Xycom XVME-653, Руководство пользователя

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Chapter 4 – Programming
4-15
Address Consistency
Address consistency, or address coherency, refers to communications between the
XVME-653/658 and the VMEbus in which both architectures’ addresses are the same for
each byte. In other words, the XVME-653/658 and the VMEbus memory images appear
the same. Address consistency is desirable for byte-oriented data such as strings or video
image data. Consider the example of transferring the string Text to the VMEbus memory
using a 32-bit transfer in Figure 4-4.
‘t’ ‘x’ ‘e’ ‘T’
‘T’
‘e’
‘x’
‘t’
‘T’ ‘e’ ‘x’ ‘t’
‘T’
‘e’
‘x’
‘t’
Pentium Register (32 bit)
VMEbus
Byte-swapping
Hardware
Address
M
M+1
M+2
M+3
XVME-653/658 VMEbus
Figure 4-4 Maintaining Address Consistency
Notice that the data byte at each address is identical. To achieve this, the data bytes need
to be swapped as they are passed from the PCI bus to the VMEbus. To maintain address
consistency, enable the byte-swapping buffers by setting setting bits 6 and 7 of the
NVRAM and DiskOnChip Port (Register 234h) to 0 (see p. 2-6).