OS3.x - Open a screen with 1x bandwidth

[dissident]

Again something special.

Is there a smart way to open a screen on OS3.x with the 1x bandwidth?

The only solution I know is poking in the graphics base like this:

  move.l	_GraphicsBase,a6
  move.b	#BANDWIDTH_1X,gb_MemType(a6)

And after that open the screen. But you have to disable multitasking with Forbid() to be sure that the bandwidth won't be changed.

Not very system friendly. I did some investigations in the autodocs, but did not find a way to change the bandwidth, after the screen is opened or before with a special tag. Looking in the vieport/viewport extra structure didn't help.

[dissident]

Do you have any hints for me, Thomas Richter?

[Ernst Blofeld]

Originally Posted by dissident

Do you have any hints for me, Thomas Richter?

You need to say his name three times for him to appear.

[aros-sg]

Maybe create your own screen bitmap with manually allocated planes that are misaligned (address multiple of 2, but not 4 or 8) such that only 1x is possible. And then open screen with custom bitmap (SA_Bitmap tag).

[dissident]

Originally Posted by aros-sg

Maybe create your own screen bitmap with manually allocated planes that are misaligned (address multiple of 2, but not 4 or 8) such that only 1x is possible. And then open screen with custom bitmap (SA_Bitmap tag).

Not a bad idea. I guess I have to explain what should be the purpose of this screen. It‘s part of a display downgrader to get BPL1MOD / BPL2MOD to 0 values. I doubt that the system is smart enough to detect misaligned bitmaps. There must be a reason for the global entry in the graphics base for the bandwidth.

[Thomas Richter]

Originally Posted by Ernst Blofeld

You need to say his name three times for him to appear.

Not really, but I was busy with PCI workings and finish a driver for PCI-based S3 graphics cards...

No, the FMode is nothing the Os allows you to set or play with. Your best option is probably BestModeID() of graphics to request modes with certain properties. However, what exactly is the application here?

[aros-sg]

Originally Posted by dissident

Not a bad idea. I guess I have to explain what should be the purpose of this screen. It‘s part of a display downgrader to get BPL1MOD / BPL2MOD to 0 values.

Open screen with a width which is multiple of 64?

I doubt that the system is smart enough to detect misaligned bitmaps. There must be a reason for the global entry in the graphics base for the bandwidth.

It needs to be smart enough, otherwise it would break old software. OpenScreenTagList (-> SA_Bitmap) exists since AOS 2.x but the AllocBitMap() only since 3.x. Before that everyone created bitmap and bitmap planes manually. Also in things like double buffering (animation player). AllocRaster() only does 16x alignment, not 32x or 64x as required by higher fetchmodes. It again would break things, if it did.

[Thomas Richter]

Originally Posted by dissident

I doubt that the system is smart enough to detect misaligned bitmaps. There must be a reason for the global entry in the graphics base for the bandwidth.

Actually, graphics goes through multiple functions for computing what the best possible FMode could be, not only depending on alignment but also on the number of reserved sprites and adapts to the requirements. But the algorithm is opaque and undocumented.