User`s guide

DWMBB Adapter
3.5 VAXBI Wrapped Read Transactions
Both the XMI and the VAXBI have unique and different ways of
ordering data read from memory. Wrapped read transactions are
used by the DWMBB to order data received from XMI memory to
an order that VAXBI devices expect.
When a VAXBI DMA read command is received with an address that is
not quadword- or octaword-aligned, the DWMBB forces XMI address bits
<2:0> to zero when it issues the command to the XMI. This causes the
read data to be returned in the same order that it resides in memory,
as shown in Figure 3–11. Before the DWMBB issues the data on the
VAXBI, it disassembles it so that the requesting VAXBI node gets only
the data requested and in the order requested. The DWMBB uses the
latched VAXBI address to determine how the Read Return Data is to be
disassembled and issued on the VAXBI, as shown in Figure 3–11 and
Table 3–5.
When a longword Read transaction is received from the VAXBI, the
DWMBB transforms the command to a quadword Read transaction on
the XMI. When the read data is received, only the requested longword is
returned to the VAXBI node; the remaining longword is not issued on the
VAXBI.
Figure 3–11 VAXBI Wrapped Read Transactions
Read data as it resides in XMI memory
63 0
Longword 1 Longword 0 QUADWORD 0 −−−
QUADWORD 1 −−−
QUADWORD 0
OCTAWORD 0
Read data as it is returned to the DWMBB from XMI memory
63 0
Longword 3 Longword 2
msb−p093−89
Longword 1 Longword 0
3–26