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6. Bus Support

because all VME device drivers are configurable. However, conflicting devices

may be a system issue.

Dynamic Bus Sizing on VMEbus Accesses

There are three address types defined in the specification:

■

A16 short

■

A24 standard

■

A32 extended

In addition, there are often data width restrictions to off-board devices.

Many implementers offer different windows with different data widths (D16 or

D32) to the same VME address.

Especially useful are windows that break the wider D32 accesses into two D16

accesses automatically. This can be achieved with the dynamic bus-sizing

capability of some architectures (for example, 68K).

Some boards require that a register be initialized to indicate data “direction”

(read/write) in addition to the AM (Address Modifier). This is inconvenient.

Wind River does not provide direct support of 64-bit addressing or data. However,

this does not preclude board specific routines from providing such support to the

user.

Dual-Port Memory

Most CPU boards have local (on-board) RAM. Creating a slave access window on

the VMEbus makes the local memory accessible by other CPUs and DMA devices

on the bus. This is also called dual porting. It is required by systems that want

backplane-shared memory to be on the local processor’s RAM. Such dual-ported

memory should fully support RMW cycles as described below.

Dual porting is also required for off board devices that DMA into target memory,

such as the Excelan EXOS-302.

It is useful if the dual-ported memory can be seen by the local processor at the

memory’s external address, although this is often not provided (and is not used by

VxWorks).

Dual-port memory is also very useful during porting; it facilitates the use of

backplane debugging.

Using processor memory for device data buffers can cause a slight amount of

increased interrupt latency. When the device is transferring data, the processor is

unable to fetch instructions or data required by ISRs because of bus contention. For

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RAM 6.0 BUX II Series Guía de usuario Pagina 15