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VXIbus Interface
When accessing the VMEbus in this manner, the source of the VMEbus address lines
is defined below.
A32
31 30 29 0
00 From 486 address bits 29-0
A24
23 0
From 486 address bits 23-0
A16
15 0
From 486 address bits 15-0
The main purpose of the direct VMEbus access mechanism, as opposed to the
E-page mechanism, is for multitasking 32-bit operating-system environments, where
multiple tasks need to make VMEbus accesses. Without this, the tasks would have to
coordinate their use of the E-page mapping registers.
When using the EPC-7 this way to perform VMEbus accesses, one would typically
set up the E-page window for interrupt acknowledge accesses. Also note that the
direct access mappings do not cover the entire VMEbus A32 address range and do
not provide all VMEbus-defined address modifier encodings, but one can use the
E-page mechanism if needed to provide these.
Byte Ordering
There are two fundamentally different ways of storing numerical values in byte loca-
tions in memory:
7 7
• Little endian, characteristic of Intel microprocessors, where the
least -significant data byte (LSB) is stored in the lowest byte address
A 3 A 2 A 1 Address
Byte 3 Byte 2 Byte 1 Byte 0
MSB LSB
• Big endian, characteristic of Motorola microprocessors and the VMEbus
environment in general, where the
most -significant data byte (MSB) is
stored in the lowest byte address
A 3 A 2 A 1 Address
Byte 3 Byte 2 Byte 1 Byte 0
LSB MSB
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