Commodore 64 Manual de usuario descargar pdf (Pagina 3)

GRAPHICS LOCATIONS

Some general information first. There are 1000 possible locations on

the Commodore 64 screen. Normally, the screen starts at location 1024

($0400 in HEXadecimal notation) and goes to location 2023. Each of

these locations is 8 bits wide. This means that it can hold any integer

number from 0 to 255. Connected with screen memory is a group of

1000 locations called COLOR MEMORY or COLOR RAM. These start at

location 55296 ($D800 in HEX) and go up to 56295. Each of the color

RAM locations is 4 bits wide, which means that it can hold any integer

number from 0 to 15. Since there are 16 possible colors that the Com-

modore 64 can use, this works out well.

In addition, there are 256 different characters that can be displayed

at any time. For normal screen display, each of the 1000 locations in

screen memory contains a code number which tells the VIC-II chip which

character to display at that screen location.

The various graphics modes are selected by the 47 CONTROL regis-

ters in the VIC-II chip. Many of the graphics functions can be controlled

by POKEing the correct value into one of the registers. The VIC-II dhip is

located starting at 53248 ($DOOOin HEX)through 53294 ($D02E in HEX).

VIDEO BANK SELECTION

The VIC-II chip can access ("see") 16K of memory at a time. Since

there is 64K of memory in the Commodore 64, you want to be able to

have the VIC-II chip see all of it. There is a way. There are 4 possible

BANKS (or sections) of 16K of memory. All that is needed is some means

of controlling which 16K bank the VIC-II chip looks at. In that way, the

chip can "see" the entire 64K of memory. The BANKSELECT bits that

allow you access to all the different sections of memory are located in

the 6526 COMPLEXINTERFACEADAPTERCHIP #2 (CIA #2). The POKE

and PEEK BASIC statements (or their machine language versions) are

used to select a bank by controlling bits 0 and 1 of PORT A of CIA#2

(location 56576 (or $DDOOHEX». These 2 bits must be set to outputs by

setting bits 0 and 1 of location 56578 ($DD02.HEX) to change banks. The

following example shows this:

POKE 56578,PEEK(56578)OR 3 :REM MAKE SURE BITS 0 AND 1 ARE

SET TO OUTPUTS

POKE 56576,(PEEK(56576)AND 252)OR A:REM CHANGE BANKS

"A" should have one of the following values:

PROGRAMMING GRAPHICS 101

1 2 3 4 5 6 7 8 ... 82 83

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Commodore 64 Manual de usuario Pagina 3