Update: ARM7 gets 40-bit, virtualization support

SAN JOSE, Calif. -- ARM Ltd. announced extensions for virtualization and 40-bit addressing to the ARMv7a architecture, two of the key new aspects of ARM's upcoming Eagle core. The extensions aim to serve a variety of new and existing markets from digital cameras, Web-connected TVs and servers.

The lack of 64-bit addressing is one of the barriers to a new initiative bringing ARM cores to servers. The 40-bit addressing will ease but not eliminate that concern for programmers willing to do the heavy lifting the ARM approach requires.

A new so-called Large Physical Address Extension translates 32-bit virtual addresses to 40-bit physical addresses. Thus programs are limited to 32-bit applications, but designers can manage page tables in ways to extend beyond the 4 GByte memory and I/O limit of ARM's 32-bit architecture.

Some observers said the approach was a temporary stopgap and could be a "programmer's nightmare." Nathan Brookwood, principal of market watcher Insight64 (Saratoga, Calif.) said he expects ARM will follow up as early as next year with a true 64-bit core.

The memory limit is particularly acute for digital cameras, said David Brash, an ARM architecture program manager who announced the extensions at the Hot Chips conference. ARM's work on servers currently remains focused primarily on 32-bit implementations, he said.

"Where the 4GByte virtual address space for a process remains sufficient, this extension allows multiple processes to be mapped into a larger physical space and remain resident in memory," said Brash in an email exchange.

"We believe this is a logical extension of the ARM architecture where a 32-bit architecture covers the needs of our application space, but demands such as memory requirements for video/still image support, and the general increase in performance and capability of ARM cores are putting pressures on the physical address space," he added.

The virtualization extensions will create a new privilege level for hypervisors. It supports two-stage address translation for operating system OS and hypervisor levels. ARM is not making virtualization available under its TrustZone security extensions in part because it maintains a goal of keeping trusted code size small. However the extensions work in tandem with TrustZone.

Web-connected TVs could use virtualization to isolate online and broadcast work, Brash said. Other uses include supporting guest OSes for device drivers.

ARM said it will release the spec for the extensions this quarter. It is currently in review with about six third party software companies including Enea, Mentor Graphics, Green Hills Software, Open Kernel Labs and VMware. It is not currently working with Wind River, now part of Intel.

ARM has yet to scope out its virtualization strategy beyond the core. For instance it has not determined how or whether it will mandate use of I/O virtualization specified by the PCI Special Interest Group.

"We are still in our infancy there," said Brash.

For the past nine months ARM has had an architectural model of the extensions available to partners, some of whom already have working hypervisors for nine months. The extensions will be supported in the ARM Amba interface in 2011.