LETTER OF THE MONTH
PAED off
In issue 263 of Linux Format, one of the stated reasons for the phasing out of 32-bit operating systems was the ‘hard’ 4GB memory limit of 32-bit processors. I had thought this for many years as well.
However, for Intel’s Pentium Pro line of processors, a technology called Physical Address Extension (PAE) was introduced that made it possible for up to a theoretical 64GB of memory to be addressed on these 32-bit systems.
I first came across this feature when working with Linux-based checkpoint firewalls, when a firewall manager that was running a 32-bit Linux kernel had 6GB of memory according to top , which prompted my research into this. The reason why I – and I suspect most people – were unaware of this technology was that no 32-bit Windows desktop operating system supported PAE, although I believe that Windows Server editions did.
So while the 386, 486, and even Pentium processors had the hard 4GB limit, from the Pentium Pro through to the first 64-bit Pentium 4 processors more memory was addressable provided the chipset and motherboard supported it. Ashley Black, Reading, Berkshire
Neil says…
We are aware of the PAE system! We’ve written about how some 32-bit builds require a Pae-supporting CPU to work at all, while there was an issue with some Pentium-m CPUS misreporting their support for it. While it’s certainly useful, my understanding is it still limits individual software tasks to a 4GB limit – it’s just that overall an OS can allocate beyond the hard 4GB limit.
It certainly helps make memory-heavy applications work far better on 32-bit systems. But be aware that there’s a hit to performance because an additional page directory table has to be traversed for every memory access. Phoronix ran some test back in 2009 and noted this was pretty significant (www.phoronix. com/scan.php?page=article&item =ubuntu_32_pae).
Windows helped confuse the situation as it artificially limited its consumer OS (and even some server editions) spins to 4GB, for compatibility issues with the no-execute (NX) CPU feature. But the entire system has to support over 4GB of memory, i.e. the CPU, motherboard chipset and physical memory slots, which older systems can struggle with.