In IBM AIX, cross-memory services let kernel code access or transfer data in an address space other than the current process or the kernel’s global address space. The kernel uses a descriptor for the target region: attach to obtain access, transfer data with the appropriate service, then detach when finished. The term also appears in z/OS and CICS, but those facilities are not interchangeable with the AIX kernel API.
What “cross-memory” means in AIX
Each process runs within an address space. A kernel operation sometimes needs to work with data that belongs to a different address space—for example, data needed for asynchronous device or kernel work. AIX calls the relevant kernel interfaces cross-memory services.
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IBM’s AIX 7.2 documentation states: “Kernel processes must be provided with a valid cross-memory descriptor to access address regions outside the kernel global address space or kernel process address space.” The descriptor is best understood as a controlled access handle to a specified region, not as a promise that two processes share one universal pointer.
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1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesCross-memory describes access across address-space boundaries. It does not mean a consumer memory accessory or a general-purpose cloud service, and it does not necessarily mean that a transfer avoids copying; behavior depends on the service and platform.
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How the AIX service sequence works
- Attach to the target region. The process containing the data uses
xmattachorxmattach64to obtain a cross-memory descriptor. IBM specifies that attachment must be performed while under a process. - Transfer data with the descriptor. Use
xmeminto transfer data from the target address space into kernel space, orxmemoutto transfer data in the other direction. IBM describesxmeminas transferring data “from an address space to kernel space.” - Detach when the operation is complete. Call
xmdetachto end descriptor-backed access.
Some transfers can run at interrupt-handler level when the referenced buffers are in memory. That qualification matters: the fact that a transfer service can be used in an interrupt handler does not remove the requirement to establish the descriptor under a process.
Where cross-memory services are useful
IBM’s AIX kernel-service documentation describes cross-memory operations for asynchronous access to user data, including work involving direct memory access (DMA), interrupt handlers, and kernel processes. For DMA preparation, AIX provides xmemdma64 to prepare a page for DMA processing. These interfaces address specific kernel and device-work requirements; they are not ordinary application-level ways to make unrelated processes share memory.
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AIX interfaces at a glance
| Interface | Role in the AIX sequence |
|---|---|
xmattach / xmattach64 |
Obtain a descriptor for access to a specified region in another address space. |
xmemin |
Transfer data from the target address space into kernel space. |
xmemout |
Transfer data from kernel space toward the target address space. |
xmdetach |
End descriptor-backed access to the region. |
xmemdma64 |
Prepare a page for DMA processing. |
Why the platform distinction matters
Cross-memory is platform-specific terminology. IBM warns that AIX cross-memory operations are machine dependent, which can make a kernel extension harder to port. Code that uses these interfaces should therefore be evaluated against the target AIX release and machine rather than assumed to work unchanged elsewhere.
IBM also uses “cross-memory services” in its mainframe documentation, but that is a related term, not the same AIX API. In IBM CICS Transaction Server 5.5, an MRO link can use cross-memory (XM) when both ends select XM. IBM says this can reduce the instructions needed to transmit messages and reduce MVS common service area storage. CICS’s guidance also calls out security considerations and requires the CICS address spaces to be nonswappable.
For that CICS MRO case, the relevant comparison is XM versus the IRC access method, considering message-transmission path length, common-area storage, security, and address-space operational requirements. Those tradeoffs belong to the CICS context; they should not be generalized into a comparison of AIX kernel services.
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