Comment by amiga386
7 hours ago
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 !!!
> 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
The Atari ST had a professional resolution of 640x400 monochrome with a 70hz refresh rate. The Amiga had a 640x200 resolution non-interlaced, or a 640x400 interlaced which was basically a kid's toy.
It was nice of the ST to offer a special black and white screen mode on a custom monitor. The Amiga had the memory bandwidth to offer the same, but chose to only offer PAL/NTSC and no other custom video signals at launch, probably to avoid making its display chip even more complicated.
The Amiga had a "professional" resolution of 1008x1024 PAL or 1008x800 NTSC, 4-colour greyscale with a 15Hz refresh rate via its A2024 monitor (which effectively sampled 4 screens worth of video output and stitched them together).
You could also buy a "flicker fixer" (later included internally in the A3000) which gave you full colour 640x512 at 25Hz or 640x400 at 30Hz if you didn't like interlace.
In 1990, the ECS chipset gave you "Productivity" mode (640x480 at 60Hz, 4 colours) and "Euro72" (640×400 at 70Hz, 4 colours) - there's a nice list here: https://amiga.lychesis.net/articles/ScreenModes.html
But different strokes for different folks. The Amiga distinguished itself by being easily genlockable to video and having a 4096 colour palette and 21kHz 4-channel 8-bit sampled sound replay in 1985. The ST distinguished itself by having MIDI ports and a choice of either monochrome monitor, or the TV (or a colour monitor based on NTSC/PAL) if you wanted to do anything in colour. It found its niche in controlling other musical devices.
But the ST did have a garish green low-res desktop by default.
I never knew anyone who used the ECS "productivity" modes. That would've required an expensive multisync monitor since other Amiga stuff would still need the 15 kHz support. The only useful feature of ECS, as far as I can recall, was support for additional chip RAM.
Later Amigas also supported VGA monitors with 31kHz horizontal refresh rate, with just a passive adaptor. I used a VGA monitor with my Amiga 1200 up until I switched to PC, and a tweaked video mode with 704×520 pixels.
If you wanted to play games on an Amiga or an Atari ST you needed to use a TV-signal compatible 15kHz monitor or a Multisync monitor. I ran desktop programs in 640×200 mode with four colours on both my ST and my Amiga 500 before I upgraded.
There were also "flicker fixer" add-ons for the Amiga, converting the interlaced modes to VGA signals: one was built into the Amiga 3000.
As a kid the 70 Hz made a much more professional impression on me than the colors, but of course the colors were much cooler.
I had an Amiga 500, then a 3000. You could really feel the slow performance with 8 and 16 color Workbench screens on the 500.
For everyone wondering why there are so many dollar values in the diagram: $ meant hex in the vernacular of the day.
(Could mean string too, but that'd a different story)
I so wish we could go back to that convention, rather than the silly '0x' convention. Every language I write in my head (and there's been a bunch) uses that.
But then there ist fast-RAM
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