Understanding DIWHIGH register

[jman]

Hello,

yet another basic concept I cannot grasp.
I "discovered" that it would be interesting to use the ECS register DIWHIGH instead of writing DIWSTRT+DIWSTOP.

There's unfortunately a basic concept I cannot understand: how to set bit values in this register, as explained here.

According to the documention I should distribute the usual window coordinates (DIWSTRT=$2c81, DIWSTOP=$2cc1) like the following scheme. I'm colouring the relevant bits to show where I think I should pick them from DIWSTRT+DIWSTOP and set in DIWHIGH):

DIWSTRT $2c81 %0010 1100 1000 0001 VSTART,HSTART
DIWSTOP $2cc1 %0010 1100 1100 0001 VSTOP,HSTOP

....................5432 1098 7654 3210 bit position
DIWHIGH.......0010 0001 0010 0001

cyan = Horizontal stop, most significant bit.
green = Vertical stop, most significant 3 bits.
red = Horizontal start, most significant bit.

Please bear with me once more and help me with this, it's driving me crazy. Am I calculating the bits correctly? (answer: no, idiot)

Why do I have to put twice the 3 msb of VSTOP? I sort of expected this register to contain all 4 coordinates (vstart&vstop, hstart&hstop)

Finally, is there a PDF of the AHRM, third edition? Probably I can look up a complete explaination in there.

Thank you, again.

[pmc]

Is the DIWHIGH register not used in addition to DIWSTRT / STOP rather than instead of...?

To me, the docs at the link you posted seem to be saying that DIWHIGH either has no effect on DIWSTRT / STOP if it's not written to at all or effects DIWSTRT / STOP to work for direct start / stop positions if it's written last.

That would seem to hold true taken in relation to the bit descriptions too ie. DIWHIGH defines the most significant start / stop bits for vertical (V8, V9, V10) and horizontal (H8) while DIWSTRT / STOP defines the less significant start / stop bits (V0-7 & H0-7)

To put it another way, doesn't DIWHIGH just override the implied V8 & H8 = 0 for start positions or implied V8 & H8 = 1 for stop positions in DIWSTRT / STOP...?

[Toni Wilen]

Originally Posted by pmc

To put it another way, doesn't DIWHIGH just override the implied V8 & H8 = 0 for start positions or implied V8 & H8 = 1 for stop positions in DIWSTRT / STOP...?

Yes. Pre-ECS "hidden" V8+ and H8 can be modified using DIWHIGH. (H8 bits require ECS Denise or AGA, V8+ bits require ECS Agnus or AGA)

Note that it is also easy to get caught in hidden trap when using DIWHIGH. For example you will get unexpected result if you set DIWSTRT V0-V7 to zero and then update DIWHIGH VSTART8 = 1 if current vertical position = 0.

[jman]

I have read again the register description and it is consistent with your suggestions.
However I don't understand how to use it to increase the size of the usable screen.
What would be useful is probably a couple of examples. I'll note this issue as I progress and check the documentation around.

Thanks.

[Toni Wilen]

Originally Posted by jman

However I don't understand how to use it to increase the size of the usable screen.

You don't.

DIWHIGH/DIWSTRT/DIWSTOP only configures location and size of display window.

If you want more scanlines or horizontal size, you need programmed screen mode (non-PAL/NTSC compatible) which is something much more complex and usually not worth the trouble.

[jman]

Originally Posted by Toni Wilen

You don't.
DIWHIGH/DIWSTRT/DIWSTOP only configures location and size of display window.

So, according to the description of the registers, DIWHIGH "allows larger start and stop ranges" and - according to what you and pmc suggested - this register will override the otherwise defined DIWSTRT+DIWSTOP display position.
Isn't it equal to say that the display size is extended?
Maybe I'm using "display size" improperly, I'm talking about how many pixels of usable display I have.

If I write:

 dc.w $2cc1,diwstrt

or:

 dc.w $2ac1,diwstrt

Am I not actually enlarging the display area? (let's forget for a moment the consequences of displaying an area larger then the image size).

Thanks :-)

[Toni Wilen]

Originally Posted by jman

So, according to the description of the registers, DIWHIGH "allows larger start and stop ranges"

It only means register range is larger (more width/height combinations possible).

Actually DIWHIGH (on PAL/NTSC modes) allows smaller display sizes than was possible on OCS machines

DIWSTRT and DIWSTOP (without DIWHIGH) already allows maximum (again, PAL/NTSC) display size.