RadiSys EPC-5A, Справочное руководство по аппаратному и программному обеспечению

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EPC-5A Hardware & Software Reference Manual
Page 60
6 6
When accessing the VMEbus in this manner, the source of the VMEbus address lines
is defined below.
A32
31 3029 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
Figure 6-2. Source of VMEbus Address Lines (Via Direct Mapping).
The main purpose of the direct VMEbus access mechanism, as opposed to the
VME memory window 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 VME memory window mapping
registers.
When using the EPC-5A this way to perform VMEbus accesses, one would typically
set up the VME memory 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
VME memory window mechanism if needed to provide these.
Byte Ordering
There are two fundamentally different ways of storing numerical values in byte loca-
tions in memory:
• 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