Shared Memory Operations

Note  There are two distinct cases for using shared memory operations. In the first case, the local VXI controller exports general-purpose memory to the A24/A32 space. In the second case, remote VXI devices export memory into A24/A32 space. Unlike the first case, the memory exported to A24/A32 space may not be general purpose, so the VISA Shared Memory services do not control memory on remote VXI devices.

A common configuration in a VXI system is to export memory to either the A24 or A32 space. The local controller usually can export such memory. This memory can then be used to buffer the data going to or from the instruments in the system. However, a common problem is preventing multiple devices from using the same memory. In other words, a memory manager is needed on this memory to prevent corruption of the data.

The VISA Shared Memory operations—viMemAlloc() and viMemFree()—provide the memory management for a specific device, namely, the local controller. Since these operations are part of the INSTR resource, they are associated with a single VXI device. In addition, because a VXI device can export memory in either A24 or A32 space (but not both), the memory pool available to these operations is defined at startup. You can determine whether the memory resides in A24 or A32 space by querying the attribute VI_ATTR_MEM_SPACE.