Developing NES games with C language (cc65) 09, scrolling

Paste the CSDN The format is not adjusted , If you want to see a good format, click below

https://happysoul.github.io/nes/nesdoug/

 

Scrolling means background scrolling , Elves are not affected .

 

NES PPU There is a scroll register ,$2005.

Write this address 2 Time , The first is X rolling , The second, Y rolling . This is v-blank Another thing that needs to happen during , from neslib Automatic processing .neslib There's this function scroll(x,y), You need to pass it to him XY Value .X Screen shift left ,Y Move the screen up .

 

I don't like Y Scrolling mode ,Y The way of scrolling is strange ,0-$EF Is the actual location ,$F0-$FF It's negative , This is not what we want . If Y>$EF,neslib It will be subtracted $F0

(and assumes that you are going to manage the maximum at $1df. I can't translate this ... The latter is machine translation ) And assume that you will $ 1df Manage maximum

 

I deal with scrolling in a different way than others .

set_scroll_x(x) and set_scroll_y(y)

y You can use any int value , High byte can tell you where you are nametable in ,even Means top ,odd Represents the bottom .

Do you have 2 Map ,even= Use the first ,odd Use the second .

（ This part of the translation is a little strange , Come back and see 、 How do you feel at the moment even and odd It should mean even number and odd number ）

 

Our code still needs to be adjusted Y Each change in to skip bytes $ f0-ff Area , Because our screen only 240 High pixel . I wrote some functions to do this for us .

 

add_scroll_y(add, old y) y add add After the value of .

sub_scroll_y(add, old y) y subtract add After the value of .

 

We pass the above return value to set_scroll_y(y) To scroll the map .

 

 

Before testing the above functions , Let's talk about mirroring first

 

scroll_y The initial value of 0xef, add 5 in the future , The new value scroll_y = 0x104

 

 

scroll_y = add_scroll_y(5, scroll_y);

 

 

scroll_y The initial value of 0xef, add 0x15 in the future , The new value scroll_y = 0x114

 

 

scroll_y = add_scroll_y(0x15, scroll_y);

 

 

Subtraction

scroll_y The initial value of 0x104, subtract 5 in the future , The new value scroll_y = 0xef

 

 

scroll_y = sub_scroll_y(5, scroll_y);

 

 

skip 0xf0 - 0xff Invalid values in this interval .

 

Horizontal scrolling （ Vertical mirror ）

I mentioned in the first introduction page ,NES Of VRAM Just enough 2 individual nametables Use , If you set it to Vertical mirroring（ Vertical mirror ）, It needs to be in crt0.s Medium ines Set the image type in the header , This is actually .cfg Link symbols in files “NES_MIRRORING”. A solder joint in the real cassette will permanently set the cassette to horizontal or vertical mirror mode .

 

Vertically mirrored nametables It's like this

 

 

0 1

0 1

 

 

nametables Of 2 and 3 In fact, that is 0 and 1 Copy （ Mirror image , The memory value is the same ）

 

It is useful to use horizontal scrolling . If it doesn't change Y value , You can scroll to the right , We can make changes in the area outside the screen that users can't see . The following is the code and screenshot of the vertical image , You can scroll freely through the arrow keys , And see the following 2 individual nametables It's the image above .

 

The numbers on the screen are sprites .

 

 

https://github.com/nesdoug/10_Scroll_H/blob/master/scroll_h.c

 

https://github.com/nesdoug/10_Scroll_H

 

 

Scroll vertically （ Horizontal mirror ）

 

Similar to vertical mirror , Except the right 2 individual nametables It's a copy on the left （ Mirror image ）

 

 

 

0 0

2 2

 

 

 

Scroll vertically ,X Don't change , You can scroll up or down , The invisible area of the user outside the screen can be modified

 

 

https://github.com/nesdoug/11_Scroll_V/blob/master/scroll_v.c

 

https://github.com/nesdoug/11_Scroll_V

 

 

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There is another kind. 4 Screen mode , But few games are used , He needs a special mapper mapper , That is, there is extra in the cassette RAM chip

for example ： Iron gloves (Gauntlet) and Rad Racer II

 

 

This is a screenshot I got from Sohu Video

https://tv.sohu.com/v/dXMvNjMzMDk3MjMvMjk3MzkyMzkuc2h0bWw=.html