xref: /src/sys/arch/mvme68k/docs/VMEbus-RAM

	$NetBSD: VMEbus-RAM,v 1.5 2023/09/30 20:15:54 andvar Exp $

NetBSD/mvme68k: VMEbus RAM card configuration
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

NetBSD-mvme68k can be configured to support additional RAM boards
accessed over the VMEbus.

This file describes where to configure your VMEbus RAM and how to
point the kernel in the direction of it.

The MVME147 board has a fairly primitive VMEbus controller chip. The
mapping of CPU address to VMEbus address is hardwired and so dictates
what can be seen where by the 68030. From the CPU's perspective, A24
space spans 0x00000000 to 0x00ffffff. However, onboard RAM also spans
this space. With 8Mb of onboard RAM, only the top 8Mb of VMEbus A24
space can be seen. With 16Mb onboard, there is no easy way to get at
A24 space at all!

The other MVME boards have a more sophisticated VMEbus controller
which can remap segments of VMEbus address space anywhere in the CPU's
address space. This document will assume the remap is `transparent',
ie. no translation is taking place. The same restriction as MVME147
applies to these boards in that, without translation, a region of
VMEbus address space is masked by onboard RAM. The size of this region
depends entirely on the size of onboard RAM.

The best place for VMEbus RAM cards is somewhere in A32D32 VMEbus address
space. Obviously, if your VMEbus RAM card doesn't respond to that space
then you'll have to locate it elsewhere. Typically, you may find it
responds to A24D16 only, in which case the CPU-relative address you need
to specify below will be in the 16MB region starting at 0xZZZZZZZZ.

For A32D32, choose an address which is reasonably close to the end of the
MVME board's RAM. That is, if you have 32MB of onboard RAM, set the
VMEbus RAM board to appear at A32:02000000.

This starting address needs to be written to the MVME board's NVRAM at
address 0xfffe0764 for MVME147, and 0xff, as follows:

	147Bug> mm fffe0764 ;L
	FFFE0764 00000000? 01000000   <cr>	<--- you type 01000000
	FFFE0768 00000000? .          <cr>
	147Bug>

Next, you need to configure the end address of VMEbus RAM. Assuming
your RAM card is 8Mb in size, this would be 0x017fffff. You need to
write this value to NVRAM address 0xfffe0768, as follows:

	147Bug> mm fffe0768 ;L
	FFFE0768 00000000? 017fffff   <cr>	<--- you type 017fffff
	FFFE076c 00000000? .          <cr>
	147Bug>

You could obviously combine the above two steps.

If you have more than one VMEbus RAM card, you must configure them so
that they appear physically contiguous in A32 address space. So, to add
another 8Mb card in addition to the card above, it should be jumpered
to start at 0x01800000. In this case, you would change NVRAM location
0xfffe0768 to be 0x01ffffff.

If NVRAM location 0xfffe0764 is zero, the kernel assumes you only have
onboard RAM and will not attempt to use any VMEbus RAM.


Some extra notes on VMEbus RAM cards
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

So... You've got your nice shiny VMEbus RAM card up and running with
NetBSD, and you're wondering why your system runs slower than it did
with less RAM!

The simple answer is "Motorola got it wrong". (Or at least that's my
opinion. If anyone can cure the following, let me know!)

In their infinite wisdom, the designers of the MVME147 decided that
they would disable the 68030's cache on *any* access to the VMEbus.
The upshot is that the cache only works for onboard RAM, not VMEbus
RAM, hence your system runs slower. As far as I can see, the only
way to cure this is to physically cut a trace on the circuit board
and use the MMU to control caching on a page-by-page basis...

Anyhow, hopefully the above instructions have finally put to rest
the most asked question about the mvme68k port.

Cheers,
Steve Woodford: scw@NetBSD.org