Commodore 64 Uživatelský manuál stáhnout pdf (Strana 9)

STANDARD CHARACTER MODE

Standard character mode is the mode the Commodore 64 is in when

you first turn it on. It is the mode you will generally program in.

Characters can be taken from ROM or from RAM, but normally they

are taken from ROM. When you want special graphics characters for a

program, all you have to do is define the new character shapes in RAM,

and tell the VIC-II chip to get its character information from there in-

stead of the character ROM. This is covered in more detail in the next

section.

In order to display characters on the screen in color, the VIC-II chip

accesses the screen memory to determine the character code for that

location on the screen. At the same time, it accesses the color memory

to determine what color you want for the character displayed. The

character code is translated by the VIC-II into the starting address of the

8-byte block holding your character pattern. The 8-byte block is located

in character memory.

The translation isn't too complicated, but a number of items are com-

bined to generate the desired address. First the character code you use

to POKE screen memory is multiplied by 8. Next add the start of char-

acter memory (see CHARACTERMEMORYsection). Then the Bank Select

Bits are taken into account by adding in the base address (see VIDEO

BANK SELECTIONsection). Below is a simple formula to illustrate what

happens:

CHARACTER ADDRESS

= SCREEN CODE*8+(CHARACTER

S ET* 2048) + (BAN K* 16384)

CHARACTER DEFINITIONS

Each character is formed in an 8 by 8 grid of dots, where each dot

may be either on or off. The Commodore 64 character images are

stored in the Character Generator ROM chip. The characters are stored

as a set of 8 bytes for each character, with each byte representing the

dot pattern of a row in the character, and each bit representing a dot.

A zero bit means that dot is off, and a one bit means the dot is on.

The character memory in ROM begins at location 53248 (when the I/O

is switched off). The first 8 bytes from location 53248 ($DOOO)to 53255

($D007) contain the pattern for the @ sign, which has a character code

value of zero in the screen memory. The next 8 bytes, from location

PROGRAMMINGGRAPHICS 107

1 2 3 4 5 6 7 8 9 10 11 12 13 14 ... 82 83

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Commodore 64 Uživatelský manuál Strana 9