OCS discussion

[commodorejohn]

A lot of it has to do with chip-RAM throughput. The 6-plane 320px modes are pushing the limits of the bandwidth alotted to the bitplane-fetch channels. 640px mode requires twice as much data for the same bit-depth, so 6-plane 640px is well beyond its capabilities. (IIRC, 4-plane 640px takes all the video bandwidth on OCS.)

As for 640px being slow, that's because when running from chip RAM, the CPU has to compete with the chipset for RAM access, and the more the video takes up, the less chance the CPU has to fetch instructions and data, so it winds up just spinning its wheels more often than in less intensive modes. (This isn't a problem when you have some fast RAM, of course, but since none of the OCS home-computer models shipped with fast RAM, and the A500's default 512KB expander is "slow RAM" that's subject to the same restrictions as chip RAM, it's understandable why programmers shied away from it.)

[commodorejohn]

Do accelerators remove this memory bandwidth contention or do they sit on top of it?

Accelerators don't change anything about the chipset, only the CPU. (Matter of fact, they don't even affect fast RAM, which is why all the really good accelerators have on-board memory that runs as fast as the new CPU.) That's why RTG makes such a difference in performance: access to chip RAM is still constrained by the chipset speed and contention, whereas Z2/Z3 access is unconstrained (and, for Zorro III, faster in general.)

[commodorejohn]

What makes "slow RAM".. slow ?

It suffers the same contention with the chipset for access as chip RAM does.

[commodorejohn]

It's a quirk of Agnus/Fat Agnus, I believe. It was fixed in later versions, but unfortunately most/all stock 500s have it.

[commodorejohn]

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.

[commodorejohn]

So why did ECS not improve this with the improved chipset and faster CPUs?  Did not the chip ram increase thus allowing 640x256(200) 32 EHB?

Because ECS doesn't actually improve chip-RAM throughput; the CPU in the A3000 is faster, but not the chipset. That's why 1280px mode has a maximum of 4 colors - it's still working with the same amount of bandwidth.

As to why that is, I'd guess it's a cost consideration...oh well.

[commodorejohn]

Wait, I can.  Check this out.  What might have been.

Any chance of getting those in non-"crappy JPEG thumbnail" form?