EM64T supported?

Chuck Swiger cswiger at mac.com
Thu Nov 17 20:50:26 GMT 2005

Dan Nelson wrote:
> In the last episode (Nov 17), Chuck Swiger said:
>>EM64T uses 64-bit wide registers and addressing, and can talk to >4GB
>>of RAM natively.  Older processors may still support >4GB of physical
>>RAM using the PSE/PSE-36 CPU extensions, but are still using 32-bit
> PAE/PAE36, right?  Note that if you enable PAE, some drivers may not be
> available.  See the PAE kernel config file for a list.

PAE is related, but I don't believe "PAE36" exists; cpuid lists these:

PSE    Page Size Extensions
PAE    Physical Address Extension
PSE-36 36-bit Page Size Extension

I believe PSE lets you choose whether your MMU uses a 4KB or a 4MB pagesize for 
virtual address translation.  PAE was the first attempt at supporting more than 
4GB of address space, but I gather it requires doing bank swapping or something 
fairly awkward that doesn't play too well with VM, whereas PSE-36 integrates 
more easily.

The other point you've made is correct, that is, a fair number of drivers don't 
understand PAE/PSE36 yet, and will not work using it-- generally because the 
hardware associated with the driver has a DMA engine which is limited to 32-bit 
addressing.  You end up having to double-buffer or use "DMA bounce buffers", 
whatever phrase you wish to use.  :-)

This link seems to have a more complete description:



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