Service manual

System Overview 1-29
The system bus to PCI bus bridge module converts system bus commands and data
addressed to I/O space to PCI commands and data; and converts PCI bus commands
and data addressed to system memory or CPUs to system bus commands and data.
An AlphaServer 4100 system has one bridge module; an AlphaServer 4000 system
can have a second bridge module.
The bridge has two major components:
• Command/address processor (CAP) chip
• Two data path chips (MDPA and MDPB)
There are two sets of these three chips, one set on each side of the module. Each set
bridges to one of the PCI buses on the PCI motherboard.
The interface on the system bus side of the bridge responds to system bus commands
addressed to the upper 64 Gbytes of I/O space. I/O space is addressed whenever bit
<39> on the system bus address lines is set. The space so defined is 512 Gbytes in
size. The first 448 Gbytes are reserved and the last 64 Gbytes, when bits <38:36>
are set, are mapped to the PCI I/O buses.
The interface on the PCI side of the bridge responds to commands addressed to
CPUs and memory on the system bus. On the PCI side, the bridge provides the
interface to the PCIs. Each PCI bus is addressed separately. The bridge does not
respond to devices communicating with each other on the same PCI bus. However,
should a device on one PCI address a device on the other PCI bus, commands,
addresses, and data run through the bridge out onto the system bus and back through
the bridge to the other PCI bus.
In addition to its bridge function, the system bus to PCI bus bridge module monitors
every transaction on the system bus for errors. It monitors the data lines for ECC
errors and the command/address lines for parity errors.
NOTE: When errors are logged, the two bridge modules on the AlphaServer 4000
are differentiated in the error log by their engineering code names, the left hand
horse and the right hand horse. The left hand horse is the B3040-AA module; it is in
the left most slot on the system bus motherboard when seen from the rear of the
drawer. The right hand horse is the B3040-AB module, and it is in the right most
slot on the system bus motherboard when seen from the rear of the drawer.