21154 bridge with 840 chipset

[Jim Sather <jsather@sbs.com>]

I am testing prototypes of a PCI/ISA style backplane in which I have placed
an Intel 21154-BC (Rev 5) bridge chip between the 64-bit CPU slot and two
64-bit 3.3V expansions slots. Plug-in CPUs that will drive this are rare,
but we have managed to procure a 64-bit plug-in CPU from Diversified
Technology based on an Intel 840 chipset. I am running diagnostics supplied
with a Syskonnect gigabyte ethernet card and pass all diagnostics tests
(indicating reliable data path) but have substantially slower measured DMA
rates (98 MBytes/sec @ 33 MHz) through the 21154 as compared to the same
diagnostic test and hardware in an unbridged connection (167 MBytes/sec @ 33
MHz). 

I have heard rumors of 840 chipset thoughput issues, but have not found any
verification of that from Intel. Is anyone aware of any 64-bit thoughput
issues with the 840 chipset, especially though a 21154-BC bridge? Any other
suggestions as to a possible source of the problem would be appreciated.