Commodore Plus 4 - Page 212

800 Programming Graphics
Custom Character Sets
The characters you see on your computer monitor or TV are formed by a pattern
of dots. Each character occupies a cell eight dots (pixels) wide and eight dots high.
Each dot is either on or off. Because each dot is individually controlled, the
graphics chip is said to be in high-resolution mode. The pattern of dots that are on
form the character you see. The patterns for the built-in characters are stored in
your computer on a permanent memory chip (the character ROM).
Each dot in the character's pattern is represented by one bit in the character
ROM. Since there are 8 bits in one byte (or memory location), it takes 8 bytes to
specify each character's pattern. Following is a diagram illustrating the character
C's 8 x 8 cell.
Bit Values
128 64 32 16 8 4 2 1
« 0 ,
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
byteO
byte 1
byte 2
byte 3
byte 4
byte 5
byte 6
byte 7
The character C is represented by 8 bytes of data calculated by adding up the bit
values of the bits it has on. For example, byte 0 is 32 + 16 + 8 + 4 = 60 ($3C).
There are two sets of built-in characters. The first is upper case/graphics and
the second is upper/lower case. You can switch between the two by pressing the
SHIFT and IS keys simultaneously.
When the built-in characters are not sufficient, you can set up a custom
character set. That is, the graphics chip can be instructed to get its character
patterns not from the character ROM, but from an area of user memory (RAM).
A character set consists of 128 characters (8 bytes each) and resides in IK (1024
bytes) of memory. (See the machine language section of this chapter for informa
tion on expanding the character set to 256 characters.)
Two locations control where the character patterns come from: 65298 ($FF12),
which controls whether the patterns come from ROM or RAM, and 65299
($FF13), which controls what locations in memory the patterns come from.
When you switch between upper case/ graphics and upper/lower case with the
SHIFT and Q keys, you are changing the value of 65299, which controls where
the patterns come from. You can disable the £8 and SHIFT key combination by
POKEing the value 128 into the memory location 1351, or by PRINTing a
CHR$(8). To reenable this feature, POKE a 0 into 1351, or PRINT a CHR$(9).