Day 17 - The Sprite Attribute Byte

The What?

	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