Screen Position with Scrolling, Sprites and 4X

[17BitRetro]

Hi all,

I'm attempting to put an AGA game together. I've settled with a 256 wide, dual playfield (16x16cols) screen, with 8 Sprites and FMode 4x @50hz. It's working out OK, plenty of raster time left, nice and smooth scrolling etc.. but one thing that really bugs me is the shift to the right with the whole display.

Reading the forums here I've played around with recreating a PAL screen mode (with BEAMCON0, HSSTRT...etc) and have managed to adjust the position with a shift back to the left on my old TV / A1200 ...but it doesn't align when using OSSC, and needs different settings for WinUAE. .. and is mega slow in FS-UAE.. which makes me think that this isn't the way to do it.

There might be a trick I'm missing, or I'm running into HW limitations and should just accept the shift without the BEAMCON0 settings, or drop the FMode back. Any comments appreciated. My AGA chipset knowledge is lacking!

DDFSTRT=$38, DDFSTOP=$A0, DIWSTRT=$2CC1, DIWSTOP=$1CC1

[Rock'n Roll]

Hmm?
DDFSTRT=$38, DDFSTOP=$A0, DIWSTRT=$2CC1, DIWSTOP=$1CC1 ; => DDFSTRT=$58, DDFSTOP=$D0
width=256, height=240

try this:
DDFSTRT=$48, DDFSTOP=$C0, DIWSTRT=$2CA1, DIWSTOP=$1CA1
DDFSTRT=$40, DDFSTOP=$B8, DIWSTRT=$2C91, DIWSTOP=$1C91

[Rock'n Roll]

How to calculate the correct ddfstrt, ddfstop values for fmode 1,2,3 (2x, 2x, 4x) for any screenposition and resolution?

I read anywhere:

fmode=0 (normal 16-bit fetch)
        dc.w    ddfstrt,$0038
        dc.w    ddfstop,$00d0
fmode=$3 (64-bit fetch)
        dc.w    ddfstrt,$0038
        dc.w    ddfstop,$00a0

What about fmode=$1,2 (32-bit fetch)?

[Rock'n Roll]

I search for a general mathematical formula to calculate the ddf-values especially for FMODE based on DIWSTRT/DIWSTOP, like this:

DIWSTRT=$2c81
DIWSTOP=$2cc1

OCS/ECS
lowres:
DDFSTRT=(DIWSTRT-(16px)-1)/2
DDFSTRT=($81-(16)-1)/2=56=$38

hires:
DDFSTRT=(DIWSTRT-(8px)-1)/2
DDFSTRT=($81-(8)-1)/2=60=$3C

AGA
lowres:
DDFSTOP=((DIWSTOP-(fetch_width)-1)/2)-8
DDFSTOP=(($1c1-(1x16px)-1)/2)-8 =208=$d0 ; 1x (fetch=0) 16Bit ; same value for 2x (fetch=1,2) 32Bit
DDFSTOP=$c8 ; 4x (fetch=3) 64Bit ; 8px earlier possible

hires:
DDFSTOP=((DIWSTOP-(fetch_width)-1)/2)-4
DDFSTOP=(($1c1-(1x16px)-1)/2)-4 =212=$d4 ; 1x (fetch=0) 16Bit ; same value for 2x (fetch=1,2) 32Bit
DDFSTOP=$cc ; 4x (fetch=3) 64Bit ; 8px earlier possible

My testresult is, only for a fetch=3 the DDFSTOP value can be set 8 pixels earlier.

remark: all DDF-results are to align like this:
lowres: DDFSTRT & $FFF8
hires: DDFSTRT & $FFFC

Maybe someone can check and confirm this or and can give more information about general DDFSTRT/DDFSTOP calculation (with FMODE)?

[a/b]

OCS/ECS/AGA DMA fetch calcs with C code in this thread, and if you prefer optimized asm it's on the last page:
https://eab.abime.net/showthread.php?t=111329

[Rock'n Roll]

perfect, now I have something to read.