Understanding Sprites

[johnnystarr]

Here's a quick look at something else I don't quite get:

 

ScanLoop:
   sta WSYNC

; graphics updates go here

   lda SpriteLine
   cmp #5
   beq .skip
   lda #%01010101
   sta GRP0
   inc SpriteLine
.skip

   dey
   bne ScanLoop

   lda #%00000010
   sta WSYNC
   sta VBLANK
   ldx #30 ; setup 30 scan lines for overscan

 

So, what I'm trying to do, is load 5 lines of sprite data so that I can make a block.

 

SpriteLine is my variable and is initalized as 0. Every iteration I am comparing this to 5, if the zero flag is set, then I know it has reached 5 so i skip that drawing.

 

For some reason, the state of ScanLoop doesn't seem to matter because this is the output:

 

I can see that it is "working" when it comes to drawing to the screen, but it isn't working as it should.

Edited August 20, 2013 by johnnystarr

[+SpiceWare]

TIA is a scanline based video chip. If you set a register, such as GRP0, on one line and don't change it then that value will continue to be displayed on all following scanlines.

 

You're setting GRP0 to %01010101, but never setting it to any other value, so the strips are drawn down the screen.

 

You need to load GRP0 with 0 to make it stop displaying.

        lda SpriteLine
       cmp #5
       beq .skip
       lda #%01010101
       .byte $2c
.skip
       lda #0
       sta GRP0
       inc SpriteLine

 

$2c is the opcode for BIT $ABSOLUTE, so it "hides" the lda #0 by treating it as the $ABSOLUTE address. It doesn't affect the registers, but it does change the N, V and Z flags.

 

There's undocumented opcodes that can also be used. NOP $ABSOLUTE can be done using .byte $0c. Like the $2c, it'll "hide" the lda #0, but it won't change any registers OR flags.

[johnnystarr]

Hmm, I applied this code and I'm just getting a scan up and down. I must be doing something wrong here:

 

Maybe you could look at my source?

 

   processor 6502
   include "macro.h"
   include "vcs.h"
   
   ORG $F000
   
; constants
; variables
SpriteLine = $80

Start:
   sei
   cld
   ldx #$FF
   txs
   lda #0
.clearmem
   sta 0,x
   dex
   bne .clearmem
   
   lda #$23
   sta COLUBK
   
   ;setup sprite
   
   lda #$20
   sta COLUP0
   
; Main gameloop
Main:
   lda #%00000010
   sta VSYNC
   sta    WSYNC
   sta WSYNC
   sta WSYNC
   
   ; set timer to handle the rest of VBLANK draw
   lda #41
   sta TIM64T
   lda #0
   sta VSYNC
   
   ; main logic goes here
   ; this is where you put all your controller logic and score, etc.
   
   
   
   
   ; -----
   
WaitForVBlank:
   lda INTIM
   bne WaitForVBlank
   ldy #191
   sta WSYNC
   sta VBLANK
   sta WSYNC

   
ScanLoop:
   sta WSYNC
   
   ; graphics updates go here
   
   lda SpriteLine
   cmp #5
   beq .skip
   lda #%01010101
   .byte $2c
.skip
   lda #0
   sta GRP0
   inc SpriteLine

   dey
   bne ScanLoop
   
   lda #%00000010
   sta WSYNC
   sta VBLANK
   ldx #30 ; setup 30 scan lines for overscan
   
Overscan:
   sta    WSYNC
   dex
   bne Overscan
   jmp Main
   
   
Sprite:
   .byte #%00011000
   .byte #%00111100
   .byte #%00011000
   
   .org $FFFC
   .word Start
   .word Start

[+SpiceWare]

Go to the table of contents and read chapters 21, 22 and 23.

[johnnystarr]

Will do.

 

I have scanned over those chapters, but I was hoping to try to figure this out so I can get what I'm doing wrong.

 

I've tried a second attempt to no avail.

 

; in above code
   lda #5
   sta SpriteLine ; initialize to 5

ScanLoop:
   sta WSYNC

; --- begin graphics

   lda SpriteLine    ; load current state
   beq .skip		 ; if is 0, branch to skip
   lda #%01010101    ; line pattern
   sta GRP0		  ; store
   dec SpriteLine    ; decrement the counter
   jmp .done		 ; jump past the skip portion

.skip
   lda #0		    ; load 0
   sta GRP0		  ; store in sprite register
.done

 

The output is just my background. When i change "beq .skip" to "bne .skip" I get the strip down the screen again.

Edited August 20, 2013 by johnnystarr

[Joe Musashi]

It might help to have a look a this example, which shows how to move a sprite vertically, i.e., how to turn it on and off on the respective lines.

 

It's made for the ball, but it would be easy to adapt it for the players.

[shazz]

why not changing to this ?

 

ScanLoop:
sta WSYNC

; graphics updates go here

lda SpriteLine
cmp #5
bne .cont
lda #0


.cont:
sta GRP0
inc SpriteLine

dey
bne ScanLoop

 

it displays the funny pattern

[johnnystarr]

Yeah, that fractal-ish pattern is neat, but not the desired outcome either. I will have to read up on the 2600 for newbies guide and try to fully understand this before I try to come up with my own routine.

 

I guess I have to "crawl" before I can "run"