National Instruments NI-VXI

Chapter 3 Software Overview

Programming Considerations

All accesses to the VXI/VMEbus address spaces are performed by

reads and writes to particular offsets within the local CPU address

space, which are made to correspond to addresses on the VXI/VMEbus

(using a hardware interface). The areas where the address space of the

local CPU is mapped onto the VXI/VMEbus are referred to as

windows. The sizes and numbers of windows present vary depending

on the hardware being used. The size of the window is always a power

of two, where a multiple of the size of the window would encompass

an entire VXI/VMEbus address space. The multiple for which a

window currently can access is determined by modifying a window

base register.

The constraints of a particular hardware platform lead to restrictions on

the area of address space reserved for windows into VXI/VMEbus

address spaces. Be sure to take into account the number and size of the

windows provided by a particular platform. If a mapped pointer is to be

incremented or decremented, the bounds for accessing within a

particular address space must be tested before accessing within the

space.

NI-VXI uses a term within this chapter called the hardware

(or window) context. The hardware context for window to VXI/VME

consists of the VXI/VME address space being accessed, the base offset

into the address space, the access privilege, and the byte order for the

accesses through the window. Before accessing a particular address,

you must set up the window with the appropriate hardware context.

You can use the MapVXIAddress function for this purpose. This

function returns a pointer that you can use for subsequent accesses to

the window with the VXIpeek and VXIpoke functions.

On most systems, VXIpeek and VXIpoke are really C macros

(#defines) that dereference the pointer. It is highly recommended to

use these functions instead of performing the direct dereference within

the application. If your application does not use VXIpeek and

VXIpoke, it might not be portable between different platforms. In

addition, VXIpeek and VXIpoke allow for compatibility between the

C language and other languages such as BASIC.