User`s guide

VxWorks

Hardware Considerations Guide, 6.0

with this buffer. Having done all this, memory buffers for DMA operations are

relatively safe from the effect of memory coherency in a copyback situation.

Many processors implement write pipes that can buffer write operations when the

bus controller is busy. Because of this, VxWorks device drivers make use of a

macro,

CACHE_PIPE_FLUSH, to flush cache contents. A CACHE_PIPE_FLUSH

operation should be inserted between any I/O write operation and a I/O read

operation. If a routine begins with an I/O read then drivers should assume that an

I/O write operation precedes it.

MMU Support

VxWorks supports several different MMUs through a virtual memory library.

Because virtual memory is inappropriate for some real-time applications,

VxWorks can be configured to not include virtual memory support. In other

applications, the MMU can provide memory protection for the text section and for

other uses.

For more information on VxWorks virtual memory support, see the following:

■

the reference entries for vmBaseLib and vmLib

■

VxWorks Programmer’s Guide (VxWorks 5.5)

■

VxWorks Kernel Programmer’s Guide (VxWorks 6.0)

■

VxWorks BSP Developer’s Guide

Floating-Point Support

Floating point is supported as a tasking extension to avoid increased context

switch times for tasks that do not use floating-point operations. By default, the

floating-point registers are not saved or restored during context switches. For tasks

that are expected to use the floating-point hardware, each such task can be

spawned with the floating-point option,

VX_FP_TASK, which causes the

floating-point registers to be saved and restored during context switches involving

the specified task. Then, context switch callouts provide the mechanism to

initialize, save, and restore the floating-point context. By switching floating-point

data and control registers in and out, each task effectively shares the single

floating-point unit.

For tasks that do not use the floating-point hardware, the VX_FP_TASK option is

not specified. In this case, floating-point registers are not saved, and the

performance cost incurred by context switch times for those tasks is reduced.