[roondar]

Originally Posted by idrougge

What I was wondering was how the 8 MHz clock mapped to the picture generation. The Amiga's 7.xx clock maps directly to the PAL/NTSC colour clock

Ah, I misunderstood your question then. Though it seems to have been answered by now.

Originally Posted by kovacm

So MC68000 in ST can always access RAM in "full speed" (only non-4 cycle instruction will cause delay/waste cycles) and it is almost 4MB/s in practice.
What about Amiga 500 and chip RAM? MC68000 also can read/write 4MB/s if there is no contention from custom chips on bus/ram? Is speed also 4MB/s?

Atari ST memory speed should be somewhere near 4MB/s for reading or writing (though in practice the CPU will never reach that because of instruction decoding & looping requirements). Don't forget that any figures are only one-way, so a copy is half speed and a 'soft sprite' operation will be at most 1/4 of this.

Amiga memory speed is more complicated. Up to 4 bitplanes in lowres / 2 bitplanes in hires the CPU is, like the Atari, not slowed down (except on non-4 cycle multiples instructions). Since its clock speed is ~7MHz, the speed is then somewhere near 3.5MB/s.

When more bitplanes are used, memory speed goes down (5 BPL=75% speed, 6 BPL = 50%, hires 3 BPL = 50%, hires 4 BPL = 0%), but only during the time bitplane DMA is used. So a 5 BPL 320x256 scrolling screen has the CPU running at ~75% speed of each visible scan line and at 100% for the remainder, or roughly 80% overall.

Memory speed for the CPU is further impacted by Audio, Disk, Copper, Sprite and Blitter DMA. Which I haven't included for clarity.

And what is speed of (real) FAST RAM in Amiga 500?

Fast RAM on an A500 always runs at full speed for the CPU, or ~3.5MB/s regardless of any custom chip usage. Note that this is more of a gain than might be thought, clever programming can keep the Blitter/Copper/etc busy while doing stuff with the CPU in Fast RAM. This can improve performance by quite a bit.

hm... Shifter in ST do not even use available 4MB/s but rather around 2MB/s for 32KB screen (320x200 in 16 color, 60Hz; and somewhere more for 640x400 mono @71Hz on SM124) - rest of available memory cycle is used for memory refreshing. Someone claim that Atari use RAM outside manufacture specification to achieve this speed (4MB/s for CPU and 4MB/s for Shifter) so this additional refreshing cycles are necessary because of this.
But, what prohibit in ST design to have e.g. 32colors (5 bitplanes)? There is enough bandwidth for this...

The same that prohibits the Amiga OCS design from having 8 BPL screenmodes: design time decisions.

My guess is that Atari didn't want to spend the time and money needed to create a better graphics chip - the one they had was significantly better than the 8 bit chips and consumer level PC hardware as it was and the Amiga wasn't on the market yet.