Re: BIOS and BARs

["David O'Shea" <daveo@corollary.com>]

Anne,

When the PCI spec went from 2.0 to 2.1, the suggestion in 2.0 that only 256bytes
of I/O Space be requested was quite silently turned into the requirement that
no more than 256 bytes of I/O space be requested in a single BAR.   How this got
through review is a very good question, since its sole purpose is to help
support
the IBM PC ISA Bus, with old ISA cards, that really no one cares about anymore.
None-the-less, it is now in the 2.1 spec that only 256 bytes of I/O space can 
be requested.   [How, the spec writers chose to deal with the perfectly legal
2.0 compliant devices that request more than 256 bytes seems to be a complete 
mystery.  The empirical evidence suggests that those perfectly compliant devices
are no longer supported.....   The BIOS people should really take out the
check, or
at a minimum, but in a BIOS option for turning it on or off....   But most
PC BIOS
lack such an option.

The request for memory is perfectly legal.  Unfortunately, on IBM PC
systems, when
you request memory below 1MB for your device, it must come from the
C8000h-E0000h
(or on some very few systems up to F0000h).    This is also the region that all
ISA option ROMs must live, and also the region where all PCI BIOS ROM's are
shadowed
on IBM PC type systems.   Most IBM PC BIOS refuse to honor a requet for memory 
below 1MB because it can only be honored with a memory region from this
small range,
and the BIOS needs that range for other stuff.   Basically, BIOS consider a
board
that asks for below 1MB memory to be "hoggy - asking for too much of a
precious resource".
Such memory requests are usually not granted, and such a BAR (though legal)
is never
satisfied by a IBM PC system BIOS.   

So, although your device seems to be a perfectly legal implementation in 2.0 and
a mostly legal implementation in 2.1, it does not appear that IBM PC BIOS will
want to play fair with your device.   Generally, people request low memory for
either one of two reasons.  Reason 1, they want to have an Option ROM that will
capable of allowing their device to be the boot device, and they wish the Option
ROM to be able to access their device using a memory format.  [Good reason, but
this never works, because of the aforementioned scarcity of low memory space.]
Reason 2, they wish to support MS-DOS without writing a device driver that uses
DMPI services...  [Less good of a reason, and again to bad anyway.]

It really is a shame that the 4K of I/O space that you request is not
granted.  On
a Corollary System (which is IBM PC compatible at boot time, your I/O
request would
be granted.   We choose to ignore the absolutely abysmal REQUIREMENT that
devices
only ask for 256 bytes of I/O space in one BAR.   This is an arbitrary
requirement
in the sense of the PCI bus standing alone.  It is only usefull for supporting 
10-bit decode devices on PCI/ISA systems that wish to do so.   It was a poor bit
of specsmenship to allow this suggestion (in 2.0) to become the requiremnt
that it
is not (in 2.1).   

So, just ship your board in a Corollary machine..... :-)  And rewrite your
option 
ROM to use your I/O space that we will grant.   [ Even we do not satisfy low
memory < 1MB
requests.   Nobody satisfies these requests in IBM PC systems.]

Good Luck,

David O'Shea
daveo@corollary.com

At 09:29 AM 11/28/96 MET, ABIVEN Anne wrote:
>
>I am now testing a PCI board under DOS. Only two base address registers are 
>set. BAR 0 requires 4kbytes of I/O space and BAR1 requires 4kbytes of 
>memory, under 1Mbyte to be seen by dos.
>At boot, the PC (the BIOS) does not give any warning message or error 
>massage but when I look at the content of my BAR1 I see that it did not 
>implement it below 1 Mbyte but on top of the 4Gbytes, so I cannot access my 
>board under DOS.
>Is it normal that the BIOS gives no warning message when it does not respect 
>the requirements given ?
>Is it normal that a PC with nothing else than this board (with 2 BARs only) 
>is not able to satisfy these requirements (4kbytes of memory)?
Uï