Subject: SCSI questions
To: None <>
From: Greg Cronau <>
List: current-users
Date: 05/13/1994 19:43:26
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

I have an HP 1.355 gig drive on the Bustek controller. I had my Viper
2150S on the same controller. I had an extra adaptec laying around, so
I decided, what the hell, let's see if both will work. They seem to
co-exist just fine and I get much better throughput on my backups.
This setup has run fine for over a month.

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, and I added an NEC CDR-74-1
CDROM drive. I put the CDROM on the adaptec controller, becuase it seemed
to be more of a "low bandwidth device". The 74-1(as opposed to the older 74)
has a switch to select SCSI-I and SCSI-II operation. I set it for SCSI-II
operation. During bootup, the probe seemed to find the drive just fine,
but when I tried to mount a disk, I got an error from the kernel saying
something to the effect "error: DMA past end of isa" or something like
that. I then switched the drive to SCSI-I mode and everything works fine.

The following questions come to mind:
1.) Why did I get that error in SCSI-II mode?
2.) I seem to vaguely recall a warning about mixing SCSI-I and SCSI-II
    devices on the same bus. This may just be computer urban legend, or
    may be OS/hardware dependant, because I know there is no problem doing
    this on a Sun for instance. Should I move the CDROM to the Bustek?
3.) Will the abiility to run the CDROM in SCSI-II mode buy me anything?
4.) My machine is EISA and the Bustek is EISA, but the adaptec is only ISA.
    Could this problem have something to do with the extra memory I added?
5.) Does Netbsd properly handle DMA to ISA devices from memory beyond the
    16mem boundary?

Weird question: Now that I have added the extra memory, I notice a couple
of operations in X that actually seem alittle *slower*. Is it possible
that somewhere in NetBSD their is code that detects the fact that:
a.) There is more than 16meg of memory, and
b.) there is an ISA device present
and then switches the system into some kind of double buffer mode to
compensate? I can't think of any reason that *more* memory should make
an X app *slower*.