RadiSys EPC-2 Especificações descarregar pdf (Página 55)

VXIbus Interface

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

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:

• Little endian, characteristic of Intel microprocessors, where the

least-significant data byte (LSB) is stored in the lowest byte address

Address + 3 Address + 2 Address + 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

Address + 3 Address + 2 Address + 1 Address

Byte 3 Byte 2 Byte 1 Byte 0

LSB MSB

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