Subject: Re: SCSI questions
To: Michael L. VanLoon -- Iowa State University <>
From: Greg Cronau <>
List: current-users
Date: 05/14/1994 00:40:57
Michael L. VanLoon -- Iowa State University
>>I am currently running Netbsd current from the Aprill 6th sources.
>>My machine is as follows:
>>	AMI Enterprise III EISA 486-66 with 12meg.
>>	Bustek 747S EISA Fast-SCSI at 0x330, int-11, DMA-5
>>	Adaptec 1542B ISA SCSI at 0x334, int-12, DMA-6
>>2 days ago I opened the machine up to make 2 addtions: I added an additional
>>8 meg of memory to bring the total to 20megs,
>>5.) Does Netbsd properly handle DMA to ISA devices from memory beyond the
>>    16mem boundary?
>No.  It has been posted repeatedly that it doesn't.  The ISA bus
>doesn't address more than 16 meg, and NetBSD doesn't yet do anything
>to compensate for that.  Your EISA BusLogic card should work fine in
>20 meg, but the ISA-bus Adaptec will only work if you have less than
>16 meg.

Ok, thanks. I must have missed the earlier posts. I knew that an ISA
bus master card couldn't get to memory beyond 16meg directly, but most PC
based UNIX that I have used, have included some buffer dancing logic to 
make sure the DMA is always below the 16meg boundary. I just didn't know
whether netbsd was capable of this.

>>Weird question: Now that I have added the extra memory, I notice a couple
>>of operations in X that actually seem alittle *slower*.
>It's possible that your caching hardware is poorly designed.  There
>are some cheap caches out there that will only cache the first 16 meg
>of RAM.  Your last 4 meg might not be getting cached.

Ah yes, I forgot about that. However the AMI Enterprise III is a fairly
topend motherboard and I can't believe that would have done something that
silly. I can also believe that on an ISA board, but this is an EISA. *But*,
I may have something mis-configured in the bios setup.