Memoirs of an X register - Still Reading - Player Missile Graphics Stuff

[RSS Bot]

Still reading...and I've come across something in De Re Atari that has me a bit curious. I have cross posted this to the forum, but, I am posting it here too for my journal....

 

In the book De Re Atari, Chris Crawford talks about moving the player vertically through memory. It's very straightforward...

 

QUOTE

VERTICAL MOTION

 

Animating this image is very easy. Vertical motion is obtained by moving the image data through the player RAM. This is, in principle, the same method used in playfield animation, but there is a big difference in practice; the move routine for vertical motion is a one-dimensional move instead of a two-dimensional move. The program does not need to multiply by 40 and it often does not need to use indirection. It could be as simple as:

 

CODE

LDX $01

LOOP LDA PLAYER,X

STA PLAYER-1,X

INX

BNE LOOP

 

 

This routine takes about 4 milliseconds to move the entire player, about half as long as the playfield animation routine which actually moves only 50 bytes where this one moves 256 bytes. If high speed is necessary, the loop can be trimmed to move only the image bytes themselves rather than the whole player; then the loop would easily run in about 100-200 microseconds. The point here is that vertical motion with players is both simpler and faster than motion with playfield objects.

 

of special note, is that next to last sentence in the paragraph.

 

If high speed is necessary, the loop can be trimmed to move only the image bytes themselves rather than the whole player; then the loop would easily run in about 100-200 microseconds.

 

What is he talking about there?

 

-Thom

 

http://www.atariage.com/forums/index.php?a...&showentry=1602