Address Consistency - Xycom XVME-678 Manual

4.8.2 Address Consistency
XVME-678/688 Manual
July 1994
Address consistency refers to the situation where the data byte at both the XVME-678/688 and VMEbus
addresses is the same for each byte address. In other words, the XVME-678/688 and VMEbus memory
images appear the same. Address consistency is desirable for byte-oriented data such as strings or video
image data. Consider the example below of transferring the string "Text" to VMEbus memory using a
32-bit transfer (Note that the 80386SX/486SLC/e processor splits the 32-bit transfer into two 16-bit
transfers) :
First 16-Bit Transfer
80386SX
Byte-swapping
VME
Data Bus
Hardware
Data Bus
Address
D15
I
::<
I
D15
Address
I I
M
'T'
D8
D8
'T' M
M+1 'e'
D7 D7
'e'
M+1
I
I
M+2
'x'
DO DO
M+2
M+3 't'
M+3
XVME·678/688 VMEbus
Memory
Second 16-Bit Transfer
Address Address
M
'T'
80386SX
Byte-swapping
VME
M
Data Bus
Hardware
Data Bus
M+1
'e'
D15
I
::<
I
D15
M+1
I I
M+2
'x'
D8 D8
'x'
M+2
M+3
't'
D7 D7
't' M+3
I I
XVME·678/688
DO
DO
VMEbus
Memory
Figure 4-12. Maintaining Address Consistency
Notice that the data byte at each address is identical. To achieve this, the data bytes needed to be swapped
as they were passed from the 678/688 internal bus to the VMEbus. The XVME-678/688 incorporates
byte-swapping automatically in hardware. No software concern is necessary if address consistency is
desired.
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