User`s guide

DWMBB Adapter
3.2.1 DWMBA Compatibility Mode
There are two different XMI-to-VAXBI adapters, the DWMBA and the
DWMBB. The basic difference is that the DWMBB has a more extensive
address space. The DWMBA compatibility mode is the default mode for
the DWMBB after power-up and XMI node reset. While in this mode the
DWMBB does not perform address translation. This mode requires that
the value loaded into the BIIC’s Starting Address Register be less than the
value of its Ending Address Register. The addressing mode is 30-bit.
The DWMBB detects all transactions from the VAXBI that fall within
the BIIC’s starting and ending address space. All legal transactions
received by the DWMBB are converted into the corresponding XMI
transactions and processed on the XMI. The XMI physical address for
a DMA transaction in DWMBA compatibility mode is identical to the
VAXBI address, VAXBI A<28:0>. The upper address bits of the extended
XMI address format, XMI A<39:29>, are forced to zero.
The DWMBB transforms a VAXBI address to the XMI address format by
first checking that the upper address bit, VAXBI A<29>, is zero and then
generating the XMI address with VAXBI A<28:0> going to XMI A<28:0>
and moving zeros to XMI A<39:29> (see Figure 3–3).
Figure 3–3 DWMBA Compatibility Mode Address
28 0
VAXBI ADDRESS
39 29
0 XMI PHYSICAL ADDRESS
msb−p084−89
28 0
FORCED TO ZERO BY DWMBB
3–8