numeric 'matrix':
A BLANK CHARACTER
MATRIX (GRID)
LOWER CASE
a
UPPERCASEA
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
matrix or 'cell' on the 6128 display, and if you don't find a character you want out of
the set of 255 that are provided, then you can re-define your own characters using the
keyword S Y M B 0 L described later in this section.
At your leisure .... Chapter 9 Page 17