Amiga Screens: A Primer

MiroslavPokorny

Maybe its me, but i believe the primary or secondary reason for screen is different screens had different resolutions and color depths, both of which dont really exist these days on any major OS.

weinzierl

How do different resolutions (pixel sizes) work? I get that everything memory related (including color mode and depth) could be switched on a raster line. In theory it should be possible to change the frequency of the video signal mid screen as well, but I have a hard time to imagine switching repeatedly every frame wouldn't have driven the monitors of the time crazy. Even multi-sync monitors needed probably a couple of frames to sync, right?

LastTrain

I don’t know much about Amiga, but from this article I recognize the Atari 8 bit / ANTIC DLI heritage at play!

amiga386

I break out this diagram every time Amiga graphics hardware comes up: https://amigadev.grimore.org/Hardware_Manual_guide/node02d4.... The Amiga had a great trick. Both the CPU and the display/audio hardware share the same RAM (called Chip RAM), so they have to arbitrate for access to it, through a chip called Agnus, which prioritises who gets to read/write memory, depending on importance. You can starve the blitter, you can starve the CPU, but you can't starve the audio or video or disk I/O. As the 68000 CPU only needed memory access every second clock cycle, the Amiga designers arranged it so that the custom chips preferred odd cycles and saved the even cycles for the 68000. So your 68000 runs at full speed. But if you need more and more work done by the custom chips, it starts to rob some of the even cycles from the 68000. If you have a 16-colour lowres screen (4 bitplanes), Denise (the chip that turns RAM values into video signals) only reads display data on odd cycles. But if you add another bitplane for 32 colours, she needs some of the even cycles. If you add another bitplane for HAM or EHB mode, she needs even more. In hires mode, it needs twice the bandwidth for twice the pixels. So if you have a 4-colour hires screen, it only needs odd cycles and the 68000 is full speed. If you go for 8 or 16 colours, it starts slowing the 68000 down. This is why the default Workbench screen is 4-colour hires. It's hires to look nice and professional like an IBM, and not like a kid's toy like the Atari ST's default lowres GEM interface. It's 4-colours to show it's colourful and not black-and-white, but it's not 8- or 16- colours because that would nearly halve the speed of the CPU !!!

mettamage

The Amiga is before my time, but I'm getting TempleOS vibes a bit, graphics-wise.

agar

I wish there was a way to recapture the feeling of magic produced by using the Amiga. From usability to programmability, everything seemed exciting, new, and with unlimited possibility. It really was like going from a black-and-white world to seeing full color. I always said that if 1% of the effort (and money) put into overcoming the PC architecture's shortcuts was put into the Amiga, the computing world would be in a very different place.