missing the last 8 lines of my overscan display

[jotd]

Hi,


I want a display that is 288 lines high. Using AGA fmode 3

    lea        _custom,a5
    move.w #0x1C81,diwstrt(a5)
    move.w #0x30D1,diwstop(a5)
    move.w #0x0038,ddfstrt(a5)
    move.w #0x00D0,ddfstop(a5)
    move.w #0x0210,bplcon0(a5)  | 8 bitplanes
    move.w #3,fmode(a5)          | 8 bitplanes
    
    clr.w bplcon1(a5) 
    clr.w bplcon2(a5)
    move.w    #-8,d0
    move.w d0,bpl1mod(a5) 
    move.w d0,bpl2mod(a5)

Works but I only get 280 lines, see the black lines on the bottom (where lives are supposed to be displayed)





I've tried playing with diw/ddf params without luck (I don't understand really how all that stuff works, it's magic to me most of the time)

[hooverphonique]

The vertical display window values are straight forward, and if you set diwstart to 1C and stop to 3C, you should get 288 vertical lines.
I just subtracted 16 from the default start of 2C, and added 16 to the default stop of 2C (which actually means 12C).
The datafetch values do not need change when changing the vertical display window values.

[a/b]

312 lines, $000-$137, so you should set the vertical end to $(1)38 instead of $(1)30.

[ross]

Without recurring to programmed mode you have 'only' 286 visible line in PAL mode.

min DDFSTRT=$1Axx

max DDFSTOP=$38xx

Technically there would also be line $138 (where however you can only use COLOR00) plus two half lines at the start and end of view (covered by blanking).

You can also use lower values for DDFSTRT and the data is fetched but of course nothing is visible before line $1A.
For DDFSTOP, however, there is a hard stop on line $138 and the fetch is not performed.

[jotd]

thanks! that's better. But WinUAE maxes the display at 283 lines now. Is that a known issue? 286 lines would be already very good.

[ross]

Originally Posted by jotd

thanks! that's better. But WinUAE maxes the display at 283 lines now. Is that a known issue? 286 lines would be already very good.

Because it is conservative, in a standard 'old' device you cannot view this many lines (this is different in modern displays).
It saves you from having exaggerated borders in normal games/demos in the default window.

Set 576 y in Host/Display for full Overscan.

[jotd]

already better. Thanks

[Photon]

This article on maximum overscan is the result of testing on real equipment, which has a paragraph on display types.

There's also a related article on centering and one discussing aspect ratio.