Changing Background AND Playfield on a per scanline basis

[johnon]

Okay, so right now, I'm trying to create a scene. Just a basic scene, and then work from there.

 

My problem right now is that no matter what I do, I can't seem to get them both to change when I want them to. The playfield always ends up screwing something up.

 

I've tried all sorts of syncing up in the middle of the background changes, etc, and nothing.

 

I'm just completely lost. Once, it changed my playfield color to purple, screwed up the playfield and then continued on to cause one of the noises to activate continuously. I couldn't tell you how. I removed the code after that and tried again.

 

In any case, here's the code without the playfield included.

 

	
       processor 6502

include "vcs.h"

include "macro.h"

org $F000

Start

sei

cld

ldx #$FF

txs

lda #0

ClearMem

sta 0,x

dex

bne ClearMem

StartOfFrame

lda #000000

sta CTRLPF

lda #0

sta VBLANK

lda #2

sta VSYNC

sta WSYNC

sta WSYNC

sta WSYNC

lda #0

sta VSYNC

ldx #0

VerticalBlank

sta WSYNC

inx

cpx #37

bne VerticalBlank

ldx #0



BackgroundTop

sta WSYNC

lda #$96

sta COLUBK

inx

cpx #64

bne BackgroundTop

BackgroundMid

sta WSYNC

lda #$9A

sta COLUBK

inx

cpx #128

bne BackgroundMid

BackgroundBot

sta WSYNC

lda #$9E

sta COLUBK

inx

cpx #192

bne BackgroundBot

; insertion point

lda #%01000010

sta VBLANK

ldx #0


Overscan

sta WSYNC

inx

cpx #30

bne Overscan

jmp StartOfFrame


org $FFFC
.word Start
.word Start

And it works fine...for the background only.

 

 

At the place labelled insertion point, what I tried doing was:

 

CloudTop

sta WSYNC

lda #$0E

sta COLUPF

cpx #2

bne CloudTop

lda #%01100000

sta PF0

 

And adding that, as is, completely wipes my background and playfield and causes the entire screen to be the color of the bottom segment of the background.

 

It's a newbie question, I know, and I may be missing something huge with the syntax here, so I'm asking for help.

 

EDIT: Would it be possible to get the playfield working in a separate .asm file, and then have it included somewhere? Idk...I'm so lost.

Edited April 1, 2010 by johnon

[Omegamatrix]

You are drawing the entire "visible" portion of the screen before the playfield. The playfield you will see therefore will be right at the very bottom of the screen.

 

The real problem is the X register. It increases all the way to 192 before the insertion piece, but once there you don't change it all. At that point the code gets stuck in an infinite loop as X=192, and that never changes, so X can never equal 2.

 

Also you can make seperate files if you wish. You just have to add an "include filexxx.x" at the top of your code where include vcs.h and such are. "filexxx.x" needs to be changed to the actual filename and extension of course.

[johnon]

You are drawing the entire "visible" portion of the screen before the playfield. The playfield you will see therefore will be right at the very bottom of the screen.

Ah, so what needs to go where in terms of order?

 

The real problem is the X register. It increases all the way to 192 before the insertion piece, but once there you don't change it all. At that point the code gets stuck in an infinite loop as X=192, and that never changes, so X can never equal 2.

There was a "ldx #0" in there after BackgroundBot, but it must have gotten cut out. *shrugs* Didn't change anything, as far as the code was concerned. =\

 

Also you can make seperate files if you wish. You just have to add an "include filexxx.x" at the top of your code where include vcs.h and such are. "filexxx.x" needs to be changed to the actual filename and extension of course.

Yeah, I just figured if I could get the playfield to work in one file and the background to work in another, then somehow I could mesh them together once I get order correct.

 

*shrugs*

 

I did quite a bit in C# before attempting this. This is a major change and I'm used to separated files with specific purposes and all sorts of other syntax, etc.

[Omegamatrix]

Ah, so what needs to go where in terms of order?

I'm just saying you're putting your playfield kinda low. It's reaching the point where it is almost off of the screen.

 

There was a "ldx #0" in there after BackgroundBot, but it must have gotten cut out. *shrugs* Didn't change anything, as far as the code was concerned. =\

You're on the right track now! Look at the loop again for "CloudTop". You still need to change X inside of that loop with an INX, or else it will always remain at 0.

 

I did quite a bit in C# before attempting this. This is a major change and I'm used to separated files with specific purposes and all sorts of other syntax, etc.

You're doing fine.

[johnon]

You're on the right track now! Look at the loop again for "CloudTop". You still need to change X inside of that loop with an INX, or else it will always remain at 0.

*facepalm*

 

Is that really what I missed?

 

*double-checks*

 

Thanks man. I thought there was some intricate thing I missed. Apparently I really just need to learn to proofread better. >.>

[+SpiceWare]

Are you using Stella to test with? If so hit ~ to go into the debugger. I suspect you would have realized the INX was missing after stepping thru the loop a couple times.

[johnon]

Are you using Stella to test with? If so hit ~ to go into the debugger. I suspect you would have realized the INX was missing after stepping thru the loop a couple times.

Yes, I am, actually.

 

Thanks for the advice! ^^