The Sprite Attribute Byte (SAB) is the third (of 4bytes) byte that you write
to OAM (SPR-RAM). This byte controls several (4) things including:
Bits 0-1: 2 Bits of the sprites colors. Will be discussed at a later time.
Bits 2-4: UNUSED.
Bit #5: 0 = Sprite in front or 1 = Sprite in back of BG.
Bit #6: 1 = Flip horizontally.
Bit #7: 1 = Flip vertically.
See's see how to do this!
Gettin' Goin'
When I demonstrate each bit (with code peices) I'll give 2 pieces, of
code, one for the $2003/$2004 method and one for the Sprite#-BYTE (DMA) method.
To flip a sprite horizontally, you just need to set bit #6 of the special byte
of OAM data (the third byte written). Assuming you have your special byte in A (you
have to, to be able to do bit operations & math):
eor #%01000000 ; will toggle bit #6 in A.
sta $2004 ; this assumes that you already entered 2 bytes to $2004
Now the DMA method:
lda Sprite1_S
eor #%01000000
sta Sprite1_S ; need to store it back so next time the correct thing happens.
sta $2004
As you see, it's not much different.
To flip a sprite vertically, you need to toggle bit #7 of the special byte.
Gettin' Goin'
Putting sprites behind backgrounds is always cool if you do it right,
like the Warp Whisle (Whistle?) in Super Mario Bros. 3 ((c)1988? Nintendo, wow I was
1 year old in 1988...) you have to get BEHIND the background and make to the end of
the stage to get the whisle, remember that?
Well, Bit #5 of the special byte is probably how that was done. Just make Bit #5
a 1 to have your sprite be drawn under/behind the background. Example of toggling bit
#5:
eor #%00100000
That will toggle bit #5, so you can use that instead of the eor in the previous code.
One Last Thing
Remember that you can always toggle more than one bit at a time.
This Day In Review
Hopefully I'll get around to talking about color attributes tomorrow.
(Bits 0-1 of special byte and the BG Attribute Tables).
Hope you liked this!,
-Mike H a.k.a GbaGuy
P.S. Could someone email me to tell me if this works or not, please?
Intro - Day 18