OCS discussion

[vidarh]

A501, yes, as does (IIRC) all trapdoor-slot RAM on the A500. I'm not 100% clear on how Zorro II is implemented on the A500, but it's not through Agnus, so it doesn't suffer from chipset contention.

As far as I remember on the A500 it's is pretty much wired up directly to the CPU and RAM. Been about 18 years since I last saw an A500 PCB up close, though

[Thorham]

With only two bits per gun color you simply get 64 colors total. The four color base palette won't improve this much (if any). Therefore HAM4 is almost useless. Lores HAM6 will most certainly look better than HAM4 for colorful images, and hires lace 16 color with palette slicing will also look much better, and also works straight out of the box.

Then there's lores HAM6 sliced palette, and this is truly superior to Hires HAM4, even if the resolution is lower.

The best way to get many colors in hires is to use a 16 color screen and use palette slicing: You can use Floyd Steinberg dithering effectively, you can change 15 colors in the border without overscan, and you can get 12 more colors by placing sprites behind the bitmap (give bitmap pixels color 0 and the sprites will show through). Best of all, it's all done in software.

[psxphill]

I see, that makes sense. What exactly was the purpose of 1280x256/200 anyway? Very odd resolution.

You need that pixel clock for productivity mode. The sync is programmable, so it would be harder to specifically block that mode than to let you use it.

[Zac67]

The A501 expansion RAM and Zorro-II RAM all becomes slow-RAM in an A500 ..?

No.

With the original A500 Agnus you'd get the infamous 'slow RAM' at $C00000. This happens because the PLCC Agnus was expanded to allow for addressing of 1 MB (much cheaper than an extra RAM controller), but C= 'forgot' to upgrade the chipset registers with the neccessary address bit, so the chips themselves can't reach that high. To make sure that the RAM isn't configured as chip RAM (which simply wouldn't work) the highest address bit is swapped so the RAM appears at $C0 instead of $08.

Since the 'slow' RAM is accessed through Agnus and the chip bus, the CPU has to wait for the bus to be available - thus it isn't any faster than real chip RAM.

For ECS they made up their minds and added the missing address bit to the registers and voilá - 1 MB chip RAM were there.

The A3000's chip RAM is slightly different (similar to AGA) as the CPU can access it 32-bit wide - effectively the CPU's getting double bandwidth. It's no real surprise that Alice is somewhat pin-compatible to the A3k's Agnus and with AGA the 32 bit access can be used by video DMA as well.

However, the CPU's got its own bus with ROMs, CIAs - and fast RAM. This is usually Zorro II connected and it's never slowed down by the chipset!

[freqmax]

A neat hack in the Amiga days would be to make a Zorro-II RAM of a A501 expansion with little or none support circutry. In essence an adapter.

[thedocbwarren]

With only two bits per gun color you simply get 64 colors total. The four color base palette won't improve this much (if any). Therefore HAM4 is almost useless. Lores HAM6 will most certainly look better than HAM4 for colorful images, and hires lace 16 color with palette slicing will also look much better, and also works straight out of the box.

Then there's lores HAM6 sliced palette, and this is truly superior to Hires HAM4, even if the resolution is lower.

The best way to get many colors in hires is to use a 16 color screen and use palette slicing: You can use Floyd Steinberg dithering effectively, you can change 15 colors in the border without overscan, and you can get 12 more colors by placing sprites behind the bitmap (give bitmap pixels color 0 and the sprites will show through). Best of all, it's all done in software.

I know this doesn't matter, but I'd like to understand what you mean.  Why would a control shift not look nice in high res if you have four index colours?  You simply shift away as needed.  You have the ability to change the value of any of the rgbs per control so you could set an index on pixel one and the rest of the image could be a control in theory.

The image I produced assumed 2 colours index (is it really 4?) and uses control RGB changes for each adjacent pixel.  Looks pretty bloody nice to me.

[Zac67]

@freqmax

An A501 is nothing more than some RAM chips, a battery and a clock chip. Since it's also limited to 512 KB there's not too much point in trying to adapt it to Z II.

I know this doesn't matter, but I'd like to understand what you mean.  Why would a control shift not look nice in high res if you have four index colours?  You simply shift away as needed.  You have the ability to change the value of any of the rgbs per control so you could set an index on pixel one and the rest of the image could be a control in theory.

The image I produced assumed 2 colours index (is it really 4?) and uses control RGB changes for each adjacent pixel.  Looks pretty bloody nice to me.

A theoretical HAM4 mode would require 2 bits for mode control: 00 = use index palette color; 01 = hold GB, modify R; 02 = hold RB, modify G, 03 = hold RG, modify B. With the 2 bits left you'd be able to use 4 palette colors. In contrast to HAM6 where the modify value is 4 bits, i.e. you can change any of three subpixel full range, you'd only have 2 bits for modify as well. How'd you want to use them? Step up / down 1 or 2 little steps?

[psxphill]

i.e. you can change any of three subpixel full range, you'd only have 2 bits for modify as well. How'd you want to use them? Step up / down 1 or 2 little steps?

Yeah 4 bits is not enough to do anything meaningful.
 
It might be possible to play around with HAM in hires on the Amiga using copper for writing the additional bitplane data registers. The copper resolution isn't great but it might be possible to do some funky stuff with it. If you can enable HAM6 in hires but disable the dma for bitplane 0 & 1 then you'd effectively have HAM4. You might then be able to use the bitplane data registers to do brightness control from the copper, which would be interesting.

[thedocbwarren]

@freqmax

An A501 is nothing more than some RAM chips, a battery and a clock chip. Since it's also limited to 512 KB there's not too much point in trying to adapt it to Z II.



A theoretical HAM4 mode would require 2 bits for mode control: 00 = use index palette color; 01 = hold GB, modify R; 02 = hold RB, modify G, 03 = hold RG, modify B. With the 2 bits left you'd be able to use 4 palette colors. In contrast to HAM6 where the modify value is 4 bits, i.e. you can change any of three subpixel full range, you'd only have 2 bits for modify as well. How'd you want to use them? Step up / down 1 or 2 little steps?

I got it, so without the four bits you only get 4 levels you can change from the previous pixel vs the full 16 levels.  It's more useful if you wanted to limit the colour range to 4 bits per rgb.  Makes sense.  I'm going to plug this into my simulator to try it out though.