Commodore 64 Uživatelský manuál stáhnout pdf (Strana 35)

bits) needed to define a sprite. The 63 bytes are arranged in 21 rows of

3 bytes each. A sprite definition looks like this:

BYTE0

BYTE3

BYTE6

BYTE1

BYTE4

BYTE7

BYTE2

BYTE5

BYTE8

BYTE 60 BYTE 61

BYTE 62

Another way to view how a sprite is created is to take a look at the

sprite definition block on the bit level. It would look something like Figure

3-2.

In a standard (HI~RES)sprite, each bit set to 1 is displayed in that

sprite's foreground color. Each bit set to 0 is transparent and will display

whatever data is behind it. This is similar to a standard character.

Multi-color sprites are similar to multi-color characters. Horizontal

resolution is traded for extra color resolution. The resolution of the sprite

becomes 12 horizontal dots by 21 vertical dots. Each dot in the sprite

becomes twice as wide, but the number of colors displayable in the

sprite is increased to 4.

SPRITE

POINTERS

Even though each sprite takes only 63 bytes to define, one more byte

is needed as a place holder at the end of each sprite. Each sprite, then,

takes up 64 bytes. This makes it easy to calculate where in memory your

sprite definition is, since 64 bytes is an even number and in binary it's an

even power.

EC'ch of the 8 sprites has a byte associated with it called the SPRITE

POINTER.The sprite pointers control where each sprite definition is lo-

cated in memory. These 8 bytes are always located as the last 8 bytes

of the 1K chunk of screen memory. Normally, on the Commodore 64,

this means they begin at location 2040 ($07F8 in HEX). However, if you

move the screen, the location of your sprite pointers will also move.

Each sprite pointer can hold a number from 0 to 255. This number

points to the definition for that sprite. Since each sprite definition takes

64 bytes, that means that the pointer can "see" anywhere in the 16K

block of memory that the VIC-II chip can access (since 256*64= 16K).

PROGRAMMING GRAPHICS 133

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Commodore 64 Uživatelský manuál Strana 35