Accelerator board + Zorro II: good idea bad idea ?

[Clad]

Hello everyone !


I remember having read that it's not a good idea to use ram on Zorro II boards along with an accelerator that comes with its own memory like a TF53X or a Dicke Olga, as it would force the whole fast mem to be limited to Z2 bus speed.


My question is: is it limited to ram, or should you avoid addressing anything Z2 with such accelerator ?


For instance, should I avoid buying a Z2 based Freeway USB or using my GVP SCSI ? Or could I theoretically keep using my SCSI drive with no penalty to performance as long as I disable the integrated ram through jumper ?


Thanks !

[thomas]

I don't think that the accelerator RAM will operate at reduced speed only because there are some Z2 expansions in the system.

It's rather so that when the CPU accesses the Z2 bus it has to wait for the Z2 bus to reply before it can continue with other work.

So if you copy data between the Z2 bus and the accelerator RAM, this copying will run at Z2 speed. Not because the accelerator becomes slow but because the CPU has to wait for the Z2 bus.

In other words, if you had a Freeway and a GVP SCSI before, these won't become any faster if you add a CPU accelerator. In fact the whole system probably does not feel any faster if your harddrive is connected to the Z2 bus.

Only calculations which the CPU can do solely with accelerator RAM will benefit from the acceleration. So you'll probably become able for example to decode JPEG images or MP3 files at a higher quality than before.

[Clad]

But you never know if any given software would pick the accelerator ram or the Z2 ram, so the system would still feel slower with configured Z2 ram would it not ? I'm not sure exactly which memory would be addressed first, but my guess would be that Z2 ram is accessed through a lower adress range and would tend to be picked first unless the software explicitly avoid or put lower priority to the Z2 range.

So there is no performance penalty to having say a Z2 SCSI drive over built in IDE on the accelerator board except of course for being reduced to Z2 speed ? In that case it's just a matter of picking your bottleneck: the Z2 available bandwitdh or your available CPU cycle.

[nogginthenog]

RAM regions in AmigaOS have a priority. Higher priority regions are used first.

Typical values (for me at least)
Chip RAM = -10
Motherboard fast RAM = 30
Accelerator RAM = 40

Of course a program can request Chip RAM.

You can see this information in SysInfo.

[Clad]

Didn't know that, that's good to know thanks !