Specifications

5-1
5 5
Chapter 5 - Programming
the VMEbus Interface
This chapter describes the EPC-8 VMEbus interface as seen by a program. Users
should avoid direct use of most of these facilities. Whenever possible, the VMEbus
interface should be accessed through the EPConnect software or other higher level
programming facilities.
The EPC-8 VMEbus interface registers are defined in Appendix F. For specific bit
definitions, refer to that Appendix.
Concepts
The VMEbus interface on the EPC-8 is primarily for master accesses out onto the
VMEbus. The only exceptions to this are the VXI-defined registers accessible in the
A16 space discussed in the previous chapter. There is no slave memory accessible in
the A32 or A24 address spaces.
VMEbus accesses are performed in either of two ways. For real-mode operating
systems, a 64K “window” is provided in high memory accessed at 0E0000-0EFFFF.
Any 64K region of the VMEbus (all address spaces) can be mapped behind this “E
page” window such that the VMEbus address space is visible to the operating system
memory map. For protected mode operating systems, the EPC-8 provides direct
mapping of the VMEbus into the PC memory space above 256 MB.
Atomic access
The EPC-8 supports atomic access to the VMEbus data for data lengths not greater
than 32 bits and only then if it is aligned to its natural boundary; for instance, 32-bit
data must be aligned on a 32-bit address.
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