Question 1:
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I am trying to apply the description of the PPU from Loopy to my code,
and I can't figure out why it's doing what it is doing. As an example, if
I wait for VBLANK, then write something to PPU memory via $2007 I have
modified the "v" and "t" registers loopy describes. Lets say I wrote
something to the image palette, so both "v" and "t" would be equal to
$3F10. When VBLANK is over, Loopy says "v=t", which in this case puts
$3F10 in "v". If the PPU uses the "v" register for read from the nametable,
how come the name table data is not read from $3F10 in this case?


The answer is in fact staring you in the face. ;)
When the PPU reads name table data, it doesn't use the value in the VRAM
register as linear index. Rather it only uses a few bits from the VRAM
register, and retranslates the addresses into an address that only has
enough width (in bits) to index into the name table.

"you can think of bits 0,1,2,3,4 of the vram address as the "x scroll"(*8)
that the ppu increments as it draws. as it wraps from 31 to 0, bit 10 is
switched. you should see how this causes horizontal wrapping between name
tables (0,1) and (2,3)."

"you can think of bits 5,6,7,8,9 as the "y scroll"(*8). this functions
slightly different from the X. it wraps to 0 and bit 11 is switched when
it's incremented from _29_ instead of 31. there are some odd side effects
from this.. if you manually set the value above 29 (from either 2005 or
2006), the wrapping from 29 obviously won't happen, and attrib data will be
used as name table data. the "y scroll" still wraps to 0 from 31, but
without switching bit 11. this explains why writing 240+ to 'Y' in 2005
appeared as a negative scroll value."


Question 2:
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>When you overwrite "t" by writing to $2005/$2006 you have lost your
>old vertical and horizontal scroll values. Does this mean I have to
>reload the scroll values on every vblank?

correct.. anytime you write to $2006 (for writing to vram or whatever),
you've just trashed your scroll values and have to reload them. if you don't
write to $2005/6 between frames, your scroll values shouldn't need to be reloaded.


Question 3:
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I can give a pretty good explanation of how the scrolling works...

First of all, the scroll should only be updated during the vblank.
The first value written to the register($2005) is the horizontal scroll value.
This can be in the range of 0 - 255.
The second value written to the register is the vertical scroll value.
This can be in the range of 0 - 239.

These are direct values. What I mean is, if you scroll the screen past
255 (wrapping back around to 0) horizontally, the screen will reset back to
the left edge. This isnt what we want. In order to fix this, we must switch
nametables AS SOON AS the scroll value resets back to 0. Once we switch
nametables, we can continue scrolling again with no problem - just toggle back
and forth between nametables each time the scroll wraps around.

Here is the scrolling code from Ultima 2:

;scroll THE SCREEN
LD9D2: 		

	LDA <$FF ;GET CONTROL REGISTER 1

	LSR A ;SHIFT OUT NAMETABLE NUMBER
	TAX ;SAVE TEMPORARILY

	LDA hscroll1 ;HORIZONTAL scroll DIVIDED BY 8

	ASL A
	ASL A
	ASL A ;MULTIPLY BY 8 TO GET THE CORRECT VALUE
	ORA hscroll2 ;ADD IN THE MISSING LOWER 3 BITS (0 - 7)
	STA hscroll ;THIS IS THE REAL HORIZONTAL scroll VALUE

	TXA ;GET BACK CONTROL REGISTER 1 
	ROL A ;THE CARRY IS THE NAMETABLE NUMBER WE WANT
	STA <$FF ;NOW WE HAVE THE CORRECT NAMETABLE SET

	LDA vscroll1 ;VERTICAL scroll VALUE DIVIDED BY 8

	ASL A
	ASL A
	ASL A ;MULTIPLY BY 8 TO GET THE CORRECT VALUE
	ORA vscroll2 ;ADD IN THE MISSING LOWER 3 BITS 
	STA vscroll ;NOW WE HAVE THE REAL VERTICAL scroll VALUE

	RTS
	
Later during the vblank, the program writes HSCROLL followed
by VSCROLL to $2005.

Actually, looking over this code it seems like you can write values higher
than 239 to the vertical scroll.

I hope this helps anyone.


Question 4
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How should I write scrolling stuff to $2005?

Like this:

ScrollScreen:

.vwait:	
	lda $2002    ;wait for vblank
	bpl .vwait
	rts

	lda ScrollH	;Horitontal scroll
	sta $2005

	lda ScrollV	;Vertical scroll
	sta $2005

	lda ppuReg0	;reg $2000 with correct name table set
	sta $2000
	rts


Question 5
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Easy way to flip name tables:

HotizontalFlip:
	lda #%00000001
	eor ppuReg0	;copy of $2000, since $2000 can't be read.
	sta ppuReg0
	rts

VerticalFlip:
	lda #%00000010
	eor ppuReg0	;copy of $2000, scine $2000 can't be read.
	sta ppuReg0
	rts

Afterwards copy ppuReg0 to $2000 after writing you scroll values.