ECS super hi-res palette limitations

[absence]

I understand the maximum of two bitplanes due to DMA bandwidth limitations (twice the speed of hi-res leaves half the amount of data per pixel), but why is the palette limited to just 64 colours rather than the usual 4096? It's half the number of bits per channel, so I guess there's another kind of bandwidth limitation at the same time. Is it known how the hardware operates at this level, or is it a black box to guess (educationally) about?

[roondar]

I'm not sure how it works internally in the chip, but the Hardware Reference Manual states:

Originally Posted by HRM

SuperHires mode has a coarser granularity of color control than either Hires or Lores modes. This is because the timing of color conversions at these very high pixel rates requires special "tricks". There are only two bits of red, green and blue color resolution per hires pixel.

So I guess there is some limitation in Denise which stops it from being possible. Perhaps someone else can give more hardware specific answer? It's certainly interesting, though!

[Toni Wilen]

AFAIK it is due to Denise internal palette RAM bandwidth limit. Chip manufacturing technology used for pre-AGA Amiga chipset simply wasn't fast enough for superhires.

This hack allows to get two pixels worth of color information using single palette RAM read (with halved color component value range per pixel).

[roondar]

Sounds logical. Thanks for the info!

[absence]

Originally Posted by Toni Wilen

This hack allows to get two pixels worth of color information using single palette RAM read (with halved color component value range per pixel).

I see, that makes sense. Does it mean software has to pack two colours together in each palette register manually?

[Toni Wilen]

Originally Posted by absence

I see, that makes sense. Does it mean software has to pack two colours together in each palette register manually?

graphics.library handles it automatically (Note that AGA KS ROMs don't support this mode anymore). 3rd edition HRM ECS extra chapter documents the "encoding".

[ross]

It can also be viewed online here: http://amigadev.elowar.com/read/ADCD.../node00A4.html
not a very comfortable mode

[absence]

That's hilarious, what a hack. Thanks for the information!

[Thomas Richter]

Originally Posted by Toni Wilen

graphics.library handles it automatically (Note that AGA KS ROMs don't support this mode anymore). 3rd edition HRM ECS extra chapter documents the "encoding".

Actually, there are no longer AGA and ECS versions of the graphics.library. That's just a single code.

Concerning the palette usage, the trick is that every color(!) is responsible for generating two pixels. That is, once a particular color is loaded into the DA converter, it generates from the two most significant bits of each channel the left pixel, and the two least significant bits of each color go into the right pixel. Thus, the Denise logic stays almost identical, the pixels are duplicated at the very last step of the DAC.

Hence 4 bits per color = 12 bits per pixel for a regular mode, but 2 bits per color = 6 bits for SHIRES, giving the data rate for 2 pixels (2*6 = 12).

A very fine hack indeed.

[AmigaHope]

They could have conceivably just added an extra CLUT buffer to Denise to get around this, and keep the 12-bit palette, but they didn't. I'm not sure why they didn't.

[ovale]

I think the problem is the CLUT max speed is 70ns. Super hires requires 35ns. In super hi res Denise works as in hi res except 2 pixels are emitted every 70 ns using that trick. BTW, Denise has no color DACs.