Commodore 64 Manual de usuario descargar pdf (Pagina 8)

access character data the ROM is switched in. It becomes an image in

the 16K bank of memory that the VIC-II chip is looking at. Otherwise,

the area is occupied by the I/O control registers, and the character ROM

is only available to the VIC-II chip.

However, you may need to get to the character ROM if you are going

to use programmable characters and want to copy some of the char-

acter ROM for some of your character definitions. In this case you must

switch out the I/O register, switch in the character ROM, and do your

copying. When you're finished, you must switch the I/O registers back in

again. During the copying process (when I/O is switched out) no inter-

rupts can be allowed to take place. This is because the I/O registers are

needed to service the interrupts. If you forget and perform an interrupt,

really strange things happen. The keyboard should not be read during

the copying process. To turn off the keyboard and other normal inter-

rupts that occur with your Commodore 64, the following POKE should be

used:

POKE 56334,PEEK(56334)AND254

(TURNS INTERRUPTS OFF)

After you are finished getting characters from the character ROM,

and are ready to continue with your program, you must turn the

keyboard scan back on by the following POKE:

POKE 56334,PEEK(56334)ORI

(TURNS INTERRUPTS ON)

The following POKE will switch out I/O and switch the CHARACTER

ROM in:

POKE l,PEEK(1)AND251

The character ROM is now in the locations from 53248-57343 ($DOOO-

$DFFF).

To switch I/O back into $DOOOfor normal operation use the following

POKE:

POKE l,PEEK(l)OR 4

106 PROGRAMMING GRAPHICS

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Commodore 64 Manual de usuario Pagina 8